Abstract
A managed riparian lowland in a glacial landscape (Holtum catchment, Denmark) was studied to quantify the relative importance of subsurface and surface flow to the recipient stream. The hydrogeological characterization combined geoelectrical methods, lithological logs, and piezometric heads with monthly flow measurements of springs, a ditch, and a drain, to determine seasonality and thereby infer flow paths. In addition, groundwater discharge through the streambed was estimated using temperature and water-stable isotopes as tracers. The lowland received large groundwater inputs with minimal seasonal variations from adjacent upland aquifers. This resulted in significant amounts of groundwater-fed surface flow to the stream, via man-made preferential flow paths comprising ditches, drainage systems, and a pond, and via two natural springs. Roughly, two thirds of the stream gain was due to surface flow to the stream, mainly via anthropogenic alterations. In contrast, direct groundwater discharge through the streambed accounted for only 4% of the stream flow gain, although bank seepage (not measured) to the straightened and deepened stream potentially accounted for an additional 17%. Comparison to analogous natural flow systems in the catchment substantiate the impact of anthropogenic alterations of riparian lowlands for the subsurface and surface flow components to their streams.
Highlights
Non-point source contamination of streams with nutrients, applied for agricultural production, stresses natural environmental systems
It can be concluded that management for agricultural purposes have changed the natural flow paths through the riparian lowland to the stream
In the present study we have addressed the connection of a groundwater system and a riparian lowland to determine the relative importance of subsurface and surface flow to a headwater stream
Summary
Non-point source contamination of streams with nutrients, applied for agricultural production, stresses natural environmental systems. Riparian lowlands may serve as natural buffers, which may attenuate the movement of nutrients to streams [2]. Management of riparian lowlands are often recommended as an integral part of watershed management practices in agricultural areas to reduce nutrient loading to streams [3,4,5]. To effectively use riparian lowlands as buffer zones, or to implement new mitigation measures in the riparian lowlands, it is imperative to first understand the flow paths. This includes how discharge of groundwater from regional aquifers is distributed among e.g., direct groundwater discharge through the streambed, Water 2019, 11, 1905; doi:10.3390/w11091905 www.mdpi.com/journal/water
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