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Advective Heat Transport in an Unconfined Aquifer Induced by the Field Injection of an Open-Loop Groundwater Heat Pump

Advective Heat Transport in an Unconfined Aquifer Induced by the Field Injection of an Open-Loop... American Journal of Environmental Sciences 6 (3): 253-259, 2010 ISSN 1553-345X © 2010 Science Publications Advective Heat Transport in an Unconfined Aquifer Induced by the Field Injection of an Open-Loop Groundwater Heat Pump Stefano Lo Russo and Glenda Taddia Department of Environment and Geo-Engineering (DITAG), Politecnico di Torino-Land, C.so Duca degli Abruzzi, 24 10129 Torino, Italy Abstract: Problem statement: The increasing diffusion of low-enthalpy geothermal open-loop Groundwater Heat Pumps (GWHP) providing buildings air conditioning requires a careful assessment of the overall effects on groundwater system, especially in the urban areas. The impact on the groundwater temperature in the surrounding area of the re-injection well is directly linked to the aquifer properties. Physical processes affecting heat transport within an aquifer include advection (or convection) and hydrodynamic thermodispersion (diffusion and mechanical dispersion). If the groundwater flows, the advective components tend to dominate the heat transfer process within the aquifer and the diffusion can be considered negligible. This study illustrates the experimental results derived from the groundwater monitoring in the surrounding area of an injection well connected to an open-loop GWHP plant which has been installed in the “Politecnico di Torino” (NW Italy) for cooling some of the university buildings. Groundwater pumping and injection interfere only with the upper unconfined aquifer. Approach: After the description of the hydrogeological setting the authors examined the data deriving from multiparameter probes installed inside the pumping well (P2), the injection well (P4) and a downgradient piezometer (S2). Data refers to the summer 2009. To control the aquifer thermal stratification some multi-temporal temperature logs have been performed in the S2. Results: After the injection of warm water in P4 the plume arrived after 30 days in the S2. That delay - 1 is compatible with the calculated plume migration velocity (1.27 m d ) and their respective distance (35 m). The natural temperature in the aquifer due to the switching-off of the GWHP plant has been reached after two month. The Electrical Conductivity (EC) values tend to vary out of phase with the temperature. The temperature logs in the S2 highlighted a thermal stratification in the aquifer due to a low vertical dispersion of the injected warm water. Conclusion: Experimental evidences seem to confirm the prevalence of heat advective transport component respect the dispersive phenomena. This hypothesis appears validated by the following evidences: (i) the calculated advective migration velocities are compatible with the calculated retardation factor and the temperature revealed in the S2, (ii) both the groundwater and the heat tend to flow horizontally due to the different values of horizontal and vertical hydraulic conductivity in the Unit 1 (thermal stratification) and (iii) the flowing water highlighted different geochemical characteristics during the time. Key words: Advective transport, heat flow, low-enthalpy GWHP, Turin, Italy INTRODUCTION Pump (ASHP) system (Blum et al., 2010). However, the benefit of utilizing groundwater may not be fully The Groundwater Heat Pump (GWHP) system is achieved everywhere because system performance an open-loop system that withdraws water from a well depends significantly on hydrologic and geological or surface water, passes it through a heat exchanger and conditions (for example, aquifer depth, groundwater discharges the water into an injection well or nearby quality, the cooling and heating pattern, building load) river (Lund et al., 2005). As an efficient use of natural (Abu-Nada et al., 2008). energy, this system utilizing the relatively stable Depending on the use mode (heating or cooling), temperature of groundwater can achieve a higher energy can be extracted or injected. Thus, the ambient coefficient of performance and offers a more energy- aquifer temperature is disturbed and cold or warm saving solution than the conventional Air-Source Heat plumes develop. In order to optimize the design and Corresponding Author: Stefano Lo Russo, Department of Environment and Geo-Engineering (DITAG), Politecnico di Torino-Land, C.so Duca degli Abruzzi, 24 10129 Torino, Italy 253 Am. J. Environ. Sci., 6 (3): 253-259, 2010 operation of GWHP systems, it is usually necessary to an injection well of an open-loop GWHP which has predict groundwater and heat flow and evaluate system been installed in the “Politecnico di Torino” for cooling performance comprehensively. The temperature some of the university buildings. The monitoring period distribution in the aquifer will affect the heat pump covers the summer 2009. Two existing wells are efficiency if the perturbed area reaches the extraction present in the site, one useable for groundwater well (or that of other heat pumps installed in the extraction (P2), the other for injection (P4). A surrounding areas) (Hecht-Mendez et al., 2010; Lo piezometer (S2) in placed 35 m downgradient respect Russo and Civita, 2009; 2010; Nam and Ooka, 2010). the P4. In these situations, it is important to understand the Through multiparameter probes installed in the effect of the heat on the groundwater system and to be pumping and injection wells and inside the piezometer able to predict consequences such as the accelerated it is possible to control the movement of the warm precipitation of dissolved substances, or changes in the plume over time during the operating period of the heat biological regimes. To this aim several simulation pump. The multi-temporal thermal logs in S2 models have been intensively developed with different highlighted the plume thermal stratification in the reliability of results. Although in several studies models aquifer and tend to confirm the hypothesis about the were successfully applied for simulating heat transport prevailing advective transport component for heat flow. within the aquifers, experimental studies which MATERIALS AND METHODS provided field data concerning heat transfer phenomena are not so diffuse (Hecht-Mendez et al., 2010). Site description: The site is located in the Turin urban In a composite medium, such as an aquifer, the district (NW Italy) at about 248 m asl. This flat area is properties of both the fluid phase and the solid phase mainly developed on the outwash plain constituted by play important roles in heat transport (Bear, 1972). The several glaciofluvial coalescing fans connected to the temperature disturbances due to injection induced are Pleistocene-Holocene expansion phases of the Susa compensated by lateral conductive heat transport and glacier. The plain extends between the external Rivoli- by convection due to moving water. In addition, heat Avigliana Morainic Amphitheatre (RAMA-Susa transfer from the aquifer system to adjacent aquitards or Glacier) on the west side and the miocenic sequences of through the unsaturated zone to the atmosphere can be the Torino Hill on the east (Fig. 1). significant process for removing heat from the aquifer. Downhole log data in the study area indicate the Physical processes affecting heat transport within an presence of two lithologic zones with distinct hydraulic aquifer include advection (or convection), mechanical properties. Unit 1-(Middle Pleistocene-Holocene; from dispersion and diffusion (usually grouped into the surface to 48 m depth). Continental alluvial cover hydrodynamic thermodispersion) (Diao et al., 2004). composed mainly of coarse gravel and sandy sediments Convection is an energy transport mechanism due to (locally cemented) derived from alluvial fans aggraded fluid motion inside the medium. When the flow field is by the Alpine rivers downstreaming towards the east. caused by external forces, the transport is said to occur The base of Unit 1 (erosional surface) dips gently by forced convection (Carslaw and Jager, 1959). Free or (0.5%) towards the north-east, overlaying Unit 2. natural convection, instead, occurs when the movement of water is due to density variations caused by temperature gradients (Sethi and Di Molfetta, 2007). The diffusion of heat depends on the thermal conductivity and heat capacity of the aquifer. Diffusion occurs by conductive transport in a solid or a liquid by a linear expression relating the heat flux to the temperature gradient. If there is a lack of groundwater flow the heat transport in the porous medium occurs only due to the diffusion. However under most conditions of groundwater natural flow, diffusion is insignificant and is neglected. At higher velocities and/or longer flow paths (higher Peclet number) mechanical dispersion is the predominant cause of mixing of the thermal plume and the effects of diffusion can be ignored. In this study we discuss the first experimental Fig. 1: Hydrogeological map of the Turin area and results derived by the groundwater monitoring around location of the site 254 Am. J. Environ. Sci., 6 (3): 253-259, 2010 Table 1: Nomenclature K dh v = ⋅ (1) Symbol Variable Unit a n dl n Total porosity (void volume/total volume) (-) n Effective porosity (always <n) (-) 2 - 1 Trans Aquifer transmissivity (m sec ) During the injection of warm water in the P4 the - 1 K Aquifer hydraulic conductivity (m sec ) hydraulic gradient increases up to 0.86% and thus the - 1 dh dL Hydraulic gradient (-) actual average velocity slightly grows up to 3.55 m - 3 ρ Density of the water (kg m ) - 1 day according with Eq. 1. - 1 - 1 C Specific heat capacity of the water (J kg K ) - 3 - 1 The average P2 pumping (and P4 injection) rates ρ C Volumetric heat capacity of the water (J m K ) w w - 1 T, T Temperature, temperature of the solid (K) s during the GWHP functioning period were 8.5 L sec . t time (sec) - 3 ρ Density of the solid material (minerals) (kg m ) - 1 - 1 Heat transport in the aquifer: For shallow unconfined C Specific heat capacity of the solid (J kg K ) - 3 - 1 aquifers five physical processes are relevant to the ρ C Volumetric heat capacity of the solid (J m K ) s s - 1 - 1 - 1 λ Effective thermal conductivity of the (J sec m K ) storage and movement of the thermal energy. These porous media (water and solid) processes are (1) advection of the injected slug due to - 3 ρ Dry bulk density ρ = (1-n)ρ (kg m ) b b s the natural gradient, (2) upward movement of the α, α Dynamic dispersivity, Longitudinal (m) heated slug due to the buoyancy of the heated water dispersivity - 1 heat (3) conduction within the aquifer, (4) heat transfer να Seepage average linear velocity (m sec ) - 1 - 3 q Heat injection (source)/extraction (sink) (J sec m ) h across the surface boundary and (5) seasonal variation 3 - 1 - 3 q Volumetric flow rate per unit volume of (m sec m ) ss in the background surface/aquifer temperatures aquifer representing sources and sinks (Palmer et al., 1992). - 3 C Concentration of the sources or sinks (kg m ) or (K) ss Owing to the analogies between solute and heat R Retardation factor (-) transport processes, the governing equations for transport in the subsurface can be represented by Unit 2-(Early Pliocene-Middle Pleistocene; from similar differential equations. The heat transport 48 m depth). Fossiliferous sandy-clayey layers with equation can be characterized by the principle of heat subordinate fine gravely and coarse sandy marine layers conservation, including conduction and convection (De or by quartz-micaceous sands with no fossil evidences. Marsily, 1986): The unconfined aquifer that extends over the entire plain, including the location of the Politecnico site, is ∂T ∂T hydraulically connected to the main surface water nρ c + 1 - nρ c = ( ) w w s s drainage network in the area (i.e., the Dora Riparia and ∂t ∂t (2) Po Rivers). This aquifer is hosted in Unit 1 and is quite div  λ + nρ c αv gradT  - div ρ n cα v T+ q ( ) ( )  m w w a  w w a h vulnerable to pollution because of its shallow depth. The potentiometric surface, 21 m below ground level, Assuming that the temperature of water and soil shows a W-to-E undisturbed gradient of dh/dl; i = are the same and that there is no net transfer from one 0.29% (Table 1 for nomenclature). The saturated phase to another, that is, thermal equilibrium (Nield and thickness of the unconfined aquifer is about 27 m. Bejan, 2006), the term on the left side of the heat The GWHP plant interferes only with the Unit 1 by transport equation can be expressed as follows: means of a 40-m deep pumping well (P2) and one downgradient 47-m deep injection well (P4). A 35-m ∂T ∂T ∂T deep piezometer (S2) monitors the aquifer and is nρ c + 1 - nρ c = ρ c (3) ( ) w w s s m m located downgradient respect P4. The respective ∂t ∂t ∂t distances are: P2-P4 = 78 m, P4-S2 = 35 m, P2-S2 = 109 m. The heat capacity of the porous medium ρ c can m m First a step drawdown test was performed in P4 to be computed as the weighted arithmetic mean of solid evaluate the hydraulic properties of the Unit 1. The test rock and pore fluid (Anderson, 2005; Hoeh and Cirpka, - 2 2 - 1 yielded a transmissivity (Trans ) of 1.55×10 m sec . 2006): - 4 - 1 The hydraulic conductivity (K = 5.74×10 m sec ) was calculated assuming and average saturated ρ c = nρ c + 1 - nρ c= ρ n c + ρ c (4) ( ) m m w w s s w w b s thickness of 27 m. The effective porosity n was assumed 0.12. The undisturbed average linear velocity - 1 Using Eq. 2 and 3 and rearranging them, Eq. 2 (Fetter, 1999) ν (1.19 m d ) is thus calculated as follows: simplifies to: 255 Am. J. Environ. Sci., 6 (3): 253-259, 2010 To be consistent with the dimensions relating the    ρ c  ∂T  λ  m m m = div + αv gradT      a contaminant and heat transport, the unit Kelvin (K) is nρ c ∂t nρ c    w w   w w    - 3 (5) equivalent to the concentration (kg m ). Thus energy input/extraction is stated similar as a mass load per unit - div v T+ ( ) nρ c w w volume of aquifer. One of the most significant effect is the advection Temperature dependency of the thermal of the thermal plume away form the injection well due parameters: Temperature has an influence on several to the natural hydraulic gradient. The advective velocity physical parameters such as density and viscosity of of the plume should be less than the natural water and thermal conductivity and heat capacity of the groundwater velocity by a factor R, the thermal porous medium. Density and viscosity would directly retardation factor, which can be derived by factoring affect the heat transport through the hydraulic out the appropriate terms from the heat transport conductivity and, consequently, the groundwater flow equation. It is given as the ratio between volumetric calculation. This influence is essentially independent of heat capacity of the porous medium (total phase) and the hydrogeological system being simulated. However volumetric heat capacity of the water (mobile phase) if the GWHPs groundwater temperature changes are (Shook, 2001): restricted to some degrees the variation of fluid density and fluid viscosity with temperature is negligible. ρ c Moreover this inaccuracy seems acceptable in view of m m R = (6) the general imprecision related to the determination of nρ c w w hydraulic conductivity, which is already reported as 27% for laboratory conditions (Butters and Duchateau, Assuming a total porosity of 0.25 and using the 6 - 3 - 1 2002). heat capacity of the aquifer ρ c 2.94×10 J m K m m 6 - 3 - 1 Temperature variations can also promote free computed by Eq. 4 (ρ C = 2.52×10 J m K and s s convection, which is a process driven by density 6 - 3 - 1 ρ C = 4.2×10 J m K (Diersch, 2005)), R would w w differences as well as salinity concentration (Nield and equal approximately 2.8 in the saturated aquifer. The Bejan, 2006). Free convection creates a buoyancy natural groundwater velocity would therefore almost effect, making a denser fluid flow below the lighter three times the migration velocity of the thermal plume. - 1 one. However, in the absence of brine currents in This is approximately 1.27 m d during the warm - 1 shallow aquifers, density changes are weak water injection and 0.43 m d after the switching-off of (Kolditz et al., 1998). Buoyancy effects begin to be the injection plant due to the variations in the hydraulic - 3 important for density differences larger than 0.8 kg m gradient. (Schincariol and Schwartz, 1990). Neglecting salinity - 3 effects, a density variation of 0.8 kg m implies a Diffusion and dispersion coefficients: In the diffusion temperature change from 0-15°C. and dispersion term of the partial differential equation Finally the temperature influences the heat capacity for heat transport (Eq. 2), we identify two parts. The and thermal conductivity (Clauser, 2003; Holzbecher, first one is the pure thermal diffusivity driven only by the temperature gradient: 1998). However even for larger differences (up to 60°C), the error in the heat transport simulation is less λ than 3% (Hecht-Mendez et al., 2010). D = (7) nρ c Based on these observations, the temperature w w dependency of the thermal parameters is not a real limitation for heat transport simulation of shallow The second term of Eq. 2, the hydrodynamic geothermal systems if the maximum differences dispersion αν , is a process driven by the differences in between injected hot water and natural conditions are flow velocities at pore scale. below 10-15°K. Density and viscosity variations with Sources and sinks: The source and sink term in the temperature can be considered negligible and also the heat transport equation represent energy input or buoyancy term in the momentum equation. For systems extraction: in which higher temperature changes are expected (>>10° K), instead, heat transport simulation should take into account the physical temperature q C = (8) ss ss ρ c w w dependencies of the thermal parameters. 256 Am. J. Environ. Sci., 6 (3): 253-259, 2010 RESULTS Monitoring: The GWHP plant started its operation on May 14th, 2009 and switch off on September, 20th 2009. Monitoring of hydraulic levels, electrical conductivity EC and temperature T in P2, P4 and S2 started in February 2009 and ended in November 2009. The data collected in P2 did not show significant changes during the monitoring period demonstrating no interference between pumping well (P2) and injection well (P4). Not being significant to evaluate the propagation of the thermal plume these data were not reported. On the contrary the data collected in P4 and Fig. 2: Monitoring data S2 are important to understand the subsurface heat transport phenomena. Analysis of those data clearly highlights three phases (Fig. 2). The phase 1 (February-May) corresponds to the period when the plant had not yet started. The T in the P4 and S2 is constant around 15°C. The relative increase in the T P4 (end of March) is connected to a plant functioning test and is not revealed in the S2. The values of EC are almost stationary. The phase 2 (May-September) corresponds to the functioning period. The T and EC values recorded in the P4 and S2 vary considerably. The oscillations in the T P4 depends on the daily and weekly cycles of the heat pump. The maximum recorded value reaches 28.7°C. The T recorded in the S2 clearly identify the aquifer response to the passage of the thermal plume with a significant delay (25-30 days) respect the injection in the P4. The T increase rate in S2 is approximately Fig. 3: Thermal logs - 1 1°C day for the first 15 days, then slows down and - 1 levels off to 0.5°C day during the last part of the Multi-temporal thermal logs: The injection of the increasing period. The highest T measured in S2 was warm water in the P4 occurs by means of discharge in 22°C (September, 17th). The oscillations in the the upper part of the water column in the well. injection temperature in the P4 are smoothed but clearly Therefore the thermal plume is originated by a point revealed also in the S2. The EC values tend to vary out source located on the top of the water table. In order to of phase with the temperature: When the T increases verify the presence of a thermal stratification in the there is a reduction of EC. plume, one thermal log has been conducted in the S2 The third phase (September-November) during the phase 1 (natural conditions). The result has corresponds to the period after the plant closure. The been compared with those derived by 3 thermal logs parameter values gradually tend to restore the baseline. conducted during the phase 3 (Fig. 3). Results The T P4 decreases sharply while the T S2 reduces highlighted a clear aquifer thermal stratification - 1 progressively losing about 1°C week and reaching the Temperature revealed in the S2 decrease with depth initial temperature (around 15°C) after two months. (Fig. 3). The progressive restoring of the initial After the injection of warm water in P4 the plume temperature vertical homogeneity occurred only several arrives in the S2 with a time delay (about 30 days) that weeks after the plant closure. is compatible with the calculated migration velocity and the respective distance. Moreover, the time lag DISCUSSION observed in S2 which is necessary to restore the natural The experimental results demonstrated the temperature in the aquifer is compatible with the lower migration velocity (due to lower hydraulic gradient) propagation of the subsurface thermal plume during the connected to the switching-off of the GWHP plant. injection of warm water and the progressive 257 Am. J. Environ. Sci., 6 (3): 253-259, 2010 disappearance of the temperature anomaly with time. In REFERENCES particular, the simultaneous monitoring of groundwater temperatures in P4 and S2 held to assess both the Abu-Nada, E., B. Akash I. Al-Hinti, A. Al-Sarkhi and overall width and the temporal evolution of the S. Nijmeh et al., 2008. Modelling of a geothermal temperature variations in different control points. standing column well. Int. J. Energy Res. These elements are very important to verify the 32: 306-317. 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Moreover comparison of field data and DOI: 10.1016/j.renene.2009.03.034 modelling assumptions can help individuating the Butters, G.L. and P. Duchateau, 2002. Continuous flow actual role and the relative weight of each subsurface method for rapid measurement of soil hydraulic parameter. properties: I. Experimental considerations. Vadose In accordance with the available budget and the Zone J., 1: 239-251. natural local conditions the efforts in the field Carslaw, H.S. and J.C. Jager, 1959. Conduction of Heat investigation should therefore concentrate to assess in Solids. 2nd Edn., Oxford University Press, New with the greater accurateness the most significant York, ISBN: 0198533683, pp: 510. parameters affecting the actual dynamics of heat Clauser, C., 2003. Numerical Simulation of Reactive transport. Flow in Hot Aquifers, SHEMAT and Processing SHEMAT. 1st Edn., Springer Verlag, Berlin, CONCLUSION pp: 331. De Marsily, G., 1986. Quantitative Hydrogeology: Results seem to confirm the prevalence of heat Groundwater hydrology for engineers. 1st Edn., advective transport component respect the dispersive Academic Press, Orlando, Florida, pp: 440. phenomena. This hypothesis appears validated by the Diao, N., Q. Li and Z. Fang, 2004. Heat transfer in following evidences: ground heat exchangers with groundwater advection. Int. J. Thermal Sci., 43: 1203-1211. • The growing of S2 temperature as a marker of the DOI: 10.1016/j.ijthermalsci.2004.04.009 warm plume transit highlighted a temporal delay Diersch, H.J.G., 2005. Reference Manual for (25-30 days) that is compatible with the P4-S2 FEFLOW®. WASY GmbH, Berlin, Germany, distance (35 m) and the calculated migration pp: 292. velocities in the different hydrodynamic conditions - 1 Fetter, C.W., 1999. Contaminant Hydrogeology. 2nd (1.27 m d during the warm water injection) Edn., Prentice-Hall, New-Jersey, USA., ISBN: 0- • Groundwater tend to flow horizontally due to the 13-751215-5, pp: 500. different values of horizontal and (lower) vertical Hecht-Mendez, J., N. Molina-Giraldo, P. Blum and hydraulic conductivity in the Unit 1. The thermal P. Bayer, 2010. Evaluating MT3DMS for heat stratification in the aquifer can be explained by the transport simulation of closed geothermal systems. prevailing advection phenomena. Heat is Ground Water. DOI: 10.1111/j.1745- transferred primarily by the flowing groundwater, 6584.2010.00678.x horizontally Hoeh, E. and O.A. Cirpka, 2006. Assessing hyporheic • The electrical conductivity appears to vary with an zone dynamics in two alluvial flood plains of the oscillatory behaviour, out of phase with respect to Southern Alps using water temperature and tracers. temperature. Uniquely difficult to explain, this Hydrology Earth Syst. Sci., 10: 553-563. 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Predicting thermal breakthrough in application of geothermal energy: 2005 worldwide heterogeneous media from tracer tests. review. Geothermics, 34: 691-727. DOI: Geothermics, 30: 573-589. DOI: 10.1016/S0375- 10.1016/j.geothermics.2005.09.003 6505(01)00015-3 Nam, Y. and R. Ooka, 2010. Numerical simulation of ground heat and water transfer for groundwater heat pump system based on real-scale experiment. Energy Build. 42: 69-75. DOI: 10.1016/j.enbuild.2009.07.012 http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png American Journal of Environmental Sciences Unpaywall

Advective Heat Transport in an Unconfined Aquifer Induced by the Field Injection of an Open-Loop Groundwater Heat Pump

American Journal of Environmental SciencesMar 1, 2010

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Abstract

American Journal of Environmental Sciences 6 (3): 253-259, 2010 ISSN 1553-345X © 2010 Science Publications Advective Heat Transport in an Unconfined Aquifer Induced by the Field Injection of an Open-Loop Groundwater Heat Pump Stefano Lo Russo and Glenda Taddia Department of Environment and Geo-Engineering (DITAG), Politecnico di Torino-Land, C.so Duca degli Abruzzi, 24 10129 Torino, Italy Abstract: Problem statement: The increasing diffusion of low-enthalpy geothermal open-loop Groundwater Heat Pumps (GWHP) providing buildings air conditioning requires a careful assessment of the overall effects on groundwater system, especially in the urban areas. The impact on the groundwater temperature in the surrounding area of the re-injection well is directly linked to the aquifer properties. Physical processes affecting heat transport within an aquifer include advection (or convection) and hydrodynamic thermodispersion (diffusion and mechanical dispersion). If the groundwater flows, the advective components tend to dominate the heat transfer process within the aquifer and the diffusion can be considered negligible. This study illustrates the experimental results derived from the groundwater monitoring in the surrounding area of an injection well connected to an open-loop GWHP plant which has been installed in the “Politecnico di Torino” (NW Italy) for cooling some of the university buildings. Groundwater pumping and injection interfere only with the upper unconfined aquifer. Approach: After the description of the hydrogeological setting the authors examined the data deriving from multiparameter probes installed inside the pumping well (P2), the injection well (P4) and a downgradient piezometer (S2). Data refers to the summer 2009. To control the aquifer thermal stratification some multi-temporal temperature logs have been performed in the S2. Results: After the injection of warm water in P4 the plume arrived after 30 days in the S2. That delay - 1 is compatible with the calculated plume migration velocity (1.27 m d ) and their respective distance (35 m). The natural temperature in the aquifer due to the switching-off of the GWHP plant has been reached after two month. The Electrical Conductivity (EC) values tend to vary out of phase with the temperature. The temperature logs in the S2 highlighted a thermal stratification in the aquifer due to a low vertical dispersion of the injected warm water. Conclusion: Experimental evidences seem to confirm the prevalence of heat advective transport component respect the dispersive phenomena. This hypothesis appears validated by the following evidences: (i) the calculated advective migration velocities are compatible with the calculated retardation factor and the temperature revealed in the S2, (ii) both the groundwater and the heat tend to flow horizontally due to the different values of horizontal and vertical hydraulic conductivity in the Unit 1 (thermal stratification) and (iii) the flowing water highlighted different geochemical characteristics during the time. Key words: Advective transport, heat flow, low-enthalpy GWHP, Turin, Italy INTRODUCTION Pump (ASHP) system (Blum et al., 2010). However, the benefit of utilizing groundwater may not be fully The Groundwater Heat Pump (GWHP) system is achieved everywhere because system performance an open-loop system that withdraws water from a well depends significantly on hydrologic and geological or surface water, passes it through a heat exchanger and conditions (for example, aquifer depth, groundwater discharges the water into an injection well or nearby quality, the cooling and heating pattern, building load) river (Lund et al., 2005). As an efficient use of natural (Abu-Nada et al., 2008). energy, this system utilizing the relatively stable Depending on the use mode (heating or cooling), temperature of groundwater can achieve a higher energy can be extracted or injected. Thus, the ambient coefficient of performance and offers a more energy- aquifer temperature is disturbed and cold or warm saving solution than the conventional Air-Source Heat plumes develop. In order to optimize the design and Corresponding Author: Stefano Lo Russo, Department of Environment and Geo-Engineering (DITAG), Politecnico di Torino-Land, C.so Duca degli Abruzzi, 24 10129 Torino, Italy 253 Am. J. Environ. Sci., 6 (3): 253-259, 2010 operation of GWHP systems, it is usually necessary to an injection well of an open-loop GWHP which has predict groundwater and heat flow and evaluate system been installed in the “Politecnico di Torino” for cooling performance comprehensively. The temperature some of the university buildings. The monitoring period distribution in the aquifer will affect the heat pump covers the summer 2009. Two existing wells are efficiency if the perturbed area reaches the extraction present in the site, one useable for groundwater well (or that of other heat pumps installed in the extraction (P2), the other for injection (P4). A surrounding areas) (Hecht-Mendez et al., 2010; Lo piezometer (S2) in placed 35 m downgradient respect Russo and Civita, 2009; 2010; Nam and Ooka, 2010). the P4. In these situations, it is important to understand the Through multiparameter probes installed in the effect of the heat on the groundwater system and to be pumping and injection wells and inside the piezometer able to predict consequences such as the accelerated it is possible to control the movement of the warm precipitation of dissolved substances, or changes in the plume over time during the operating period of the heat biological regimes. To this aim several simulation pump. The multi-temporal thermal logs in S2 models have been intensively developed with different highlighted the plume thermal stratification in the reliability of results. Although in several studies models aquifer and tend to confirm the hypothesis about the were successfully applied for simulating heat transport prevailing advective transport component for heat flow. within the aquifers, experimental studies which MATERIALS AND METHODS provided field data concerning heat transfer phenomena are not so diffuse (Hecht-Mendez et al., 2010). Site description: The site is located in the Turin urban In a composite medium, such as an aquifer, the district (NW Italy) at about 248 m asl. This flat area is properties of both the fluid phase and the solid phase mainly developed on the outwash plain constituted by play important roles in heat transport (Bear, 1972). The several glaciofluvial coalescing fans connected to the temperature disturbances due to injection induced are Pleistocene-Holocene expansion phases of the Susa compensated by lateral conductive heat transport and glacier. The plain extends between the external Rivoli- by convection due to moving water. In addition, heat Avigliana Morainic Amphitheatre (RAMA-Susa transfer from the aquifer system to adjacent aquitards or Glacier) on the west side and the miocenic sequences of through the unsaturated zone to the atmosphere can be the Torino Hill on the east (Fig. 1). significant process for removing heat from the aquifer. Downhole log data in the study area indicate the Physical processes affecting heat transport within an presence of two lithologic zones with distinct hydraulic aquifer include advection (or convection), mechanical properties. Unit 1-(Middle Pleistocene-Holocene; from dispersion and diffusion (usually grouped into the surface to 48 m depth). Continental alluvial cover hydrodynamic thermodispersion) (Diao et al., 2004). composed mainly of coarse gravel and sandy sediments Convection is an energy transport mechanism due to (locally cemented) derived from alluvial fans aggraded fluid motion inside the medium. When the flow field is by the Alpine rivers downstreaming towards the east. caused by external forces, the transport is said to occur The base of Unit 1 (erosional surface) dips gently by forced convection (Carslaw and Jager, 1959). Free or (0.5%) towards the north-east, overlaying Unit 2. natural convection, instead, occurs when the movement of water is due to density variations caused by temperature gradients (Sethi and Di Molfetta, 2007). The diffusion of heat depends on the thermal conductivity and heat capacity of the aquifer. Diffusion occurs by conductive transport in a solid or a liquid by a linear expression relating the heat flux to the temperature gradient. If there is a lack of groundwater flow the heat transport in the porous medium occurs only due to the diffusion. However under most conditions of groundwater natural flow, diffusion is insignificant and is neglected. At higher velocities and/or longer flow paths (higher Peclet number) mechanical dispersion is the predominant cause of mixing of the thermal plume and the effects of diffusion can be ignored. In this study we discuss the first experimental Fig. 1: Hydrogeological map of the Turin area and results derived by the groundwater monitoring around location of the site 254 Am. J. Environ. Sci., 6 (3): 253-259, 2010 Table 1: Nomenclature K dh v = ⋅ (1) Symbol Variable Unit a n dl n Total porosity (void volume/total volume) (-) n Effective porosity (always <n) (-) 2 - 1 Trans Aquifer transmissivity (m sec ) During the injection of warm water in the P4 the - 1 K Aquifer hydraulic conductivity (m sec ) hydraulic gradient increases up to 0.86% and thus the - 1 dh dL Hydraulic gradient (-) actual average velocity slightly grows up to 3.55 m - 3 ρ Density of the water (kg m ) - 1 day according with Eq. 1. - 1 - 1 C Specific heat capacity of the water (J kg K ) - 3 - 1 The average P2 pumping (and P4 injection) rates ρ C Volumetric heat capacity of the water (J m K ) w w - 1 T, T Temperature, temperature of the solid (K) s during the GWHP functioning period were 8.5 L sec . t time (sec) - 3 ρ Density of the solid material (minerals) (kg m ) - 1 - 1 Heat transport in the aquifer: For shallow unconfined C Specific heat capacity of the solid (J kg K ) - 3 - 1 aquifers five physical processes are relevant to the ρ C Volumetric heat capacity of the solid (J m K ) s s - 1 - 1 - 1 λ Effective thermal conductivity of the (J sec m K ) storage and movement of the thermal energy. These porous media (water and solid) processes are (1) advection of the injected slug due to - 3 ρ Dry bulk density ρ = (1-n)ρ (kg m ) b b s the natural gradient, (2) upward movement of the α, α Dynamic dispersivity, Longitudinal (m) heated slug due to the buoyancy of the heated water dispersivity - 1 heat (3) conduction within the aquifer, (4) heat transfer να Seepage average linear velocity (m sec ) - 1 - 3 q Heat injection (source)/extraction (sink) (J sec m ) h across the surface boundary and (5) seasonal variation 3 - 1 - 3 q Volumetric flow rate per unit volume of (m sec m ) ss in the background surface/aquifer temperatures aquifer representing sources and sinks (Palmer et al., 1992). - 3 C Concentration of the sources or sinks (kg m ) or (K) ss Owing to the analogies between solute and heat R Retardation factor (-) transport processes, the governing equations for transport in the subsurface can be represented by Unit 2-(Early Pliocene-Middle Pleistocene; from similar differential equations. The heat transport 48 m depth). Fossiliferous sandy-clayey layers with equation can be characterized by the principle of heat subordinate fine gravely and coarse sandy marine layers conservation, including conduction and convection (De or by quartz-micaceous sands with no fossil evidences. Marsily, 1986): The unconfined aquifer that extends over the entire plain, including the location of the Politecnico site, is ∂T ∂T hydraulically connected to the main surface water nρ c + 1 - nρ c = ( ) w w s s drainage network in the area (i.e., the Dora Riparia and ∂t ∂t (2) Po Rivers). This aquifer is hosted in Unit 1 and is quite div  λ + nρ c αv gradT  - div ρ n cα v T+ q ( ) ( )  m w w a  w w a h vulnerable to pollution because of its shallow depth. The potentiometric surface, 21 m below ground level, Assuming that the temperature of water and soil shows a W-to-E undisturbed gradient of dh/dl; i = are the same and that there is no net transfer from one 0.29% (Table 1 for nomenclature). The saturated phase to another, that is, thermal equilibrium (Nield and thickness of the unconfined aquifer is about 27 m. Bejan, 2006), the term on the left side of the heat The GWHP plant interferes only with the Unit 1 by transport equation can be expressed as follows: means of a 40-m deep pumping well (P2) and one downgradient 47-m deep injection well (P4). A 35-m ∂T ∂T ∂T deep piezometer (S2) monitors the aquifer and is nρ c + 1 - nρ c = ρ c (3) ( ) w w s s m m located downgradient respect P4. The respective ∂t ∂t ∂t distances are: P2-P4 = 78 m, P4-S2 = 35 m, P2-S2 = 109 m. The heat capacity of the porous medium ρ c can m m First a step drawdown test was performed in P4 to be computed as the weighted arithmetic mean of solid evaluate the hydraulic properties of the Unit 1. The test rock and pore fluid (Anderson, 2005; Hoeh and Cirpka, - 2 2 - 1 yielded a transmissivity (Trans ) of 1.55×10 m sec . 2006): - 4 - 1 The hydraulic conductivity (K = 5.74×10 m sec ) was calculated assuming and average saturated ρ c = nρ c + 1 - nρ c= ρ n c + ρ c (4) ( ) m m w w s s w w b s thickness of 27 m. The effective porosity n was assumed 0.12. The undisturbed average linear velocity - 1 Using Eq. 2 and 3 and rearranging them, Eq. 2 (Fetter, 1999) ν (1.19 m d ) is thus calculated as follows: simplifies to: 255 Am. J. Environ. Sci., 6 (3): 253-259, 2010 To be consistent with the dimensions relating the    ρ c  ∂T  λ  m m m = div + αv gradT      a contaminant and heat transport, the unit Kelvin (K) is nρ c ∂t nρ c    w w   w w    - 3 (5) equivalent to the concentration (kg m ). Thus energy input/extraction is stated similar as a mass load per unit - div v T+ ( ) nρ c w w volume of aquifer. One of the most significant effect is the advection Temperature dependency of the thermal of the thermal plume away form the injection well due parameters: Temperature has an influence on several to the natural hydraulic gradient. The advective velocity physical parameters such as density and viscosity of of the plume should be less than the natural water and thermal conductivity and heat capacity of the groundwater velocity by a factor R, the thermal porous medium. Density and viscosity would directly retardation factor, which can be derived by factoring affect the heat transport through the hydraulic out the appropriate terms from the heat transport conductivity and, consequently, the groundwater flow equation. It is given as the ratio between volumetric calculation. This influence is essentially independent of heat capacity of the porous medium (total phase) and the hydrogeological system being simulated. However volumetric heat capacity of the water (mobile phase) if the GWHPs groundwater temperature changes are (Shook, 2001): restricted to some degrees the variation of fluid density and fluid viscosity with temperature is negligible. ρ c Moreover this inaccuracy seems acceptable in view of m m R = (6) the general imprecision related to the determination of nρ c w w hydraulic conductivity, which is already reported as 27% for laboratory conditions (Butters and Duchateau, Assuming a total porosity of 0.25 and using the 6 - 3 - 1 2002). heat capacity of the aquifer ρ c 2.94×10 J m K m m 6 - 3 - 1 Temperature variations can also promote free computed by Eq. 4 (ρ C = 2.52×10 J m K and s s convection, which is a process driven by density 6 - 3 - 1 ρ C = 4.2×10 J m K (Diersch, 2005)), R would w w differences as well as salinity concentration (Nield and equal approximately 2.8 in the saturated aquifer. The Bejan, 2006). Free convection creates a buoyancy natural groundwater velocity would therefore almost effect, making a denser fluid flow below the lighter three times the migration velocity of the thermal plume. - 1 one. However, in the absence of brine currents in This is approximately 1.27 m d during the warm - 1 shallow aquifers, density changes are weak water injection and 0.43 m d after the switching-off of (Kolditz et al., 1998). Buoyancy effects begin to be the injection plant due to the variations in the hydraulic - 3 important for density differences larger than 0.8 kg m gradient. (Schincariol and Schwartz, 1990). Neglecting salinity - 3 effects, a density variation of 0.8 kg m implies a Diffusion and dispersion coefficients: In the diffusion temperature change from 0-15°C. and dispersion term of the partial differential equation Finally the temperature influences the heat capacity for heat transport (Eq. 2), we identify two parts. The and thermal conductivity (Clauser, 2003; Holzbecher, first one is the pure thermal diffusivity driven only by the temperature gradient: 1998). However even for larger differences (up to 60°C), the error in the heat transport simulation is less λ than 3% (Hecht-Mendez et al., 2010). D = (7) nρ c Based on these observations, the temperature w w dependency of the thermal parameters is not a real limitation for heat transport simulation of shallow The second term of Eq. 2, the hydrodynamic geothermal systems if the maximum differences dispersion αν , is a process driven by the differences in between injected hot water and natural conditions are flow velocities at pore scale. below 10-15°K. Density and viscosity variations with Sources and sinks: The source and sink term in the temperature can be considered negligible and also the heat transport equation represent energy input or buoyancy term in the momentum equation. For systems extraction: in which higher temperature changes are expected (>>10° K), instead, heat transport simulation should take into account the physical temperature q C = (8) ss ss ρ c w w dependencies of the thermal parameters. 256 Am. J. Environ. Sci., 6 (3): 253-259, 2010 RESULTS Monitoring: The GWHP plant started its operation on May 14th, 2009 and switch off on September, 20th 2009. Monitoring of hydraulic levels, electrical conductivity EC and temperature T in P2, P4 and S2 started in February 2009 and ended in November 2009. The data collected in P2 did not show significant changes during the monitoring period demonstrating no interference between pumping well (P2) and injection well (P4). Not being significant to evaluate the propagation of the thermal plume these data were not reported. On the contrary the data collected in P4 and Fig. 2: Monitoring data S2 are important to understand the subsurface heat transport phenomena. Analysis of those data clearly highlights three phases (Fig. 2). The phase 1 (February-May) corresponds to the period when the plant had not yet started. The T in the P4 and S2 is constant around 15°C. The relative increase in the T P4 (end of March) is connected to a plant functioning test and is not revealed in the S2. The values of EC are almost stationary. The phase 2 (May-September) corresponds to the functioning period. The T and EC values recorded in the P4 and S2 vary considerably. The oscillations in the T P4 depends on the daily and weekly cycles of the heat pump. The maximum recorded value reaches 28.7°C. The T recorded in the S2 clearly identify the aquifer response to the passage of the thermal plume with a significant delay (25-30 days) respect the injection in the P4. The T increase rate in S2 is approximately Fig. 3: Thermal logs - 1 1°C day for the first 15 days, then slows down and - 1 levels off to 0.5°C day during the last part of the Multi-temporal thermal logs: The injection of the increasing period. The highest T measured in S2 was warm water in the P4 occurs by means of discharge in 22°C (September, 17th). The oscillations in the the upper part of the water column in the well. injection temperature in the P4 are smoothed but clearly Therefore the thermal plume is originated by a point revealed also in the S2. The EC values tend to vary out source located on the top of the water table. In order to of phase with the temperature: When the T increases verify the presence of a thermal stratification in the there is a reduction of EC. plume, one thermal log has been conducted in the S2 The third phase (September-November) during the phase 1 (natural conditions). The result has corresponds to the period after the plant closure. The been compared with those derived by 3 thermal logs parameter values gradually tend to restore the baseline. conducted during the phase 3 (Fig. 3). Results The T P4 decreases sharply while the T S2 reduces highlighted a clear aquifer thermal stratification - 1 progressively losing about 1°C week and reaching the Temperature revealed in the S2 decrease with depth initial temperature (around 15°C) after two months. (Fig. 3). The progressive restoring of the initial After the injection of warm water in P4 the plume temperature vertical homogeneity occurred only several arrives in the S2 with a time delay (about 30 days) that weeks after the plant closure. is compatible with the calculated migration velocity and the respective distance. Moreover, the time lag DISCUSSION observed in S2 which is necessary to restore the natural The experimental results demonstrated the temperature in the aquifer is compatible with the lower migration velocity (due to lower hydraulic gradient) propagation of the subsurface thermal plume during the connected to the switching-off of the GWHP plant. injection of warm water and the progressive 257 Am. J. Environ. Sci., 6 (3): 253-259, 2010 disappearance of the temperature anomaly with time. In REFERENCES particular, the simultaneous monitoring of groundwater temperatures in P4 and S2 held to assess both the Abu-Nada, E., B. Akash I. Al-Hinti, A. Al-Sarkhi and overall width and the temporal evolution of the S. Nijmeh et al., 2008. Modelling of a geothermal temperature variations in different control points. standing column well. Int. J. Energy Res. These elements are very important to verify the 32: 306-317. 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Published: Mar 1, 2010

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