Aquifer Thermal Energy Storage (ATES) in phreatic aquifers, the role of redox conditions

Aquifer Thermal Energy Storage (ATES) generally is applied in relatively thick sandy layers under reduced conditions. But at many places in the world such optimal conditions for ATES do not exist. Therefore this research investigates the potential of ATES in (shallow) phreatic aquifers. By applying ATES in these conditions, the redox boundary should be taken into account. At greater depth the groundwater is reduced and often contains dissolved iron, at shallow depth the groundwater is oxygen/nitrate rich. Due to mixing of the oxidized water with the reduced water by extraction and injection during ATES operation, the dissolved oxygen and nitrate come into contact with the dissolved iron with precipitation of iron oxides and hydroxides as a result. These precipitates can cause well clogging, which especially will occur at the injection wells. Two cases in the north of Belgium (Flanders) are studied for the potential and restrictions of the application of ATES in aquifers where a redox boundary occurs. The first case investigates the potential of ATES in a thick phreatic sandy aquifer (Brussels Sands), but with a deep redox boundary in Leuven. In the second case the applicability of ATES is investigated in a shallow clay-rich sand layer (Berchem Sands) with a very shallow redox boundary in Antwerp. It turns out that in both cases the redox conditions play an important role in the feasibility of an ATES system. With the aid of accurate in situ hydrochemical measurements, groundwater analyses and mineralogical analyses, which are incorporated in a reactive transport model, the occurring processes around the ATES systems can be comprehended and predicted.status: publishe