How to backflush a vadose zone well - Concept and experimental testing

Managed Aquifer Recharge (MAR) is an approach to halt the depletion of stressed aquifers, store seasonal excess water and prevent damages during flood events. Commonly, MAR is conducted by using shallow infiltration basins or groundwater wells, which inject the water directly into the saturated zone. Most basins, however, demand large areas, whereas groundwater wells potentially require deep drilling and bridge the unsaturated zone, which is often considered an indispensable barrier for groundwater protection (e.g. DWA A-138 in Germany). Vadose Zone Wells (VZW) represent a trade-off and allow for high infiltration rates combined with small area demand. This makes them an interesting alternative, where land is scarce and direct injection into the aquifer is not possible, e.g. due to deep groundwater tables or legal restrictions. Nevertheless, they are rarely considered, as the infiltration of turbid or nutrient-rich water leads to gradual clogging and a subsequent decline in infiltration capacity. Common well redevelopment measures require backflushing by pumping, which is not possible, when soil matrix surrounding the well is unsaturated.

In this study, we present a new approach for backflushing a VZW. Our laboratory experiments indicate, that the method can sustainably return the infiltration capacity to a certain percentage of its initial value (~50 %), even after multiple cycles of clogging and backflushing. It can be applied for VZW's with and without a gravel pack and will allow a wider application of these wells in the future, for example to compensate for the land sealing in urban areas.