Factors controlling nitrate concentration in space and time in wells distributed along an aquifer/river interface (Káraný, Czechia)

Abstract

Nitrate is necessary for agricultural productivity, but it is also one of the most common water contaminants in developed countries. The riverbank filtration (RBF) systems in Karaný (Czechia), with nearly 500 wells regularly distributed along 22 km of the alluvium aquifer/river interface, presents a unique field laboratory where the adverse effects of intensive agriculture on groundwater can be traced from >50 years of nitrate concentration records. The largest control over nitrate concentrations in the RBF systems has been the mixing of the river-water component (low in nitrate) with the local recharge, which has a nitrate content of 100–250 mg/L. The mixing is mainly controlled by the intensity of the effective precipitation. Increases in nitrate concentration are caused by rapid hydraulic pulse propagation during the high-recharge periods to the discharge areas. In contrast, during dry periods, the river-water fraction dilutes nitrate in wells. The lowest nitrate content occurs above the weirs on the river, as well as in those areas where the alluvial aquifer is partly fed by older and/or denitrified groundwater from deeper zones of the underlying sandstone aquifer. High nitrate concentrations occur in wells where the low saturated thickness of the alluvial aquifer limits the inflow of river water. This study shows that factors affecting nitrate concentrations in the alluvium are numerous, and that long-term sampling is necessary to distinguish the oscillations caused by variability in the recharge intensity from decade-long trends controlled by the fertilizer load and nitrate lag time in the aquifer.