Transport of groundwater contaminant through high density ATES regions
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Aquifer Thermal Energy Storage (ATES) systems are a renewable energy technology which provides sustainable heating and cooling for the built environment. These systems store the surplus of thermal energy in summer and the deficit of thermal energy in winter in the groundwater to be able to recover this energy in the opposite season. The demand for ATES systems in cities is large, while aquifers beneath cities are often polluted. Therefore, the transport of contaminants through high density ATES regions is studied in this thesis. The numerical SEAWAT model from the work of Jaxa-Rozen et al. (2015) was used. This model is created to study the temperature in the subsurface. To make the model suitable for studying contaminant spreading and dilution by ATES wells, a tracer contaminant was added as a second species and a method to model internal ATES transport was developed. First, the interaction between two doublets was studied for line and checkerboard patterns. In these 2x2 configurations, the distance between the wells, the well discharge and the ambient groundwater flow were systematically changed. Secondly, the spreading and dilution through an ATES region was studied with 4x4 well configurations. The results showed that the spreading and dilution by the implementation of ATES systems increases drastically. The policy distances in the Netherlands now inhibit the negative interference of thermal performance, but do not inhibit the external ATES transport between systems. However, the impact can be managed/limited by choosing a suitable well configuration. This study showed that the external ATES transport between wells of the same type is dominated by dispersion, which causes small spreading rates in a timeframe of one year. The transport between two opposite type wells is governed by short circuit flow, which causes large spreading rates in a timeframe of one season. Therefore, line pattern doublets are preferred if spreading of contaminants needs to be limited. In practice, this means that it would be best if the same type of wells are clustered and the distance between the opposite wells is kept at least at the policy distance. This is also beneficial for the thermal efficiency of the wells. The downside of placing wells in line pattern is that the hydraulic head differences become large, causing increased groundwater flow. This increased groundwater flow can cause increased leaching from constant contaminant sources into the groundwater. Further study needs to focus on expanding this knowledge on constant contaminant sources and the implementation of that knowledge in ATES planning.