Hydrology and water quality aspects of rhine bank groundwater in The Netherlands

Abstract

In deltaic areas with deep polders and in valleys or deltas with heavy groundwater exploitation, bank infiltration is a rapidly expanding mechanism of groundwater recharge. Rhine bank filtrate can be distinguished best from autochthonous groundwater in The Netherlands by way of its natural lower 18O content. Tritium is a good measure of the portion of water younger than 25 years, at least in well mixed Rhine bank filtrate. This portion carries the bulk of the contaminations originating from the Rhine, as evidenced by strong positive correlations between 3H and e.g. halogenated hydrocarbons, taste and mutagenic activity according to the Ames test. The composition of Rhine bank filtrate is further governed by the redox level of the hydrogeochemical system, varying in The Netherlands from suboxic to anoxic with fermentation, and by the underground distance to the influent Rhine branch. Anoxic bank filtrate with strong SO 4-reduction and CH 4 > 1 mg l −1 exhibited a significantly different composition from anoxic bank filtrate without fermentation and without SO 4-reduction. The following parameters are involved: HCO 3, total dissolved solids, SiO 2; KMnO 4-consumption, NH 4, siderite, baryte and vivianite saturation indices, I, total hardness, total PO 4, Fe, dissolved organic carbon, Ba, pH, dichloroethane, Mo, VOCl, Mn, V, dichlorobenzene and 1,2-dichloroethane, in order of decreasing significance. A longer underground detention time or travel distance, leads to a small decrease in the share of bank filtrate, and a rather strong decrease in K, Li, F, temperature, Mo (only in anoxic, SO 4-(meta)stable bank filtrate), UV-extinction, AOCl, X 7OCl and mutagenic activity. Coli bacteria and viruses could not be detected, not even in samples from wells at 50 m distance from the Rhine, with a minimum travel time of 30 days.