Stormwater MAR in pre-alpine catchments

Managed aquifer recharge (MAR) is a state-of-the-art strategy to maintain groundwater levels and sustain groundwater-dependent ecosystem services. Increasing rainfall intensity and prolonged drought periods caused by climate change put the availability of river water for MAR at risk. To adapt, MAR has to include stormwater as a source. This requires to infiltrate high volumes of water with high recharge rates, increase buffer and retention capacity and times, and maintain water quality. In the project Smart-SWS we have developed a stormwater-based MAR scheme which consists of an infiltration ditch connected to a stream and a geotechnical barrier set in the aquifer. Water quality issues were addressed with a combination of catchment risk analysis, on-line monitoring backed by extended laboratory analyses, and tailored treatment techniques.

The site is located downstream of a flood retention basin (FRB) in the Bavarian alpine foreland. The upstream catchment is 16 km². The maximum discharge from the FRB at a 100-year event is 4.7 m³/s. The aquifer is composed of highly conductive quaternary gravel with a thickness of 10 to 20 m in the storage area. The hydrogeological model was transformed into a numerical groundwater model which was used to find the optimal position for the infiltration ditch (1400 m long) and the geotechnical groundwater flow barrier (1100 m long, top 3 m b.s.l.).

A first real-world proof-of-concept for the design and model was established during the 100-year flooding event in June 2024. The recorded water levels were modeled with an infiltration rate of 1.1 m³/s along a 630 m relief diversion from the main stream which runs along the planned infiltration ditch. Water quality during the flooding event was better than expected and met the criteria for infiltration.

A modeled extreme drought situation (no recharge for six months) showed that Smart-SWS will be able to still buffer up to 130.000 m³ of groundwater which is sufficient to supply drinking water for 2.600 capita. The geotechnical barrier prevents flooded basements in the village and helps to sustain ecosystem functions in the nearby wetlands.