Assessment of temporal and spatial variation of nitrate removal in riparian zones.

The aim of this study was to monitor long-term temporal and spatial groundwater NO(3) (-) removal efficiencies in different riparian zones via a limited number of sampling wells. Groundwater NO(3) (-) concentrations were measured fortnightly or monthly over a period of two years using transects of ground water sampling wells. Depending on the level of the NO(3) (-) load (up to 120mgNL(-1) at the input side of the riparian zone a distance of 10 to 30m was needed to remove NO(3) (-) from the groundwater below 11.3mgNL(-1). Considering all seasons, the mixed vegetation and grass riparian site succeeded to remove groundwater NO(3) (-) efficiently (92-100% within a distance of 30m. The forested riparian zone removed 72-90% of the total NO(3) (-) input within a distance of 30m. Evidence emerged that NO(3) (-) could also be removed actively at depths up to 2m, due to the presence of organically enriched layers of alluvial deposits or roots. Our four dimensional approach (three dimensional space and time), in combination with a limited number of sampling wells, was shown to be a useful monitoring tool to assess the variability of NO(3) (-) removal in riparian zones.