Can Borehole Heat Exchangers Trigger Cross-Contamination between Aquifers?

Alessandro Casasso,Simone Pescarmona,Natalia Ferrantello,Rajandrea Sethi,Carlo Bianco

doi:10.3390/w12041174

Abstract

Borehole heat exchangers (BHEs) commonly reach depths of several tens of meters and cross different aquifers. Concerns have been raised about the possibility of boreholes to act as preferential pathways for contaminant transport among aquifers (cross-contamination). This article employs numerical modelling of contaminant transport in the subsurface to address these concerns. A common hydrogeological setup is simulated, composed of three layers: A shallow contaminated and a deep uncontaminated aquifer separated by an aquitard, all crossed by a permeable borehole. The hydraulic conductivity of the borehole and, to a lesser extent, the vertical hydraulic gradient between the aquifers are the key factors of cross-contamination. Results of the numerical simulations highlight that, despite the severe conditions hypothesized in our modelling study, the cross-contamination due to the borehole is negligible when filled with a slightly permeable material such as a geothermal grout properly mixed and injected. A good agreement was found with analytical formulas used for estimating the flow rate leaking through the borehole and for studying the propagation of leaked contaminant into the deep aquifer.

Highlights

Ground source heat pumps (GSHPs) gained increasing popularity in recent years due to the economic and environmental benefits they achieve as they are used to replace conventional techniques for heating and cooling of buildings and for domestic hot water production [1,2,3,4,5,6,7]
GSHPs are composed of a heat pump, which uses the underground as a heat source or sink, based on two different operating principles: open-loop systems, exchanging heat with groundwater abstracted by a well, and closed-loop systems, exchanging heat with the ground through the circulation of a heat carrier fluid in a pipe loop buried into the ground [8,9]
Closed-loop geothermal systems equipped with borehole heat exchangers (BHEs) are the most diffused type of GSHP thanks to the lower maintenance requirements and the possibility to install them even in the absence of an aquifer

Summary

Introduction

Ground source heat pumps (GSHPs) gained increasing popularity in recent years due to the economic and environmental benefits they achieve as they are used to replace conventional techniques for heating and cooling of buildings and for domestic hot water production [1,2,3,4,5,6,7]. BHEs are composed of one or two U-pipes installed in a borehole with a depth usually ranging between 50 and 200 m At such depths, BHEs generally intercept different aquifers and, if incorrectly grouted during the completion, they tend to become preferential pathways through the less permeable layers (aquitards), making deep aquifer more vulnerable to contamination [10]. BHEs generally intercept different aquifers and, if incorrectly grouted during the completion, they tend to become preferential pathways through the less permeable layers (aquitards), making deep aquifer more vulnerable to contamination [10] This concern led to restriction and bans of BHE installation in different protection areas of some European countries, as reported in a recent report by Prestor et al [11]

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Conclusion