Helmholtz Centre for Environmental Research - UFZ, Department Catchment Hydrology, Theodor-Lieser-Straße 4, 06120 Halle, Germany.
Helmholtz Centre for Environmental Research - UFZ, Department of Hydrogeology, Permoserstr. 15, 04318 Leipzig, Germany.
Sci Total Environ. 2018 Feb 15;615:773-783. doi: 10.1016/j.scitotenv.2017.09.297. Epub 2017 Oct 17.
Elevated nitrate concentrations are a thread for water supply and ecological integrity in surface water. Nitrate fluxes obtained by standard monitoring protocols at the catchment outlet strongly integrate spatially and temporally variable processes such as mobilization and turnover. Consequently, inference of dominant nitrate sources is often problematic and challenging in terms of effective river management and prioritization of measures. Here, we combine a spatially highly resolved assessment of nitrate concentration and fluxes along a mesoscale catchment with four years of monitoring data at two representative sites. The catchment is characterized by a strong land use gradient from pristine headwaters to lowland sub-catchments with intense agricultural land use and wastewater sources. We use nitrate concentrations in combination with hydrograph separation and isotopic fingerprinting methods to characterize and quantify nitrate source contribution. The hydrological analysis revealed a clear dominance of base flow during both campaigns. However, the absolute amounts of discharge differed considerably from one another (outlet: 1.42ms in 2014, 0.43ms in 2015). Nitrate concentrations are generally low in the pristine headwaters (<3mgL) and increase downstream (15 to 16mgL) due to the contribution of agricultural and wastewater sources. While the agricultural contribution did not vary in terms of nitrate concentration and isotopic signature between the years, the wastewater contribution strongly increased with decreasing discharge. Wastewater-borne nitrate load in the entire catchment ranged between 19% (2014) and 39% (2015). Long-term monitoring of nitrate concentration and isotopic composition in two sub-catchment exhibits a good agreement with findings from spatially monitoring. In both datasets, isotopic composition indicates that denitrification plays only a minor role. The spatially highly resolved monitoring approach helped to pinpoint hot spots of nitrate inputs into the stream while the long-term information allowed to place results into the context of intra-annual variability.
硝酸盐浓度升高是地表水供水和生态完整性的一个威胁。在集水区出口处通过标准监测方案获得的硝酸盐通量强烈综合了空间和时间变化的过程，如动员和转化。因此，在有效的河流管理和措施优先排序方面，推断主要硝酸盐源通常是有问题和具有挑战性的。在这里，我们将在一个中尺度集水区进行的硝酸盐浓度和通量的空间高度解析评估与两个代表性地点的四年监测数据相结合。该集水区的特点是从原始源头到低地子流域具有强烈的土地利用梯度，这些子流域具有强烈的农业土地利用和废水来源。我们使用硝酸盐浓度结合水文分离和同位素指纹分析方法来表征和量化硝酸盐源的贡献。水文分析显示，在两次活动中，基流都明显占主导地位。然而，排水量的绝对值彼此差异很大（出水口：2014 年为 1.42ms，2015 年为 0.43ms）。由于农业和废水来源的贡献，原始源头的硝酸盐浓度一般较低（<3mgL），下游浓度增加（15 至 16mgL）。虽然农业贡献在年度之间的硝酸盐浓度和同位素特征方面没有变化，但废水贡献随着排水量的减少而强烈增加。整个集水区的废水中的硝酸盐负荷在 19%（2014 年）至 39%（2015 年）之间。两个子流域的硝酸盐浓度和同位素组成的长期监测与空间监测的结果非常吻合。在两个数据集，同位素组成表明反硝化作用仅起着次要作用。空间高度解析的监测方法有助于确定硝酸盐输入到溪流的热点，而长期信息则将结果置于年内变化的背景下。
