Competing controls on groundwater oxygen concentrations revealed in multidecadal time series from riverbank filtration sites

Abstract

Dissolved oxygen (DO) is an important indicator of groundwater quality, but long time series of groundwater DO concentration are rare. Here we describe and analyze multidecadal time series of groundwater DO data from five Swiss aquifers that are recharged by riverbank filtration (RBF), and relate temporal features of the DO time series to potential forcing factors. Features found in the DO time series include long‐term decreases and abrupt increases. Some features occur simultaneously in hydrologically unconnected aquifers, suggesting that external forcing partially determines DO concentrations at RBF sites. The data indicate that: (i) the DO concentration in the losing river is not a critical determinant of groundwater DO concentration; (ii) increasing river‐water and groundwater temperatures, by affecting both the physical solubility of oxygen and DO consumption in the hyporheic zone, probably cause the long‐term decline in DO concentration observed in most aquifers investigated; and (iii) a complex interaction between hydrological factors such as groundwater pumping rate and river discharge results in abrupt changes in groundwater DO concentration. Climate models predict higher temperatures and more frequent flood events in central Europe, implying that groundwater DO concentrations at many RBF sites will continue to decrease in the long term, but that irregular high‐discharge events, by scouring and unclogging riverbeds, will probably prevent the occurrence of long periods of hypoxia. Nonetheless, the risk of short periods of hypoxia at RBF sites is likely to increase.