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Pollution as a threat factor to urban food security in metropolitan K ano, N igeria

Pollution as a threat factor to urban food security in metropolitan K ano, N igeria Introduction According to the declaration of the World Food Summit ( ), the safety of food is second only to its sufficiency in determining food security. Therefore, for people to be food secured, they must have access to sufficient, nutritious, and safe food that may not expose the consumer to health risk. Urban agriculture is a common practice in many cities around the world and in Kano, Nigeria, it is implied centuries old (Dokaji ). Although time has modified the system, yet the principal distinguishing feature remains the use of stream water to irrigate land at the banks. Principal of these streams are Challawa, Getsi, Jakara, and Salanta . The main objective is to produce fruits and vegetables for the consumption of the city dwellers (Binns et al. ). Researches have been conducted on the merits of the system especially on its socioeconomic benefits in terms of job creation, income supplementation, and price subsidy for vegetables (Lynch et al. ; Olofin and Tanko ). Most of these researches, however, failed to engage with a greater understanding of urban farming in relation to specific issues, which importantly include health and environmental concerns (Lynch et al. ). The overall impacts of this land use system on human population in terms of their health and well‐being could be enormous. The sustainability of urban farming in Kano could be very doubtful because of the risks it poses to the people and the environment. Pollution of environmental component with especially heavy metals is of prime concern in this system of production. This is principally because of the use of water derived from streams that flow through the city and often through densely populated and/or industrialized areas as irrigation water while wastes from landfills and sewage sludges are used as fertilizer. These make the land and its products susceptible to pollution with domestic and industrial discharges as reported by Tanko ( ). This is in addition to exposure of the lands to exhausts from automobiles, which has been shown to be a source of heavy metal contamination (Bada et al. ). The concern over metals such as As, Cd, Cr, and Pb arises from their proven potency to be chronic human toxicants when inhaled or ingested (Pharmacopeial Forum ). Previous studies have established the presence of heavy metals in some of the effluents that are being discharged into the Jakara river from the Bompai industrial estate (Sahoo and Klopka ). Binns et al. ( ) have also detected their presence in the waters of the Jakara river while Ya'u ( ) and Wakawa et al. ( ) have detected these metals in some of the plants tissue grown in or around the Jakara and Challawa rivers. Similarly, Dawaki and Alhassan ( ) have detected the presence of some metals in the soils of the Jakara valley. None of these previous studies has, however, synchronized investigations into the various components of the production system in order to project the extent to which it could constitute a human health risk factor. The main objective of this study therefore was to evaluate the potential hazard this system of production poses to the safety of the immediate population that consume the products in terms of their health and well‐being. Methodology The study area This research was conducted at the banks of the Jakara , Challawa, and Watari Rivers within the metropolitan Kano and its suburb in an area lying between latitude 11 o 59′ and 12 o 08′N and longitude 8 o 34′ and 8 o 42′E. Kano is in the dry, subhumid agroecological zone of Nigeria (Ojanuga ). It is characterized by tropical wet and dry climate classified as Aw by W. Koppen (Olofin ). The dominant geology is basement complex (Olofin ; KNARDA (Kano Agricultural and Rural Development Agency) ). The dominant soil of the area is Eutric Cambisol (FDLAR: Federal Department of Agriculture and Land Resources ). The dominant crops being irrigated in the areas are leafy vegetables such as lettuce and spinach. Sampling and sample treatment Selection of sampling site was based on difference in terms of effluent source into irrigation water. Jakara predominantly receives domestic effluents while Challawa receives industrial effluents. Watari is not associated with any form of wastewater and was therefore selected as a control. Three irrigation sites (Yansama, Sharada, and Rafin Kuka); (Akija, Airport Road, and Magami); and (Lambu, Langel, and BUK) were selected to represent up, mid, and downstream sectors of the Challawa, Jakara, and the Watari rivers, respectively (Fig. ). Global Positioning System (GPS) and ground reconnaissance were used for identification of sites and georeferencing. Map of metropolitan K ano showing river network and sampling points. From each zone, the first 1 ha under cultivation was sampled. Stratified grid sampling method (Adepetu et al. ) was employed in soil sampling. 100 m long transect was taken parallel to the course of the river on the side with higher irrigation activity. Soil sample was collected at each 20 m from the surface 0–20 cm, being the rooting zone for most vegetables. Another transect was taken at 20 m parallel to the first and was sampled similarly. The procedure was repeated three more times, thus giving a total of 25 soil samples that were composited into five thereby making a total of 15 samples per river bank. Three water samples were collected randomly in plastic containers across the width of the river at each sampling point based on the assumption that the water is well mixed (Kirda ). As the dominant crops being grown under this land use system are leafy vegetables, Lettuce ( Lactuca sativa L .) was sampled by picking the latest matured leaves of three randomly selected plants at each sampling point. Soil samples were air‐dried, crushed with mortar, and sieved with 2 mm sieve. Plant tissues were carefully washed off of any soils with distilled water, oven‐dried at 70°C to constant weight and were ground and stored in tight polythene bags (Jaiswal ) prior to analyses. Water samples were refrigerated prior to analyses. Laboratory techniques Soil samples were subjected to laboratory analyses using established procedures. The pH was determined using the 1:2.5 soil‐distilled water ratio using EL model 720 pH meter. The electrical conductivity (EC) was determined using 1:2.5 soil–distilled water ratio using Beckman Conductivity Bridge (Model: RC ‐ 18A; Beckman Instruments Inc., Cedar Grove, NJ). Particle size analysis was done using the hydrometer method as reported in Jaiswal ( ). The Walkley‐Black wet‐oxidation technique as described in Jaiswal ( ) was used to determine the organic carbon content of the soil. The ammonium acetate extraction technique (Adepetu et al. ) was used in extracting the exchangeable bases and determination of cation exchange capacity (CEC). Two metals classified for oral ingestion by Pharmacopeial Forum ( ) as Class 1 (elements that should be totally absent) (Pb and Cd) and two metals classified as Class 2 (elements that should be limited) (Cu and Cr) were selected for pollution assessment. The double acid digestion technique (Anderson ) was used in sample extraction for total metal determination in soil. Soluble and exchangeable forms of these metals in soil were determined because of their potency for bioavailability using an adapted sequential extraction technique of Kashem et al. ( ). One gram of each sample was mixed with 100 cm 3 of deionized water. The resultant mixture was shaken for 8 h at 10,000 rpm, left to stand overnight, and then centrifuged for 15 min. The supernatant liquid was decanted and used in the determination of soluble metals in the soil. The residue from the soluble metal fraction was mixed with 100 cm 3 of ammonium oxalate and shaken for 8 h at 10,000 rpm and left to stand overnight, centrifuged for 15 min, and decanted. The solution was used to determine the fraction of metals in exchangeable form. pH of water was determined at point of collection while total dissolved solids (TDS) and the EC of the water samples were determined directly in the sampling containers using an automated digital conductivity meter (Jenway Digital Conductivity Meter Model 4520; Jenway Scientific Equipment, Staffodshire, UK). Total metals in water were extracted with HNO 3 as described by Bäckström et al. ( ). The dried leaves were ashed at 500°C and digested with HCl and KNO 3 , distilled water added and filtered through Whattman No. 4 filter paper. The content of the metals in them was determined from the filtrate. All metals concentrations throughout the work were determined using Atomic Absorption Spectrophotometry (AAS); except Na and K that were determined using flame photometry. Data analyses To estimate the potential hazard, the human population is exposed to as a result of this system, the following factors were taken into consideration; Pollution index (PI) was used in assessing level of pollution for each of the sites. It was computed with the equation of Lacatusu ( ) in which; PI = C ( WS ) C ( RS ) Where C (WS) is the concentration of the metal in wastewater‐irrigated site and C (RS) is the concentration of the metal in a reference site. The control site and the maximum allowable soil limit set by DPR: Department of Petroleum Resources ( ) were used as references. Metal transfer factor (MTF) was used in assessing the tendency of the metals to accumulate in plants' tissues via absorption from the soil. It was computed using the equation of Gosh and Singh ( ) given as: MTF = C Plant C Soil Where C Plant is the metal concentration in plants on dry weight basis and C Soil is the sum of soluble and exchangeable metal concentrations in the soil. Estimated daily intake (EDI) from oral consumption of vegetables by the city's population was used in assessing human exposure risk. It was computed using the equation of Cui et al. ( ) given as: EDI = D × M Where D is daily vegetable intake (g/day) which for the city of Kano is taken to be 255 g/day based on the survey of Kushwaha et al. ( ). M is the mean metal concentration in the edible fraction of the vegetable on fresh weight basis. The dry weight concentration was converted to fresh weight concentration by dividing with a moisture correction factor calculated from the difference between fresh and dried sample weights. Data obtained were also subjected to both descriptive and inferential statistics. Means were compared using analysis of variance (ANOVA) as obtained in SAS package 6.0 (SAS Institute Inc., Cary, NC). Significantly different means were separated using least significant difference (LSD). Pearson moment correlation was also used to establish relationship between metals in water and in soil. Results and Discussions Physicochemical properties of the soil Table shows the physicochemical properties of the soils of the three areas. The texture of the soils is sandy loam to loamy sand. The pH across the three areas can be interpreted as slightly alkaline (Esu ). The mean EC values across the three areas were significantly different from one another with the highest value recorded at Jakara. The soils were neither sodic nor saline, but the alkaline pH and the high EC values at Jakara may potent salinity hazard; a problem that was also noted by Binns et al. ( ). The organic carbon contents across the three sites were all significantly different from one another. They ranked from low in Watari and Challawa to medium in Jakara (Esu ). Mean values of the physicochemical properties of the soils of the three areas Parameter Locations SE Challawa Jakara Watari pH 7.77a 7.42b 7.48b 0.05 EC (mS/m) 1.79b 4.01a 0.58c 0.24 Sand (%) 76.29ab 80.48a 73.41b 1.08 Silt (%) 16.88a 14.56a 18.48a 0.91 Clay (%) 6.80ab 4.96b 8.11a 0.44 Textural Class Sandy loam Loamy sand Sandy loam Ca (cmol/kg) 11.70a 12.67a 7.80b 0.66 Mg (cmol/kg) 2.85a 2.46a 2.15a 0.19 K (cmol/kg) 0.79b 1.85a 0.47c 0.12 Na (cmol/kg) 1.11b 3.20a 0.25c 0.21 CEC (cmol/kg) 19.15a 23.11a 12.63b 1.07 Org. Carbon (g/kg) 8.07b 11.27a 7.16b 0.56 N (g/kg) 0.76b 1.32a 0.74b 0.04 Avail. P (mg/kg) 77.67b 213.52a 31.73c 7.80 EC, electrical conductivity; CEC, cation exchange capacity. Means followed by the same letter in the same row are not significantly different ( P ≤ 0.05). The calcium and magnesium contents of the soils as shown ranged from medium at Watari to high at both Jakara and Challawa using the rankings of Esu ( ) and Landon ( ). The K values for all the three sites were within the high to very high concentrations. The mean Na values were within the medium to high ranges based on the ranking of Landon ( ). The CEC values were within the range classified as medium using the ranking of Landon ( ). The N contents of the soils were high by the ranking of Landon ( ) with the value at the Jakara site being significantly higher than both Watari and Challawa. The P values were within the medium at Watari to excessively high ranges at the Jakara and the Challawa sites. The means were all significantly different from one another, with the Jakara site having the most excessive value. The fact that Jakara site had the highest amount of N, P, and organic carbon might not be unconnected with its association with domestic wastewater. Typical wastewater effluent from domestic sources could supply all of the nitrogen and much of phosphorus and potassium requirements of many crops thereby reducing the need for farmers to invest in chemical fertilizers (FAO ). Physicochemical properties and metal contents of the irrigation water The characteristics of the irrigation waters and level of pollutants they contained are shown in Table . The pH values of the water in all the locations were within the 6.5–8.4 range recommended for use in irrigation by the Food and Agriculture Organization (FAO ). The waters of Challawa and Watari rivers were within the safe EC limit while Jakara river was above the increased hazard limit. By the recommendation of Joshi et al. ( ), water with TDS <450 mg/L is considered good and that with >2000 mg/L is unsuitable for irrigation purpose. By this standard therefore the Jakara river may be considered as unsafe for irrigation. Proximity of the water to settlements might have contributed to the total solids dissolved in them. Most of the dissolved solids in the Challawa river could have come from tannery waste particles. The vicinity of the Jakara river has large amounts of solid waste deposited by urban activities; and this might have accounted for its high amounts of dissolved solids while the increasing distance away from the city of the Watari river might have accounted for its much lesser concentration of dissolved solids. Some physicochemical properties and metal contents of the irrigation water Site pH EC (mS/m) TDS (g/L) Pb (mg/L) Cu (mg/L) Cd (mg/L) Cr (mg/L) Challawa 7.57a 1.96b 1.78b 14.56a 8.43a 3.33ab 83.76a Jakara 7.29a 6.71a 4.35a 13.79a 7.28a 4.58a 37.61b Watari 7.60a 0.40c 0.24c 8.04a 6.51a 1.11b 5.98c SE 0.18 0.49 0.50 3.47 2.57 0.408 7.45 FAO ( ) recommended threshold 8.4 0.75 450 5.00 0.20 0.01 0.10 EC, electrical conductivity; TDS, total dissolved solids; FAO, Food and Agriculture Organization. Means followed by the same letter in the same column are not significantly different at 0.05 LSD. The concentrations of all the metals in all the waters have exceeded the threshold limits shown in Table . The Pb concentrations in the rivers were not significantly different, although the concentration at the Challawa river was higher. In addition to contamination by vehicular discharges and atmospheric deposition, industrial effluents might have contributed very much to the higher concentration at the Challawa river. Mohsen and Mohsen ( ) have highlighted the effect of industrial effluents as the main cause of the high levels of lead in the waters they analyzed. The Cd concentration at the Jakara river was significantly higher than the other rivers. Its presence may be because of dissolution from its content in products such as batteries and alloys which are discarded as municipal solid wastes. They are subsequently washed into river bodies as alleged by Wild ( ). This may explain the higher concentration associated with Challawa and Jakara rivers. Other factors such as the presence of the metal in agrochemicals could have contributed to its levels in the control. The tendency for higher Cu values in domestic and industrial wastewaters has been highlighted by Binns et al. ( ) and Mohsen and Mohsen ( ). This therefore explains the higher concentration at the Jakara and Challawa sites. The source of Cu in the control could have been runoff from fields treated with copper containing agrochemicals (Binns et al. ). The mean Cr concentrations were all different with the highest value recorded at the Challawa river. Cr is an element that is associated with industrial waste waters especially the tanning industries (Maldonado et al. ). This fact explains the higher concentrations at the Challawa river which receives additional discharge from the Sharada industrial layout where the largest tanning factory in the city is located. The results here for all the metals disagreed with previous findings of Wakawa et al. ( ) and Binns et al. ( ) for the Challawa and the Jakara rivers, respectively. Most of the values here are higher. This may be expected as spatial and temporal variations of concentration of ions in water is a well accepted fact in water quality study as suggested by Binns et al. ( ). Soil pollution Total metal concentrations in soil The total concentrations of Pb, Cd, Cr, and Cu for the soils at the banks of the sectors of the three rivers are shown in Table . There is a decrease in the total concentrations of all the metals across the sectors of Challawa river with downstream flow. Similar observation can be made with the total concentrations of the metals across the sectors of Jakara with the exception of Cr which had its highest concentration at the midstream sector. The reverse phenomenon was, however, observed at the control Watari river where concentrations of all the metals tended to increase with downstream flow. Significant differences were recorded in the total concentrations of the metals across the sectors of the rivers except for Cr at Challawa and Jakara. Total concentrations of the metals for the three sectors of the three sites Parameter Challawa Jakara Watari Yan Sama Sharada Sabuwar Gandu SE Akija Airport Rd. Magami SE Lambu Langel New Site SE Pb (mg/kg) 98.26a 75.01a 31.07b 5.23 132.77a 92.31ab 53.87b 7.16 23.95b 29.12b 75.44a 3.90 Cd (mg/kg) 11.43a 8.81b 7.56b 0.36 13.37a 12.30ab 9.77b 0.36 0.40b 0.43b 0.94a 0.05 Cr (mg/kg) 184.23 185.26 127.21 8.58 127.12 130.94 79.76 7.69 3.70b 5.36b 8.49a 0.40 Cu (mg/kg) 10.44a 1.45b 2.96b 0.91 10.88a 5.82b 1.26c 1.03 1.99b 5.65a 7.54a 0.47 Means followed by the same letter in the same row for the same area are not significantly different ( P ≤ 0.05). The major reasons for this pattern of distribution may be attributed to the spatial distribution of the locations and the major sources of the contaminants into the environment. The sites of this work are located at increasing distance from the municipal center on Challawa and Jakara rivers and at decreasing distance on the control Watari river. This pattern of distribution therefore confirms the effect of proximity to municipality as a major factor in pollution of soil resources. This may be due to increasing effect of motor vehicles that serve as sources for over the air deposition of especially Pb. Similar observations were made by Bada et al. ( ). Their values for Pb ranged from about 264.94 mgPb/kg at a distance of 1 m from the road edge to as low as 4.79 mgPb/kg at 50 m distance away. Earlier works at the Jakara basin established similar trends. Dawaki and Alhassan ( ) recorded values of Pb that are lower but following the same trend of declining concentration with distance away from the municipal. The Cu concentration in all the sites may be attributable to its presence in domestic and industrial sewage sludges (World Bank Report ; Wild ). The effect of domestic waste water on the concentration of Cu is further highlighted by the correspondingly higher concentrations detected with proximity to municipality. The Cu values are in close agreement with the findings of Audu and Peacock ( ) in the soils of the Jakara dam irrigation site. Similar behavior of Cu was also observed by Dawaki and Alhassan ( ). Cr on the other hand is strongly associated with industrial effluents, especially the tanning industry as well as the textile and iron and steel industries at various levels (Wild ). This factor alone might have significantly contributed to the excessively high concentration at especially the Challawa which has the highest concentration of all the aforementioned industries, particularly tanning and textile industries. The World Bank Report ( ) has alleged that the largest chunk of Nigeria's tanning industries is located in Kano, especially at the Bompai and Challawa industrial estates. High concentrations at the Jakara sectors may be due to possible long period of application of sewage sludge as shown by Maldonado et al. ( ) and Mapanda et al. ( ). The significantly lower means obtained at the control may justify the claims of both industrial and domestic wastewaters being contributors to Cr deposits in soils. The values recorded here agree with the work of Dawaki and Alhassan ( ), and Audu and Peacock ( ) in the Jakara basin and the work of Wakawa et al. ( ) at the Challawa basin where concentrations were as high as 246 mg/kg. The presence of cadmium in soils could be attributed to four factors: addition through phosphatic fertilizers; its use in batteries, alloys, pigments, and plastics; its discharge through industrial sewage from tanneries; and geologic addition through rock weathering. Factors (i) and (iv) might be responsible for its detection in Watari basin while (ii) and (iii) might, respectively, be the reasons for its presence in Jakara and Challawa basin as well as its declining trend downstream. Fish and Johnson ( ) have attributed their values of Cd to rapid releases from fertilizer during the first few hours of application. Earlier works at both the Jakara valley and the Challawa basin established very low Cd contents in the soils. Dawaki and Alhassan ( ) established the highest mean concentration to be as low as 0.03 mg/kg in the Jakara basin while Awode et al. ( ) reported value range of 0.94–5.27 mg/kg Cd at the Challawa basin. Exchangeable and soluble metal concentrations in soil The exchangeable and soluble concentrations of the metals are shown in Tables and , respectively. Unlike the total concentrations, the mean values of these two forms of the metals were not showing any specific pattern of distribution across the sectors of all the three sites. Exchangeability and solubility for all the metals were higher, although in some cases not significantly different at the midstream sector of the Challawa river. At the Jakara river, however, the exchangeable and soluble forms of the metals were higher at the upstream sector except for Cr while at the Watari river, the downstream sector recorded the highest of both forms of the metals. Exchangeable concentrations of the metals for the three sectors of the three sites Parameter Challawa Jakara Watari Yan Sama Sharada Sabuwar Gandu SE Akija Airport Rd. Magami SE Lambu Langel New Site SE Pb (mg/kg) 26.18a 12.83ab 9.20b 1.52 27.89a 19.34a 7.71b 1.48 7.01b 6.82b 17.63a 0.89 Cd (mg/kg) 1.37 0.96 1.04 0.06 1.63a 1.07ab 0.91b 0.07 0.01b 0.03ab 0.07a 0.01 Cr (mg/kg) 32.45 33.00 22.34 1.44 19.86ab 22.11a 13.31b 0.92 1.34 1.42 2.31 0.12 Cu (mg/kg) 2.28a 0.29b 0.52b 0.21 2.16a 1.21a 0.19b 0.23 0.22b 0.70a 0.77a 0.06 Means followed by the same letter in the same row for the same area are not significantly different ( P ≤ 0.05). Soluble concentrations of the metals for the three sectors of the three sites Parameter Challawa Jakara Watari Yan Sama Sharada Sabuwar Gandu SE Akija Airport Rd. Magami SE Lambu Langel New Site SE Pb (mg/kg) 9.97 11.48 6.62 0.85 12.20a 7.11ab 4.38b 0.79 1.87b 1.54b 4.93a 0.28 Cd (mg/kg) 0.53 0.52 0.52 0.04 0.72a 0.63ab 0.47b 0.03 0.00 0.00 0.00 0.00 Cr (mg/kg) 17.45 18.00 12.29 0.77 5.00b 9.44a 6.56ab 0.46 0.46b 0.57b 0.82a 0.03 Cu (mg/kg) 0.97a 0.14b 0.33ab 0.09 0.46a 0.37a 0.02b 0.06 0.10b 0.29b 0.77a 0.05 Means followed by the same letter in the same row for the same area are not significantly different ( P ≤ 0.05). The principal factors that affect the behavior of metals in soils are pH, nature and amount of clays and associated oxides, organic matter and nature of humic substances as well as total concentration (Basta et al. ; Wahba and Zaghloul ). As most of the factors that affect the solubility and exchange behavior of metals in soil are low to medium (especially clay and organic matter) as shown in Table that probably explains the low soluble and exchangeable concentrations despite the higher values in the total concentration. This is in agreement with the theory of Basta et al. ( ). The effect of total concentration of Pb on its exchangeable form is noticed across all the sectors of the three sites. Despite the higher amounts of organic matter and clay at the downstream sector of the Challawa site, the exchangeable concentration was significantly lower because of the significantly lower amounts of the total Pb in the soil. This was similar to the situation at the midstream sector of the Jakara site. The lack of response of the exchangeable fraction to other factors in the soil is obvious from the result. Despite the high concentration at the Jakara site, especially at the upstream only a small fraction of it was in exchangeable form even with the high organic matter content. The most probable explanation for this is that Pb may be strongly adsorbed by the organic fraction and thereby reducing its availability at the exchange complex. Yusuf ( ) and Adekola et al. ( ) reported much higher amount of Pb as sorbed by organic matter than the amount found in exchangeable form. Of particular interest in this result are low values of soluble Pb at the Jakara site in comparison to its total and exchangeable concentrations. Whereas higher amounts of total and exchangeable Pb were recorded at the sectors of the site in comparison to the Challawa site (Tables and ), yet higher soluble Pb was detected at the Challawa site. Except where concentration played a more prominent role such as the downstream sector of the Jakara site, the probable explanation for this behavior of Pb may be its association with the significantly high P at the Jakara site (Table ) possibly in the form of phosphate ions. Cao et al. ( ) showed that phosphate is effective in immobilizing lead in contaminated soils via formation of stable lead phosphate minerals which are particularly the most insoluble form of lead minerals in soils under a wide range of environmental conditions. The effect of clay, organic matter, and pH could be observed on the exchangeable and soluble concentrations of Cd especially at the Challawa and the Jakara sectors. At the Jakara site, the exchangeable and soluble concentrations were additionally affected by total concentration. Similar effect of concentration, organic matter and clay could be observed at the Watari site especially at the upstream sector. Cd is a highly mobile ion (Tokaliolu et al. ) but here it is showing relatively low mobility to the exchange complex in comparison to the total amount in the soil across all the sites. The most probable explanation for the relatively low amounts in this study may be due to sorption by the organic matter which is a possible reverse of behavior for Cd especially in soils where pH is slightly alkaline to alkaline as explained by Basta et al. ( ). Cu is a metal that is predominantly adsorbed at the top soil by clay and/or organic matter, and this adsorption increases with increasing pH (Wild ). This fact may validate the results here especially when compared with the organic matter contents of the areas shown in Table . The exchangeable values of the metal tended to be high in most of the locations with appreciable organic matter and the pH values were slightly alkaline. Reichmann ( ) stated that the amount of organic matter in the soil can be a more important determining factor on Cu solubility than pH or any other factor. Despite the higher total concentration and exchangeable forms at the Jakara site, lower amount was detected in solution compared with the Challawa sectors. This is because some of what was detected as total and exchangeable at the site might have probably been sorbed by the high organic matter content in the site. The equally lower pH at the site compared with the Challawa basin might have facilitated the sorption process. The most important factors that could have affected the concentrations of the exchangeable and soluble Cr according to this result might have been the total concentration and pH. This was typified in all the sites but more especially at the Challawa site. The predominant form of Cr in soil is Cr(III) which is highly stable. As explained by Zayed and Terry ( ) with increasing pH their solubility tends to increase. The variability of Cr solubility and exchangeability here corroborates well with the findings of Ogbo and Okhuoya ( ) in crude oil contaminated soils remediated with mushroom plants and the theory of Basta et al. ( ). Soil PI Soil pollution ranges were established by PI where values higher than 1 defines a pollution range and those <1 define a contamination range. Pollution range scale of slight (1.1–2.0), moderate (2.1–4.0), severe (4.1–8.0), very severe (8.1–16.0), and excessive (>16.0) of Lacatusu ( ) was used in ranking the soil pollution levels. This was based on the cumulative means of the metals for the three sites (Table ). The result is as depicted in Figure . The Challawa and Jakara sites could be described as slightly polluted with Cu, slightly to moderately polluted with Pb, and excessively polluted with Cd and Cr, respectively, when viewed in light of the control. But when viewed in light of the maximum limit for Nigerian soil (DPR ), the pollution level for Pb was slight in Jakara and only contamination level at Challawa and Watari, while Cr and Cd were still at excessive levels in the two sites and only contamination level at Watari. The Cu levels in all the three sites were only at contamination levels in light of the maximum limit (ML) for soils in Nigeria. The PIs are consistent with the values shown in Table where Pb, Cd, and Cr contents of Jakara were higher than the ML of DPR ( ). The same applies to the Cr and Cd values at Challawa. The values for all the metals at the control were lower than the ML similar to the Pb value at Challawa. This goes further to portray the impact of use of wastewater on the soils as well as proximity to the municipal where over the air deposition and runoff from dump sites, highways, and mechanic villages could be making significant contributions. Similar results were obtained by Nwachukwu et al. ( ) when municipal sites were compared with a control site where source was predominantly parent material. Cumulative mean total concentrations of the metals in soils of the three sites Site Pb (mg/kg) Cd (mg/kg) Cr (mg/kg) Cu (mg/kg) Challawa 68.12a 9.06b 165.66a 4.95 Jakara 92.98a 11.81a 112.61b 5.99 Watari 42.84 0.59c 5.85c 5.06 ML 85 0.8 100 36 SE± 3.66 0.44 6.89 0.48 Means followed by the same letter in the same column not significantly different ( P ≤ 0.05). ML = maximum limit = Limit set by DPR ( ) for surfaces of Nigerian soils. Pollution index for the two sites as compared with the control and ML . ML , maximum limit. Relationship between metals in water and in soil Table shows the correlation between the metals concentration in irrigation water and the total concentration detected in soil. The result shows that highly significant correlation exists between Pb, Cr, and Cd concentration in water and the concentration in soil. Negative but insignificant correlation exists between the Cu concentration in water and that in soil. Relationship between metals in water and total metals in soil Pb in H 2 O Cu in H 2 O Cr in H 2 O Cd in H 2 O Soil Pb 0.487 ** Soil Cu −0.369 Soil Cr 0.787 ** Soil Cd 0.556 ** **Significant at P ≤ 0.001 level of significance. It has been reported that heavy metal contamination in soils may have been caused by the use of wastewater and irrigation with water from industrial and urban areas have aggravated the levels of metals in soils (Maldonado et al. ). Significant relationship was established by their work with Pb, Cd, Cr, Ni, and Cu. But as they pointed out there is a variation in the nature of relationship with metal type and the concentration in soil even within areas with wastewater use history. This might explain the negative correlation between Cu in water and in the soil. Metal transfer factor The ability of the metals to be accumulated in plants was assessed based on the MTF using the cumulative sum of the soluble and exchangeable metal fractions across the three sites as presented in Table . The MTF for the individual metals is depicted in Figure . Cumulative mean exchangeable, soluble, sum of exchangeable/soluble, and plant total metal contents for the three site Site Pb (mg/kg) Cd (mg/kg) Cr (mg/kg) Cu (mg/kg) Exch. Soluble Exch. + Soluble Plant total Exch. Soluble Exch. + Soluble Plant total Exch. Soluble Exch. + Soluble Plant total Exch. Soluble Exch. + Soluble Plant total Challawa 16.07 9.36 25.43 28.39b 1.12 0.52 1.64 0.88a 29.26 15.91 45.17 27.99a 1.03 0.48 1.51 1.89b Jakara 18.31 7.90 26.21 41.99a 1.20 0.60 1.80 1.03a 18.43 7.00 25.43 28.63a 1.18 0.27 1.45 2.66a Watari 10.48 2.78 13.26 3.05c 0.39 0.00 0.39 0.37b 1.69 0.61 2.30 0.25b 0.57 0.39 0.96 1.03c SE 0.81 0.46 4.19 0.05 0.03 0.15 1.13 0.62 3.71 0.11 0.04 0.23 ML 0.1 0.2 20 73 ML = maximum level allowed for fresh vegetables by FAO/WHO ( ). Soil‐Plant metal transfer for the three sites. The highest mobility and transferability were shown by the metals at the Jakara sites except for Cd at Watari. Cu and Cd were remarkably well mobilized and transferred despite their low exchangeable and soluble forms especially at the Challawa and the Watari sites. The transferability of Cr, especially at the Challawa site was poor despite its high concentration. The accumulation of a particular element in plants depends on several factors most especially the nature of the metal, the soil conditions, the plant species, and the stage of growth (Cui et al. ). This proves the disparities between the different metals with the regards to their transferability and accumulation in the test crop. The relatively high transfer rate of Cu and Cd in comparison to their low concentrations may suggest an ability to bind more to enzyme sites when they simultaneously enter plants with for example Cr. The MTF values here generally agree with the values of Zhuang et al. ( ) and Khan et al. ( ) for leafy vegetables. The results also generally agreed with mechanism of soil absorption order: Pb 2+ >Cu 2+ >Cd 2+ of Nan et al. ( ). Plant accumulation and EDI of metals from lettuce consumption The cumulative total metal contents of plants in the three sites are presented in Table and contents in the plant tissues in the various sectors of the three rivers are given in Table . The comparative EDIs by consumers of the test crop for the various metals are depicted in Figure A–D. Significantly different amounts of Cu and Pb were accumulated by plants in the sectors of the rivers. Higher amounts tended to be accumulated in the upstream sectors of Challawa and Jakara and the downstream sectors of the control Watari river. No significant difference was found in the Cr content of the samples in the sectors of Jakara and Challawa although there was a downstream decline in the means while at the control the highest amount was found at the downstream. This trend is similar to that of Cd across all the sectors of the rivers. The concentrations of all the metals except Cu in all the sectors of the sites have exceeded the maximum level recommended for fresh vegetables by FAO/WHO ( ). Total concentrations of metals in plant tissues for the three sectors of the sites Location Cu (μg/kg) Cr (μg/kg) Pb (μg/kg) Cd (μg/kg) Challawa Yan Sama 1.46b 27.56 ns 35.18a 0.83 ns Sharada 1.62b 28.21 ns 22.22c 0.97 ns Rafin Kuka 2.60a 28.20 ns 27.78ab 0.83 ns SE± 0.11 1.39 1.06 0.06 Jakara Akija 3.09a 29.49a 44.44a 1.39a Airport Road 2.28b 28.85a 25.93b 0.96b Magami 2.60b 27.57a 12.74c 0.76b SE± 0.10 1.07 3.57 0.05 Watari Lambu 0.98b 0.21b 2.59b 0.29b Langel 0.82b 0.24ab 2.04c 0.34ab New Site 1.30a 0.31a 4.52a 0.49a SE± 0.07 0.02 0.27 0.04 FAO/WHO ML 73.00 20.00 0.30 0.1 Means followed by the same letter in the same column and for the same basin are not significantly different at 5% LSD. ML = maximum level allowed for fresh vegetables by FAO/WHO ( ). (A) Mean daily intake of Cr as compared to EPA/IRIS ( ). (B) Mean daily intake of Pb as compared to WHO PTDI ( ). (C) Mean daily intake of Cd as compared to WHO PTDI ( ). (D) Mean daily intake of Cu as compared to WHO PTDI ( ). IRIS, integrated risk information system; PTDI, provisional tolerable daily intake; US‐EPA, United States‐Environmental Protection Agency; WHO, world health organisation. The pattern of metal accumulation on sector by sector basis was consistent with the distribution of metals in the soils of the area as shown in Tables . Comparatively higher amounts of exchangeable and soluble forms of the individual metals translate into higher amounts in the tissues of the plant especially at the Jakara sites where soil conditions shown in Table favors the mobilization of metals from total concentration to potentially bioavailable forms in the soil. Cadmium is a highly mobile metal, easily absorbed by the plants through root surface, moves to wood tissue, and transfers to upper parts of plants; and evidently there is a direct relationship between the levels of Cadmium in the root zone and its absorption by plant (Mohsen and Mohsen ). Akinola and Ekiyoyo ( ) have confirmed the ease with which this metal is absorbed by plants as concentrations in washed leaves of Talinum traingulare and Telfaria occidentalis from industrialized areas of Lagos were accumulating Cd in excess of 4.03 mg/kg. The high Pb content of the Jakara soil may have also affected the Cu and Cd absorption. Significant relationship has been found between Pb concentrations in soil and Cu and Cd uptakes by crops. Nan et al. ( ) found that Cd and Cu content in grain wheat was high in locations with high Pb and Zn. Dudka et al. ( ) showed that Cd content of plants treated with Pb + Cd was greater than that of plants treated with Cd alone. The behavior of Cd in this result is similar to the observation of Abdu ( ) in the study of transfer behavior of Zn and Cd in different vegetables including lettuce. The remarkably better soil condition which facilitated higher soluble and exchangeable fractions in the soils of Jakara might have also facilitated the higher metals accumulation in the samples from the Jakara site. In the study of Abdu ( ), plants from sites with higher CEC, clay, organic matter, and comparatively lower pH, seemingly accumulated higher concentrations of the tested metals (Zn and Cd). This, however, does not apply to the Cd situation at the Watari site where despite low concentration and poor soil conditions yet was remarkably accumulated. The probable explanation may be the effect of other cations not tested, especially Zn. As reported by Zayed and Terry ( ), Cr is readily immobilized in soils by adsorption, reduction, and precipitation processes, with only a fraction of the total Cr concentrations available for plant uptake. When taken up by plants, >99% of the absorbed Cr is retained in the roots where it is reduced to Cr(III) species in a short time. Because most plants have low Cr concentrations, even when grown on Cr rich soils, the food chain is well protected against Cr toxicity. These facts about it might have explained its low values in plants despite its higher values in the soils. Akinola and Ekiyoyo ( ) were able to extract as high as 47 μg/g in roots and none in shoots of Telfaria occidentalis from a total concentration of 3647 μg/g in the soil. Organic matter can also regulate the availability of Cr(III) which is the dominant form in soil through chelation reaction, and the complex compounds formed with organic material may be unavailable to plants (Ramachandran and D'Souza ). Higher Pb concentration in soil has correspondingly produced higher concentration in plants despite the high pH across the basins which are suppose to minimize solubility as argued by Akinola and Ekiyoyo ( ). This deviation from the fact could be attributed to hyper accumulation as postulated by Mumba et al. ( ) who detected concentration as much 0.12 μg/g in cabbages grown in soils that have undetectable amounts of the metal. When the plant concentrations were translated into EDIs and compared with provisional tolerable daily intake (PTDI) recommended by FAO/WHO ( ) codex alimentarious for Pb, Cd, and Cu; and the US‐EPA ( ) reference dose for chronic oral exposure (RfD) for Cr; daily consumers of lettuce from Challawa and Jakara sites are liable to consume far in excess of the recommended daily allowable doses of Cd, Pb and Cr and less of the Cu dose; while consumers of lettuce from the control may take slightly in excess of the allowable daily dose of Cd and Pb; and far less of the Cu and Cr allowable daily doses. There was a variation in the degree of exposure with variation in sites and sectors within sites as can be seen from the figures. At the Jakara except for Pb, exposure risks decreases downstream while at the control exposure risk increases downstream. The pattern is more inconsistent at the industrial wastewater sites (Challawa). The general trend for all the metals in terms of exposure risk is such that domestic wastewater (Jakara) > industrial wastewater (Challawa) > control (Watari). The results are consistent with the metal contents in the soil, although the effect of over the air addition to the foliage may not be ruled out. Awode et al. ( ) ascribed their excessive Cd content in plants of the Challawa basin to concentrations aggravated by discharges from wear and tear of vehicle parts used to convey sand from the river, as this is a common activity along the bank of the river. The results here disagreed with the findings of Abdu ( ) for Cd in which he portrayed the consumer population as in no immediate risk. They are, however, more consistent with the findings of Lacatusu and Lacatusu ( ) for vegetables and fruits grown in pollution prone areas especially for the Cd and Pb. Conclusion Going by the data generated in this study, it is evident that urban agriculture has multiple of advantages and disadvantages. Whereas no one can doubt its significant contribution to the economy of the farmers and the state especially in terms of generating employment and income, as well as subsidizing the cost of vegetables in the city; yet its overall consequences on the environment from the point of view of the findings here could be disastrous in the long term. The use of wastewater and municipal wastes have resulted in soils that are fertile in some areas due to high levels of N, P, and K as well as high levels of toxic materials. Many of these metals have not reached levels beyond international standard in the soils, but the water levels are far in excess of the recommended averages. This is additional to the fact that plants' concentrations have resulted in exposure of the human consumers to levels beyond permissible daily averages. Because water is the principal agent in the introduction of these substances into the other components of the environment and the tendency of these metals for long‐term persistence in the environment, the risks posed to the environment and the human population may not be far‐fetched. This is coupled with the fact that other sources such as air deposit through vehicular releases, agrochemicals, and solid waste disposal may also be making significant contribution. The impact of agrochemicals may also not be overruled because it may be the principal source of contamination into the control site as all the locations are away from the city's pollution. Acknowledgments The authors wish to acknowledge the efforts of staff at the Soil and Water Laboratories of Geography and Soil Sciences Departments of Bayero and Ahmadu Bello Universities, Kano and Zaria, respectively. Conflict of Interest None declared. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Food and Energy Security Wiley

Pollution as a threat factor to urban food security in metropolitan K ano, N igeria

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© 2013 John Wiley & Sons Ltd and the Association of Applied Biologists.
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Abstract

Introduction According to the declaration of the World Food Summit ( ), the safety of food is second only to its sufficiency in determining food security. Therefore, for people to be food secured, they must have access to sufficient, nutritious, and safe food that may not expose the consumer to health risk. Urban agriculture is a common practice in many cities around the world and in Kano, Nigeria, it is implied centuries old (Dokaji ). Although time has modified the system, yet the principal distinguishing feature remains the use of stream water to irrigate land at the banks. Principal of these streams are Challawa, Getsi, Jakara, and Salanta . The main objective is to produce fruits and vegetables for the consumption of the city dwellers (Binns et al. ). Researches have been conducted on the merits of the system especially on its socioeconomic benefits in terms of job creation, income supplementation, and price subsidy for vegetables (Lynch et al. ; Olofin and Tanko ). Most of these researches, however, failed to engage with a greater understanding of urban farming in relation to specific issues, which importantly include health and environmental concerns (Lynch et al. ). The overall impacts of this land use system on human population in terms of their health and well‐being could be enormous. The sustainability of urban farming in Kano could be very doubtful because of the risks it poses to the people and the environment. Pollution of environmental component with especially heavy metals is of prime concern in this system of production. This is principally because of the use of water derived from streams that flow through the city and often through densely populated and/or industrialized areas as irrigation water while wastes from landfills and sewage sludges are used as fertilizer. These make the land and its products susceptible to pollution with domestic and industrial discharges as reported by Tanko ( ). This is in addition to exposure of the lands to exhausts from automobiles, which has been shown to be a source of heavy metal contamination (Bada et al. ). The concern over metals such as As, Cd, Cr, and Pb arises from their proven potency to be chronic human toxicants when inhaled or ingested (Pharmacopeial Forum ). Previous studies have established the presence of heavy metals in some of the effluents that are being discharged into the Jakara river from the Bompai industrial estate (Sahoo and Klopka ). Binns et al. ( ) have also detected their presence in the waters of the Jakara river while Ya'u ( ) and Wakawa et al. ( ) have detected these metals in some of the plants tissue grown in or around the Jakara and Challawa rivers. Similarly, Dawaki and Alhassan ( ) have detected the presence of some metals in the soils of the Jakara valley. None of these previous studies has, however, synchronized investigations into the various components of the production system in order to project the extent to which it could constitute a human health risk factor. The main objective of this study therefore was to evaluate the potential hazard this system of production poses to the safety of the immediate population that consume the products in terms of their health and well‐being. Methodology The study area This research was conducted at the banks of the Jakara , Challawa, and Watari Rivers within the metropolitan Kano and its suburb in an area lying between latitude 11 o 59′ and 12 o 08′N and longitude 8 o 34′ and 8 o 42′E. Kano is in the dry, subhumid agroecological zone of Nigeria (Ojanuga ). It is characterized by tropical wet and dry climate classified as Aw by W. Koppen (Olofin ). The dominant geology is basement complex (Olofin ; KNARDA (Kano Agricultural and Rural Development Agency) ). The dominant soil of the area is Eutric Cambisol (FDLAR: Federal Department of Agriculture and Land Resources ). The dominant crops being irrigated in the areas are leafy vegetables such as lettuce and spinach. Sampling and sample treatment Selection of sampling site was based on difference in terms of effluent source into irrigation water. Jakara predominantly receives domestic effluents while Challawa receives industrial effluents. Watari is not associated with any form of wastewater and was therefore selected as a control. Three irrigation sites (Yansama, Sharada, and Rafin Kuka); (Akija, Airport Road, and Magami); and (Lambu, Langel, and BUK) were selected to represent up, mid, and downstream sectors of the Challawa, Jakara, and the Watari rivers, respectively (Fig. ). Global Positioning System (GPS) and ground reconnaissance were used for identification of sites and georeferencing. Map of metropolitan K ano showing river network and sampling points. From each zone, the first 1 ha under cultivation was sampled. Stratified grid sampling method (Adepetu et al. ) was employed in soil sampling. 100 m long transect was taken parallel to the course of the river on the side with higher irrigation activity. Soil sample was collected at each 20 m from the surface 0–20 cm, being the rooting zone for most vegetables. Another transect was taken at 20 m parallel to the first and was sampled similarly. The procedure was repeated three more times, thus giving a total of 25 soil samples that were composited into five thereby making a total of 15 samples per river bank. Three water samples were collected randomly in plastic containers across the width of the river at each sampling point based on the assumption that the water is well mixed (Kirda ). As the dominant crops being grown under this land use system are leafy vegetables, Lettuce ( Lactuca sativa L .) was sampled by picking the latest matured leaves of three randomly selected plants at each sampling point. Soil samples were air‐dried, crushed with mortar, and sieved with 2 mm sieve. Plant tissues were carefully washed off of any soils with distilled water, oven‐dried at 70°C to constant weight and were ground and stored in tight polythene bags (Jaiswal ) prior to analyses. Water samples were refrigerated prior to analyses. Laboratory techniques Soil samples were subjected to laboratory analyses using established procedures. The pH was determined using the 1:2.5 soil‐distilled water ratio using EL model 720 pH meter. The electrical conductivity (EC) was determined using 1:2.5 soil–distilled water ratio using Beckman Conductivity Bridge (Model: RC ‐ 18A; Beckman Instruments Inc., Cedar Grove, NJ). Particle size analysis was done using the hydrometer method as reported in Jaiswal ( ). The Walkley‐Black wet‐oxidation technique as described in Jaiswal ( ) was used to determine the organic carbon content of the soil. The ammonium acetate extraction technique (Adepetu et al. ) was used in extracting the exchangeable bases and determination of cation exchange capacity (CEC). Two metals classified for oral ingestion by Pharmacopeial Forum ( ) as Class 1 (elements that should be totally absent) (Pb and Cd) and two metals classified as Class 2 (elements that should be limited) (Cu and Cr) were selected for pollution assessment. The double acid digestion technique (Anderson ) was used in sample extraction for total metal determination in soil. Soluble and exchangeable forms of these metals in soil were determined because of their potency for bioavailability using an adapted sequential extraction technique of Kashem et al. ( ). One gram of each sample was mixed with 100 cm 3 of deionized water. The resultant mixture was shaken for 8 h at 10,000 rpm, left to stand overnight, and then centrifuged for 15 min. The supernatant liquid was decanted and used in the determination of soluble metals in the soil. The residue from the soluble metal fraction was mixed with 100 cm 3 of ammonium oxalate and shaken for 8 h at 10,000 rpm and left to stand overnight, centrifuged for 15 min, and decanted. The solution was used to determine the fraction of metals in exchangeable form. pH of water was determined at point of collection while total dissolved solids (TDS) and the EC of the water samples were determined directly in the sampling containers using an automated digital conductivity meter (Jenway Digital Conductivity Meter Model 4520; Jenway Scientific Equipment, Staffodshire, UK). Total metals in water were extracted with HNO 3 as described by Bäckström et al. ( ). The dried leaves were ashed at 500°C and digested with HCl and KNO 3 , distilled water added and filtered through Whattman No. 4 filter paper. The content of the metals in them was determined from the filtrate. All metals concentrations throughout the work were determined using Atomic Absorption Spectrophotometry (AAS); except Na and K that were determined using flame photometry. Data analyses To estimate the potential hazard, the human population is exposed to as a result of this system, the following factors were taken into consideration; Pollution index (PI) was used in assessing level of pollution for each of the sites. It was computed with the equation of Lacatusu ( ) in which; PI = C ( WS ) C ( RS ) Where C (WS) is the concentration of the metal in wastewater‐irrigated site and C (RS) is the concentration of the metal in a reference site. The control site and the maximum allowable soil limit set by DPR: Department of Petroleum Resources ( ) were used as references. Metal transfer factor (MTF) was used in assessing the tendency of the metals to accumulate in plants' tissues via absorption from the soil. It was computed using the equation of Gosh and Singh ( ) given as: MTF = C Plant C Soil Where C Plant is the metal concentration in plants on dry weight basis and C Soil is the sum of soluble and exchangeable metal concentrations in the soil. Estimated daily intake (EDI) from oral consumption of vegetables by the city's population was used in assessing human exposure risk. It was computed using the equation of Cui et al. ( ) given as: EDI = D × M Where D is daily vegetable intake (g/day) which for the city of Kano is taken to be 255 g/day based on the survey of Kushwaha et al. ( ). M is the mean metal concentration in the edible fraction of the vegetable on fresh weight basis. The dry weight concentration was converted to fresh weight concentration by dividing with a moisture correction factor calculated from the difference between fresh and dried sample weights. Data obtained were also subjected to both descriptive and inferential statistics. Means were compared using analysis of variance (ANOVA) as obtained in SAS package 6.0 (SAS Institute Inc., Cary, NC). Significantly different means were separated using least significant difference (LSD). Pearson moment correlation was also used to establish relationship between metals in water and in soil. Results and Discussions Physicochemical properties of the soil Table shows the physicochemical properties of the soils of the three areas. The texture of the soils is sandy loam to loamy sand. The pH across the three areas can be interpreted as slightly alkaline (Esu ). The mean EC values across the three areas were significantly different from one another with the highest value recorded at Jakara. The soils were neither sodic nor saline, but the alkaline pH and the high EC values at Jakara may potent salinity hazard; a problem that was also noted by Binns et al. ( ). The organic carbon contents across the three sites were all significantly different from one another. They ranked from low in Watari and Challawa to medium in Jakara (Esu ). Mean values of the physicochemical properties of the soils of the three areas Parameter Locations SE Challawa Jakara Watari pH 7.77a 7.42b 7.48b 0.05 EC (mS/m) 1.79b 4.01a 0.58c 0.24 Sand (%) 76.29ab 80.48a 73.41b 1.08 Silt (%) 16.88a 14.56a 18.48a 0.91 Clay (%) 6.80ab 4.96b 8.11a 0.44 Textural Class Sandy loam Loamy sand Sandy loam Ca (cmol/kg) 11.70a 12.67a 7.80b 0.66 Mg (cmol/kg) 2.85a 2.46a 2.15a 0.19 K (cmol/kg) 0.79b 1.85a 0.47c 0.12 Na (cmol/kg) 1.11b 3.20a 0.25c 0.21 CEC (cmol/kg) 19.15a 23.11a 12.63b 1.07 Org. Carbon (g/kg) 8.07b 11.27a 7.16b 0.56 N (g/kg) 0.76b 1.32a 0.74b 0.04 Avail. P (mg/kg) 77.67b 213.52a 31.73c 7.80 EC, electrical conductivity; CEC, cation exchange capacity. Means followed by the same letter in the same row are not significantly different ( P ≤ 0.05). The calcium and magnesium contents of the soils as shown ranged from medium at Watari to high at both Jakara and Challawa using the rankings of Esu ( ) and Landon ( ). The K values for all the three sites were within the high to very high concentrations. The mean Na values were within the medium to high ranges based on the ranking of Landon ( ). The CEC values were within the range classified as medium using the ranking of Landon ( ). The N contents of the soils were high by the ranking of Landon ( ) with the value at the Jakara site being significantly higher than both Watari and Challawa. The P values were within the medium at Watari to excessively high ranges at the Jakara and the Challawa sites. The means were all significantly different from one another, with the Jakara site having the most excessive value. The fact that Jakara site had the highest amount of N, P, and organic carbon might not be unconnected with its association with domestic wastewater. Typical wastewater effluent from domestic sources could supply all of the nitrogen and much of phosphorus and potassium requirements of many crops thereby reducing the need for farmers to invest in chemical fertilizers (FAO ). Physicochemical properties and metal contents of the irrigation water The characteristics of the irrigation waters and level of pollutants they contained are shown in Table . The pH values of the water in all the locations were within the 6.5–8.4 range recommended for use in irrigation by the Food and Agriculture Organization (FAO ). The waters of Challawa and Watari rivers were within the safe EC limit while Jakara river was above the increased hazard limit. By the recommendation of Joshi et al. ( ), water with TDS <450 mg/L is considered good and that with >2000 mg/L is unsuitable for irrigation purpose. By this standard therefore the Jakara river may be considered as unsafe for irrigation. Proximity of the water to settlements might have contributed to the total solids dissolved in them. Most of the dissolved solids in the Challawa river could have come from tannery waste particles. The vicinity of the Jakara river has large amounts of solid waste deposited by urban activities; and this might have accounted for its high amounts of dissolved solids while the increasing distance away from the city of the Watari river might have accounted for its much lesser concentration of dissolved solids. Some physicochemical properties and metal contents of the irrigation water Site pH EC (mS/m) TDS (g/L) Pb (mg/L) Cu (mg/L) Cd (mg/L) Cr (mg/L) Challawa 7.57a 1.96b 1.78b 14.56a 8.43a 3.33ab 83.76a Jakara 7.29a 6.71a 4.35a 13.79a 7.28a 4.58a 37.61b Watari 7.60a 0.40c 0.24c 8.04a 6.51a 1.11b 5.98c SE 0.18 0.49 0.50 3.47 2.57 0.408 7.45 FAO ( ) recommended threshold 8.4 0.75 450 5.00 0.20 0.01 0.10 EC, electrical conductivity; TDS, total dissolved solids; FAO, Food and Agriculture Organization. Means followed by the same letter in the same column are not significantly different at 0.05 LSD. The concentrations of all the metals in all the waters have exceeded the threshold limits shown in Table . The Pb concentrations in the rivers were not significantly different, although the concentration at the Challawa river was higher. In addition to contamination by vehicular discharges and atmospheric deposition, industrial effluents might have contributed very much to the higher concentration at the Challawa river. Mohsen and Mohsen ( ) have highlighted the effect of industrial effluents as the main cause of the high levels of lead in the waters they analyzed. The Cd concentration at the Jakara river was significantly higher than the other rivers. Its presence may be because of dissolution from its content in products such as batteries and alloys which are discarded as municipal solid wastes. They are subsequently washed into river bodies as alleged by Wild ( ). This may explain the higher concentration associated with Challawa and Jakara rivers. Other factors such as the presence of the metal in agrochemicals could have contributed to its levels in the control. The tendency for higher Cu values in domestic and industrial wastewaters has been highlighted by Binns et al. ( ) and Mohsen and Mohsen ( ). This therefore explains the higher concentration at the Jakara and Challawa sites. The source of Cu in the control could have been runoff from fields treated with copper containing agrochemicals (Binns et al. ). The mean Cr concentrations were all different with the highest value recorded at the Challawa river. Cr is an element that is associated with industrial waste waters especially the tanning industries (Maldonado et al. ). This fact explains the higher concentrations at the Challawa river which receives additional discharge from the Sharada industrial layout where the largest tanning factory in the city is located. The results here for all the metals disagreed with previous findings of Wakawa et al. ( ) and Binns et al. ( ) for the Challawa and the Jakara rivers, respectively. Most of the values here are higher. This may be expected as spatial and temporal variations of concentration of ions in water is a well accepted fact in water quality study as suggested by Binns et al. ( ). Soil pollution Total metal concentrations in soil The total concentrations of Pb, Cd, Cr, and Cu for the soils at the banks of the sectors of the three rivers are shown in Table . There is a decrease in the total concentrations of all the metals across the sectors of Challawa river with downstream flow. Similar observation can be made with the total concentrations of the metals across the sectors of Jakara with the exception of Cr which had its highest concentration at the midstream sector. The reverse phenomenon was, however, observed at the control Watari river where concentrations of all the metals tended to increase with downstream flow. Significant differences were recorded in the total concentrations of the metals across the sectors of the rivers except for Cr at Challawa and Jakara. Total concentrations of the metals for the three sectors of the three sites Parameter Challawa Jakara Watari Yan Sama Sharada Sabuwar Gandu SE Akija Airport Rd. Magami SE Lambu Langel New Site SE Pb (mg/kg) 98.26a 75.01a 31.07b 5.23 132.77a 92.31ab 53.87b 7.16 23.95b 29.12b 75.44a 3.90 Cd (mg/kg) 11.43a 8.81b 7.56b 0.36 13.37a 12.30ab 9.77b 0.36 0.40b 0.43b 0.94a 0.05 Cr (mg/kg) 184.23 185.26 127.21 8.58 127.12 130.94 79.76 7.69 3.70b 5.36b 8.49a 0.40 Cu (mg/kg) 10.44a 1.45b 2.96b 0.91 10.88a 5.82b 1.26c 1.03 1.99b 5.65a 7.54a 0.47 Means followed by the same letter in the same row for the same area are not significantly different ( P ≤ 0.05). The major reasons for this pattern of distribution may be attributed to the spatial distribution of the locations and the major sources of the contaminants into the environment. The sites of this work are located at increasing distance from the municipal center on Challawa and Jakara rivers and at decreasing distance on the control Watari river. This pattern of distribution therefore confirms the effect of proximity to municipality as a major factor in pollution of soil resources. This may be due to increasing effect of motor vehicles that serve as sources for over the air deposition of especially Pb. Similar observations were made by Bada et al. ( ). Their values for Pb ranged from about 264.94 mgPb/kg at a distance of 1 m from the road edge to as low as 4.79 mgPb/kg at 50 m distance away. Earlier works at the Jakara basin established similar trends. Dawaki and Alhassan ( ) recorded values of Pb that are lower but following the same trend of declining concentration with distance away from the municipal. The Cu concentration in all the sites may be attributable to its presence in domestic and industrial sewage sludges (World Bank Report ; Wild ). The effect of domestic waste water on the concentration of Cu is further highlighted by the correspondingly higher concentrations detected with proximity to municipality. The Cu values are in close agreement with the findings of Audu and Peacock ( ) in the soils of the Jakara dam irrigation site. Similar behavior of Cu was also observed by Dawaki and Alhassan ( ). Cr on the other hand is strongly associated with industrial effluents, especially the tanning industry as well as the textile and iron and steel industries at various levels (Wild ). This factor alone might have significantly contributed to the excessively high concentration at especially the Challawa which has the highest concentration of all the aforementioned industries, particularly tanning and textile industries. The World Bank Report ( ) has alleged that the largest chunk of Nigeria's tanning industries is located in Kano, especially at the Bompai and Challawa industrial estates. High concentrations at the Jakara sectors may be due to possible long period of application of sewage sludge as shown by Maldonado et al. ( ) and Mapanda et al. ( ). The significantly lower means obtained at the control may justify the claims of both industrial and domestic wastewaters being contributors to Cr deposits in soils. The values recorded here agree with the work of Dawaki and Alhassan ( ), and Audu and Peacock ( ) in the Jakara basin and the work of Wakawa et al. ( ) at the Challawa basin where concentrations were as high as 246 mg/kg. The presence of cadmium in soils could be attributed to four factors: addition through phosphatic fertilizers; its use in batteries, alloys, pigments, and plastics; its discharge through industrial sewage from tanneries; and geologic addition through rock weathering. Factors (i) and (iv) might be responsible for its detection in Watari basin while (ii) and (iii) might, respectively, be the reasons for its presence in Jakara and Challawa basin as well as its declining trend downstream. Fish and Johnson ( ) have attributed their values of Cd to rapid releases from fertilizer during the first few hours of application. Earlier works at both the Jakara valley and the Challawa basin established very low Cd contents in the soils. Dawaki and Alhassan ( ) established the highest mean concentration to be as low as 0.03 mg/kg in the Jakara basin while Awode et al. ( ) reported value range of 0.94–5.27 mg/kg Cd at the Challawa basin. Exchangeable and soluble metal concentrations in soil The exchangeable and soluble concentrations of the metals are shown in Tables and , respectively. Unlike the total concentrations, the mean values of these two forms of the metals were not showing any specific pattern of distribution across the sectors of all the three sites. Exchangeability and solubility for all the metals were higher, although in some cases not significantly different at the midstream sector of the Challawa river. At the Jakara river, however, the exchangeable and soluble forms of the metals were higher at the upstream sector except for Cr while at the Watari river, the downstream sector recorded the highest of both forms of the metals. Exchangeable concentrations of the metals for the three sectors of the three sites Parameter Challawa Jakara Watari Yan Sama Sharada Sabuwar Gandu SE Akija Airport Rd. Magami SE Lambu Langel New Site SE Pb (mg/kg) 26.18a 12.83ab 9.20b 1.52 27.89a 19.34a 7.71b 1.48 7.01b 6.82b 17.63a 0.89 Cd (mg/kg) 1.37 0.96 1.04 0.06 1.63a 1.07ab 0.91b 0.07 0.01b 0.03ab 0.07a 0.01 Cr (mg/kg) 32.45 33.00 22.34 1.44 19.86ab 22.11a 13.31b 0.92 1.34 1.42 2.31 0.12 Cu (mg/kg) 2.28a 0.29b 0.52b 0.21 2.16a 1.21a 0.19b 0.23 0.22b 0.70a 0.77a 0.06 Means followed by the same letter in the same row for the same area are not significantly different ( P ≤ 0.05). Soluble concentrations of the metals for the three sectors of the three sites Parameter Challawa Jakara Watari Yan Sama Sharada Sabuwar Gandu SE Akija Airport Rd. Magami SE Lambu Langel New Site SE Pb (mg/kg) 9.97 11.48 6.62 0.85 12.20a 7.11ab 4.38b 0.79 1.87b 1.54b 4.93a 0.28 Cd (mg/kg) 0.53 0.52 0.52 0.04 0.72a 0.63ab 0.47b 0.03 0.00 0.00 0.00 0.00 Cr (mg/kg) 17.45 18.00 12.29 0.77 5.00b 9.44a 6.56ab 0.46 0.46b 0.57b 0.82a 0.03 Cu (mg/kg) 0.97a 0.14b 0.33ab 0.09 0.46a 0.37a 0.02b 0.06 0.10b 0.29b 0.77a 0.05 Means followed by the same letter in the same row for the same area are not significantly different ( P ≤ 0.05). The principal factors that affect the behavior of metals in soils are pH, nature and amount of clays and associated oxides, organic matter and nature of humic substances as well as total concentration (Basta et al. ; Wahba and Zaghloul ). As most of the factors that affect the solubility and exchange behavior of metals in soil are low to medium (especially clay and organic matter) as shown in Table that probably explains the low soluble and exchangeable concentrations despite the higher values in the total concentration. This is in agreement with the theory of Basta et al. ( ). The effect of total concentration of Pb on its exchangeable form is noticed across all the sectors of the three sites. Despite the higher amounts of organic matter and clay at the downstream sector of the Challawa site, the exchangeable concentration was significantly lower because of the significantly lower amounts of the total Pb in the soil. This was similar to the situation at the midstream sector of the Jakara site. The lack of response of the exchangeable fraction to other factors in the soil is obvious from the result. Despite the high concentration at the Jakara site, especially at the upstream only a small fraction of it was in exchangeable form even with the high organic matter content. The most probable explanation for this is that Pb may be strongly adsorbed by the organic fraction and thereby reducing its availability at the exchange complex. Yusuf ( ) and Adekola et al. ( ) reported much higher amount of Pb as sorbed by organic matter than the amount found in exchangeable form. Of particular interest in this result are low values of soluble Pb at the Jakara site in comparison to its total and exchangeable concentrations. Whereas higher amounts of total and exchangeable Pb were recorded at the sectors of the site in comparison to the Challawa site (Tables and ), yet higher soluble Pb was detected at the Challawa site. Except where concentration played a more prominent role such as the downstream sector of the Jakara site, the probable explanation for this behavior of Pb may be its association with the significantly high P at the Jakara site (Table ) possibly in the form of phosphate ions. Cao et al. ( ) showed that phosphate is effective in immobilizing lead in contaminated soils via formation of stable lead phosphate minerals which are particularly the most insoluble form of lead minerals in soils under a wide range of environmental conditions. The effect of clay, organic matter, and pH could be observed on the exchangeable and soluble concentrations of Cd especially at the Challawa and the Jakara sectors. At the Jakara site, the exchangeable and soluble concentrations were additionally affected by total concentration. Similar effect of concentration, organic matter and clay could be observed at the Watari site especially at the upstream sector. Cd is a highly mobile ion (Tokaliolu et al. ) but here it is showing relatively low mobility to the exchange complex in comparison to the total amount in the soil across all the sites. The most probable explanation for the relatively low amounts in this study may be due to sorption by the organic matter which is a possible reverse of behavior for Cd especially in soils where pH is slightly alkaline to alkaline as explained by Basta et al. ( ). Cu is a metal that is predominantly adsorbed at the top soil by clay and/or organic matter, and this adsorption increases with increasing pH (Wild ). This fact may validate the results here especially when compared with the organic matter contents of the areas shown in Table . The exchangeable values of the metal tended to be high in most of the locations with appreciable organic matter and the pH values were slightly alkaline. Reichmann ( ) stated that the amount of organic matter in the soil can be a more important determining factor on Cu solubility than pH or any other factor. Despite the higher total concentration and exchangeable forms at the Jakara site, lower amount was detected in solution compared with the Challawa sectors. This is because some of what was detected as total and exchangeable at the site might have probably been sorbed by the high organic matter content in the site. The equally lower pH at the site compared with the Challawa basin might have facilitated the sorption process. The most important factors that could have affected the concentrations of the exchangeable and soluble Cr according to this result might have been the total concentration and pH. This was typified in all the sites but more especially at the Challawa site. The predominant form of Cr in soil is Cr(III) which is highly stable. As explained by Zayed and Terry ( ) with increasing pH their solubility tends to increase. The variability of Cr solubility and exchangeability here corroborates well with the findings of Ogbo and Okhuoya ( ) in crude oil contaminated soils remediated with mushroom plants and the theory of Basta et al. ( ). Soil PI Soil pollution ranges were established by PI where values higher than 1 defines a pollution range and those <1 define a contamination range. Pollution range scale of slight (1.1–2.0), moderate (2.1–4.0), severe (4.1–8.0), very severe (8.1–16.0), and excessive (>16.0) of Lacatusu ( ) was used in ranking the soil pollution levels. This was based on the cumulative means of the metals for the three sites (Table ). The result is as depicted in Figure . The Challawa and Jakara sites could be described as slightly polluted with Cu, slightly to moderately polluted with Pb, and excessively polluted with Cd and Cr, respectively, when viewed in light of the control. But when viewed in light of the maximum limit for Nigerian soil (DPR ), the pollution level for Pb was slight in Jakara and only contamination level at Challawa and Watari, while Cr and Cd were still at excessive levels in the two sites and only contamination level at Watari. The Cu levels in all the three sites were only at contamination levels in light of the maximum limit (ML) for soils in Nigeria. The PIs are consistent with the values shown in Table where Pb, Cd, and Cr contents of Jakara were higher than the ML of DPR ( ). The same applies to the Cr and Cd values at Challawa. The values for all the metals at the control were lower than the ML similar to the Pb value at Challawa. This goes further to portray the impact of use of wastewater on the soils as well as proximity to the municipal where over the air deposition and runoff from dump sites, highways, and mechanic villages could be making significant contributions. Similar results were obtained by Nwachukwu et al. ( ) when municipal sites were compared with a control site where source was predominantly parent material. Cumulative mean total concentrations of the metals in soils of the three sites Site Pb (mg/kg) Cd (mg/kg) Cr (mg/kg) Cu (mg/kg) Challawa 68.12a 9.06b 165.66a 4.95 Jakara 92.98a 11.81a 112.61b 5.99 Watari 42.84 0.59c 5.85c 5.06 ML 85 0.8 100 36 SE± 3.66 0.44 6.89 0.48 Means followed by the same letter in the same column not significantly different ( P ≤ 0.05). ML = maximum limit = Limit set by DPR ( ) for surfaces of Nigerian soils. Pollution index for the two sites as compared with the control and ML . ML , maximum limit. Relationship between metals in water and in soil Table shows the correlation between the metals concentration in irrigation water and the total concentration detected in soil. The result shows that highly significant correlation exists between Pb, Cr, and Cd concentration in water and the concentration in soil. Negative but insignificant correlation exists between the Cu concentration in water and that in soil. Relationship between metals in water and total metals in soil Pb in H 2 O Cu in H 2 O Cr in H 2 O Cd in H 2 O Soil Pb 0.487 ** Soil Cu −0.369 Soil Cr 0.787 ** Soil Cd 0.556 ** **Significant at P ≤ 0.001 level of significance. It has been reported that heavy metal contamination in soils may have been caused by the use of wastewater and irrigation with water from industrial and urban areas have aggravated the levels of metals in soils (Maldonado et al. ). Significant relationship was established by their work with Pb, Cd, Cr, Ni, and Cu. But as they pointed out there is a variation in the nature of relationship with metal type and the concentration in soil even within areas with wastewater use history. This might explain the negative correlation between Cu in water and in the soil. Metal transfer factor The ability of the metals to be accumulated in plants was assessed based on the MTF using the cumulative sum of the soluble and exchangeable metal fractions across the three sites as presented in Table . The MTF for the individual metals is depicted in Figure . Cumulative mean exchangeable, soluble, sum of exchangeable/soluble, and plant total metal contents for the three site Site Pb (mg/kg) Cd (mg/kg) Cr (mg/kg) Cu (mg/kg) Exch. Soluble Exch. + Soluble Plant total Exch. Soluble Exch. + Soluble Plant total Exch. Soluble Exch. + Soluble Plant total Exch. Soluble Exch. + Soluble Plant total Challawa 16.07 9.36 25.43 28.39b 1.12 0.52 1.64 0.88a 29.26 15.91 45.17 27.99a 1.03 0.48 1.51 1.89b Jakara 18.31 7.90 26.21 41.99a 1.20 0.60 1.80 1.03a 18.43 7.00 25.43 28.63a 1.18 0.27 1.45 2.66a Watari 10.48 2.78 13.26 3.05c 0.39 0.00 0.39 0.37b 1.69 0.61 2.30 0.25b 0.57 0.39 0.96 1.03c SE 0.81 0.46 4.19 0.05 0.03 0.15 1.13 0.62 3.71 0.11 0.04 0.23 ML 0.1 0.2 20 73 ML = maximum level allowed for fresh vegetables by FAO/WHO ( ). Soil‐Plant metal transfer for the three sites. The highest mobility and transferability were shown by the metals at the Jakara sites except for Cd at Watari. Cu and Cd were remarkably well mobilized and transferred despite their low exchangeable and soluble forms especially at the Challawa and the Watari sites. The transferability of Cr, especially at the Challawa site was poor despite its high concentration. The accumulation of a particular element in plants depends on several factors most especially the nature of the metal, the soil conditions, the plant species, and the stage of growth (Cui et al. ). This proves the disparities between the different metals with the regards to their transferability and accumulation in the test crop. The relatively high transfer rate of Cu and Cd in comparison to their low concentrations may suggest an ability to bind more to enzyme sites when they simultaneously enter plants with for example Cr. The MTF values here generally agree with the values of Zhuang et al. ( ) and Khan et al. ( ) for leafy vegetables. The results also generally agreed with mechanism of soil absorption order: Pb 2+ >Cu 2+ >Cd 2+ of Nan et al. ( ). Plant accumulation and EDI of metals from lettuce consumption The cumulative total metal contents of plants in the three sites are presented in Table and contents in the plant tissues in the various sectors of the three rivers are given in Table . The comparative EDIs by consumers of the test crop for the various metals are depicted in Figure A–D. Significantly different amounts of Cu and Pb were accumulated by plants in the sectors of the rivers. Higher amounts tended to be accumulated in the upstream sectors of Challawa and Jakara and the downstream sectors of the control Watari river. No significant difference was found in the Cr content of the samples in the sectors of Jakara and Challawa although there was a downstream decline in the means while at the control the highest amount was found at the downstream. This trend is similar to that of Cd across all the sectors of the rivers. The concentrations of all the metals except Cu in all the sectors of the sites have exceeded the maximum level recommended for fresh vegetables by FAO/WHO ( ). Total concentrations of metals in plant tissues for the three sectors of the sites Location Cu (μg/kg) Cr (μg/kg) Pb (μg/kg) Cd (μg/kg) Challawa Yan Sama 1.46b 27.56 ns 35.18a 0.83 ns Sharada 1.62b 28.21 ns 22.22c 0.97 ns Rafin Kuka 2.60a 28.20 ns 27.78ab 0.83 ns SE± 0.11 1.39 1.06 0.06 Jakara Akija 3.09a 29.49a 44.44a 1.39a Airport Road 2.28b 28.85a 25.93b 0.96b Magami 2.60b 27.57a 12.74c 0.76b SE± 0.10 1.07 3.57 0.05 Watari Lambu 0.98b 0.21b 2.59b 0.29b Langel 0.82b 0.24ab 2.04c 0.34ab New Site 1.30a 0.31a 4.52a 0.49a SE± 0.07 0.02 0.27 0.04 FAO/WHO ML 73.00 20.00 0.30 0.1 Means followed by the same letter in the same column and for the same basin are not significantly different at 5% LSD. ML = maximum level allowed for fresh vegetables by FAO/WHO ( ). (A) Mean daily intake of Cr as compared to EPA/IRIS ( ). (B) Mean daily intake of Pb as compared to WHO PTDI ( ). (C) Mean daily intake of Cd as compared to WHO PTDI ( ). (D) Mean daily intake of Cu as compared to WHO PTDI ( ). IRIS, integrated risk information system; PTDI, provisional tolerable daily intake; US‐EPA, United States‐Environmental Protection Agency; WHO, world health organisation. The pattern of metal accumulation on sector by sector basis was consistent with the distribution of metals in the soils of the area as shown in Tables . Comparatively higher amounts of exchangeable and soluble forms of the individual metals translate into higher amounts in the tissues of the plant especially at the Jakara sites where soil conditions shown in Table favors the mobilization of metals from total concentration to potentially bioavailable forms in the soil. Cadmium is a highly mobile metal, easily absorbed by the plants through root surface, moves to wood tissue, and transfers to upper parts of plants; and evidently there is a direct relationship between the levels of Cadmium in the root zone and its absorption by plant (Mohsen and Mohsen ). Akinola and Ekiyoyo ( ) have confirmed the ease with which this metal is absorbed by plants as concentrations in washed leaves of Talinum traingulare and Telfaria occidentalis from industrialized areas of Lagos were accumulating Cd in excess of 4.03 mg/kg. The high Pb content of the Jakara soil may have also affected the Cu and Cd absorption. Significant relationship has been found between Pb concentrations in soil and Cu and Cd uptakes by crops. Nan et al. ( ) found that Cd and Cu content in grain wheat was high in locations with high Pb and Zn. Dudka et al. ( ) showed that Cd content of plants treated with Pb + Cd was greater than that of plants treated with Cd alone. The behavior of Cd in this result is similar to the observation of Abdu ( ) in the study of transfer behavior of Zn and Cd in different vegetables including lettuce. The remarkably better soil condition which facilitated higher soluble and exchangeable fractions in the soils of Jakara might have also facilitated the higher metals accumulation in the samples from the Jakara site. In the study of Abdu ( ), plants from sites with higher CEC, clay, organic matter, and comparatively lower pH, seemingly accumulated higher concentrations of the tested metals (Zn and Cd). This, however, does not apply to the Cd situation at the Watari site where despite low concentration and poor soil conditions yet was remarkably accumulated. The probable explanation may be the effect of other cations not tested, especially Zn. As reported by Zayed and Terry ( ), Cr is readily immobilized in soils by adsorption, reduction, and precipitation processes, with only a fraction of the total Cr concentrations available for plant uptake. When taken up by plants, >99% of the absorbed Cr is retained in the roots where it is reduced to Cr(III) species in a short time. Because most plants have low Cr concentrations, even when grown on Cr rich soils, the food chain is well protected against Cr toxicity. These facts about it might have explained its low values in plants despite its higher values in the soils. Akinola and Ekiyoyo ( ) were able to extract as high as 47 μg/g in roots and none in shoots of Telfaria occidentalis from a total concentration of 3647 μg/g in the soil. Organic matter can also regulate the availability of Cr(III) which is the dominant form in soil through chelation reaction, and the complex compounds formed with organic material may be unavailable to plants (Ramachandran and D'Souza ). Higher Pb concentration in soil has correspondingly produced higher concentration in plants despite the high pH across the basins which are suppose to minimize solubility as argued by Akinola and Ekiyoyo ( ). This deviation from the fact could be attributed to hyper accumulation as postulated by Mumba et al. ( ) who detected concentration as much 0.12 μg/g in cabbages grown in soils that have undetectable amounts of the metal. When the plant concentrations were translated into EDIs and compared with provisional tolerable daily intake (PTDI) recommended by FAO/WHO ( ) codex alimentarious for Pb, Cd, and Cu; and the US‐EPA ( ) reference dose for chronic oral exposure (RfD) for Cr; daily consumers of lettuce from Challawa and Jakara sites are liable to consume far in excess of the recommended daily allowable doses of Cd, Pb and Cr and less of the Cu dose; while consumers of lettuce from the control may take slightly in excess of the allowable daily dose of Cd and Pb; and far less of the Cu and Cr allowable daily doses. There was a variation in the degree of exposure with variation in sites and sectors within sites as can be seen from the figures. At the Jakara except for Pb, exposure risks decreases downstream while at the control exposure risk increases downstream. The pattern is more inconsistent at the industrial wastewater sites (Challawa). The general trend for all the metals in terms of exposure risk is such that domestic wastewater (Jakara) > industrial wastewater (Challawa) > control (Watari). The results are consistent with the metal contents in the soil, although the effect of over the air addition to the foliage may not be ruled out. Awode et al. ( ) ascribed their excessive Cd content in plants of the Challawa basin to concentrations aggravated by discharges from wear and tear of vehicle parts used to convey sand from the river, as this is a common activity along the bank of the river. The results here disagreed with the findings of Abdu ( ) for Cd in which he portrayed the consumer population as in no immediate risk. They are, however, more consistent with the findings of Lacatusu and Lacatusu ( ) for vegetables and fruits grown in pollution prone areas especially for the Cd and Pb. Conclusion Going by the data generated in this study, it is evident that urban agriculture has multiple of advantages and disadvantages. Whereas no one can doubt its significant contribution to the economy of the farmers and the state especially in terms of generating employment and income, as well as subsidizing the cost of vegetables in the city; yet its overall consequences on the environment from the point of view of the findings here could be disastrous in the long term. The use of wastewater and municipal wastes have resulted in soils that are fertile in some areas due to high levels of N, P, and K as well as high levels of toxic materials. Many of these metals have not reached levels beyond international standard in the soils, but the water levels are far in excess of the recommended averages. This is additional to the fact that plants' concentrations have resulted in exposure of the human consumers to levels beyond permissible daily averages. Because water is the principal agent in the introduction of these substances into the other components of the environment and the tendency of these metals for long‐term persistence in the environment, the risks posed to the environment and the human population may not be far‐fetched. This is coupled with the fact that other sources such as air deposit through vehicular releases, agrochemicals, and solid waste disposal may also be making significant contribution. The impact of agrochemicals may also not be overruled because it may be the principal source of contamination into the control site as all the locations are away from the city's pollution. Acknowledgments The authors wish to acknowledge the efforts of staff at the Soil and Water Laboratories of Geography and Soil Sciences Departments of Bayero and Ahmadu Bello Universities, Kano and Zaria, respectively. Conflict of Interest None declared.

Journal

Food and Energy Security – Wiley

Published: May 1, 2013

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