Fecal recontamination of infiltrated water in Dutch Managed Aquifer Research systems; 20 years of field research

The western part of the Netherlands depends largely on river water from the Meuse and Rhine rivers for drinking water supply, which is infiltrated into Managed Aquifer Recharge (MAR) systems in the Dutch coastal dunes. Pre-treated river water is infiltrated through open basins and recovered after soil passage for further treatment. This soil passage is a crucial step in drinking water treatment where unwanted microorganisms are efficiently removed. Field studies by Schijven et al (1998,1999), among others, showed that log removal during saturated transport in these systems is more than sufficient to produce microbiologically safe drinking water. Nevertheless, in subsequent years, fecal indicator organisms were still found with some regularity in the abstracted water. It was suspected that this (re)contamination was caused by short circuit flow with feces-contaminated water from ground level through the unsaturated zone. Conducting several field experiments confirmed this hypothesis and provided unique insight into transport behavior and log removal in the unsaturated zone and the role of preferential flow under field conditions. Results of the field trials have been implemented in guidelines for design and management of MAR systems and whether to allow grazing livestock near the MAR-systems for nature management. In this contribution we present the results of a series of field trials, spanning 20 years, providing unique insight in removal of fecal indicator bacteria in unsaturated dune soils and increasing insight in processes responsible for recontamination of infiltrated river water.