Geochemical changes under variably saturated conditions during artificial recharge via ponded infiltration — A field study

Abstract

Artificial groundwater recharge by percolation through the unsaturated zone is an important technique to enhance the water quality for drinking water supplies. Purification of the infiltration water results mainly from microbially mediated redox-reactions that involve the degradation of a wide range of organic substances. The aim of this study was to identify the spatial and temporal distribution of the redox zones that develop beneath an artificial recharge pond, a system that is characterised by regular hydraulic changes between the saturated and unsaturated condition within every operational cycle. The most significant hydraulic changes result from the formation and removal of a clogging layer at the pond’s bottom. Geochemical analyses of suction cup water as well as oxygen and hydraulic measurements showed that generally nitrate and manganese reducing conditions dominated below the pond as long as water saturated conditions prevailed. Iron and sulphate reduction occurred only in patchily distributed zones directly below the clogging layer and resulted from chemical and physical heterogeneity. When the sediment below the clogging layer became unsaturated, atmospheric oxygen penetrated from the pond fringes into this region, allowing re-oxidation of previously formed sulphide minerals.