Contaminant spreading in areas with a high density of Seasonal Aquifer Thermal Energy Storage (SATES) systems
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Seasonal Aquifer Thermal Energy Storage (SATES) is an increasingly popular type of renewable energy. Hereby summer heat and winter cold is stored in the subsurface for use in the opposite seasons. SATES systems are realized in high density in urban areas, where large amounts of contaminants are present. It is not fully understood to what extent interactions between the different SATES systems cause spreading of contaminants and to what extent SATES systems are a threat for the water quality, as groundwater protection zones are often present close by the urban area. In this study, analytical solutions, a theoretical model and a case study model of the city of Utrecht are used to describe, quantify and explain contaminant spreading in a high density SATES system area. Furthermore, these results are used to place the effects it in a regional perspective. Model simulations showed that SATES systems contribute significantly to contaminant spreading, by two mechanisms. 1. Recirculation is the extraction of (contaminated) water by a SATES well and re-injection in the other SATES well. More wells within a SATES system and a larger distance between the extraction and injection wells increase the spreading effect of recirculation. As there is no straightforward way to model recirculation, the most realistic method, cross coupling of wells, is chosen out of several options. 2. SATES induced head changes increase spreading significantly, also in the vertical direction, and contribute to contamination dilution from a dense non-aqueous phase liquid (DNAPL). Interaction between SATES systems is determined by overlapping capture zones. SATES induced head changes enlarge capture zones and increase the hydrological interaction of SATES systems. This results in more contaminant spreading. Spreading is increased in such a degree, that on a timescale of decades all contaminations in a cluster of SATES systems are mixed to a single contamination plume. Contaminant travel times within a SATES system area are very small, but a buffer zone without SATES systems between the contaminated area and drinking water wells can extend the travel time significantly. In a regional perspective, other extraction wells (e.g. on building sites) are at least as important for the water quality as SATES systems, because of their purely extracting character and their much larger discharge. This study shows how contaminant spreading is significantly increased in a high density SATES system area. It provides hands-on analytical relationships to describe the impact of several well variables on the amount of spreading. Besides, this report contains methods to properly model SATES specific processes that affect contaminant spreading. Also, the modeled contaminant spreading is placed in a larger perspective. Future research should focus on how this knowledge can be applied in regional subsurface planning.