Open-Loop Groundwater Heat Pump System: dynamic behavior and cross-correlation between the groundwater temperature and electrical conductivity

Groundwater Heat Pumps Systems (GWHPs), utilizing relatively stable groundwater temperature, are an efficient use of natural energy, can achieve a higher performance coefficient, and a thermal plume of colder or warmer re-injected groundwater, known as the Thermal Affected Zone (TAZ), develops. The plume propagation occurs primarily through advection (Lo Russo & Taddia, 2010), and tends to “degrade” following conductive heat transport, and convection within moving water (Hecht-Mendez et al., 2010). Plumes of colder or warmer re-injected groundwater are considered a potential anthropogenic geothermal resource or pollution. In fact, a thermal plume might pose a risk to groundwater use downgradient. This study is aimed to explore the auto and cross-correlation between some groundwater parameters (temperature, hydraulic levels and electrical conductivity) measured through specific multiparameter probes in a real field test-site. The data under consideration derived from the groundwater monitoring in the surrounding area of an injection well connected to an open-loop GWHP plant installed in the “Politecnico di Torino” (NW Italy). The plant is used for cooling some of the university buildings. Groundwater pumping and injection interfere only with the upper unconfined aquifer. The GWHP plant is constituted by a 40-m deep pumping well (P2) and a downgradient 47-m deep injection well (P4). A 35-m deep piezometer (S2) is located downgradient respect P4. The abstraction and injection well as the piezometer are screened along the whole saturated zone of the exploited unconfined aquifer. The continuous monitoring of the hydraulic levels, electrical conductivity EC and temperature T in P2, P4 and S2 is performed along the whole year, including the period of the switching off of the cooling plant. The analysis of correlation presented concentrates on the hourly data collected in the summer 2015. The GWHP plant started its operation on April 8th, 2015 and switch off on September, 3th 2015. Data collected in P4 and S2 are important to understand the subsurface heat transport phenomena and the possible trend between EC and T. The study of these correlations can be useful to characterize the heat transport in the aquifer and detecting the real velocity of migration of the thermal plume under the real operating conditions. Lo Russo S. & Taddia G. 2010. Advective heat transport in an unconfined aquifer induced by the field injection of an open-loop groundwater heat pump. American Journal of Environmental Sciences 6(3), 253-259 DOI: 10.3844/ajessp.2010.253.259 Hecht-Mendez J., Molina-Giraldo N., Blum P. & Bayer P. 2010. Evaluating MT3DMS for heat transport simulation of closed geothermal systems. Ground Water. DOI: 10.1111/j.1745-6584.2010.00678.x