From baseline to post operation: two year monitoring of thermo-hydraulic induced geochemical effects of a cyclic HT-ATES field test at the “TestUM” test site

Seasonal ATES systems enable the efficient integration of climate-neutral heat sources into urban heat-supply systems. However, secure and efficient operation presupposes the detection as well as realistic evaluation and prediction of induced hydraulic, thermal, geochemical and microbial effects and their impacts on operation and environment.

To provide the database for extending field-scale process understanding and deriving suitable monitoring strategies, a cyclic HT-ATES field test was conducted. In six fortnight-long charging periods ~300 m³ of water were infiltrated (~15 L/min; ~80 °C) into the storage aquifer (6-15 bgs) and recovered directly or after 21 storage days. Induced hydrogeochemical effects and their reversibility were tracked with a temporal and spatially high-resolute monitoring of ~90 measurement points.

Within ~7 m around the „hot well“, superimposition of formerly stratified calcium and sulphate concentrations in combination with the spatial spreading-patterns of elevated silica concentrations point towards the build-up of a density-driven convection cell, which was also predicted by accompanying numerical thermo-hydraulic simulations. In storage periods, but more so in post operation, decreases in temperature go along with a decline of previously elevated concentrations of e.g. silica, potassium, selenium and vanadium. Moreover, potassium and selenium concentration-peaks drop after the first cycles, indicating depletion of their releasable pools. Although simulated tracers indicate passage of infiltrated water, no induced temperature or concentration changes were monitored 30 m downstream so far.

Overall, highly dynamic flow conditions dominate the hot well’s vicinity and despite scale dependent low heat recovery rates, reversibility of induced effects keeps the wider surrounding geochemically unaffected.