Spatial patterns of water quality impairments from point source nutrient loads in Germany's largest national River Basin (Weser River)

We employed the well-established Horton-Strahler, hierarchical, stream-order (omega) scheme to investigate scaling of nutrient loads (P and N) from similar to 845 wastewater treatment plants (WWTPs) distributed along the river network in urbanized Weser River, the largest national basin in Germany (similar to 46K km(2); similar to 8.4 million population). We estimated hydrologic and water quality impacts at the reach- and basin-scales, at two steady river discharge conditions (median flow, Q(R50); low-flow, Q(R90)). Of the five WWTPs class-sizes (1 <= k <= 5), similar to 68% discharge to small low-order streams (omega < 3). We found large variations in capacity to dilute WWTP nutrient loads because of variability in (1) treated wastewater discharge (Q(U)) within and among different class-sizes, and (2) river discharge (Q(R)) within low-order streams (omega < 3) resulting from differences in drainage areas. For Q(R50), reach-scalewater quality impairment assessed by nutrient concentration was likely at 136 (similar to 16%) locations for P and 15 locations (similar to 2%) for N. About 90% of these locationswere lower-order streams (omega < 3). At Q(R50) and only with dilution, basin-scale cumulative nutrient loads from multiple upstream WWTPs increase impaired locations to 266 (similar to 32% of total) for P. Considering in-stream uptake decreased P-impaired streams to 225 (similar to 27%), suggesting the dominant role of dilution in the Weser River basin. Role of in-stream uptake diminished along the flow paths, while dilution in larger streams (4 <= omega <= 7) minimizes the impact of WWTP loads. Under Q(R90) conditions [(Q(R50)/Q(R90)) similar to 2.5], water quality impaired locations will likely double for the basin-scale analyses. Long-term water quality data suggested that diffuse sources are the primary contributors for water quality impairments in large streams. Our data-modeling synthesis approach is transferable to other urbanized river basins and extends understanding of point source impacts on water quality across spatial scales. (C) 2019 The Authors. Published by Elsevier B.V.