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

Soohyun Yang; Olaf Büttner; Rohini Kumar; Christoph Jäger; James W Jawitz; P S C Rao; Dietrich Borchardt

doi:10.1016/j.scitotenv.2019.134145

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

Sci Total Environ. 2019 Dec 20:697:134145. doi: 10.1016/j.scitotenv.2019.134145. Epub 2019 Aug 30.

Authors

Soohyun Yang¹, Olaf Büttner², Rohini Kumar³, Christoph Jäger⁴, James W Jawitz⁵, P S C Rao⁶, Dietrich Borchardt²

Affiliations

¹ Lyles School of Civil Engineering, Purdue University, West Lafayette, IN 47907, USA.
² Department Aquatic Ecosystems Analysis and Management, Helmholtz Centre for Environmental Research - UFZ, Magdeburg, Germany.
³ Department Computational Hydrosystems, Helmholtz Centre for Environmental Research - UFZ, Leipzig, Germany.
⁴ Department Aquatic Ecosystems Analysis and Management, Helmholtz Centre for Environmental Research - UFZ, Magdeburg, Germany; Centre for Research and Development, Rosenheim Technical University of Applied Sciences, Rosenheim, Germany.
⁵ Soil and Water Sciences Department, University of Florida, Gainesville, FL 32611, USA.
⁶ Lyles School of Civil Engineering, Purdue University, West Lafayette, IN 47907, USA; Agronomy Department, Purdue University, West Lafayette, IN 47907, USA. Electronic address: SureshRao@purdue.edu.

PMID: 32380617
DOI: 10.1016/j.scitotenv.2019.134145

Abstract

We employed the well-established Horton-Strahler, hierarchical, stream-order (ω) scheme to investigate scaling of nutrient loads (P and N) from ~845 wastewater treatment plants (WWTPs) distributed along the river network in urbanized Weser River, the largest national basin in Germany (~46K km²; ~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), ~68% discharge to small low-order streams (ω < 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 (ω < 3) resulting from differences in drainage areas. For Q_R50, reach-scale water quality impairment assessed by nutrient concentration was likely at 136 (~16%) locations for P and 15 locations (~2%) for N. About 90% of these locations were lower-order streams (ω < 3). At Q_R50 and only with dilution, basin-scale cumulative nutrient loads from multiple upstream WWTPs increase impaired locations to 266 (~32% of total) for P. Considering in-stream uptake decreased P-impaired streams to 225 (~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 ≤ ω ≤ 7) minimizes the impact of WWTP loads. Under Q_R90 conditions [(Q_R50/Q_R90) ~ 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.

Keywords: Dilution; Eutrophication; Hydrological alteration; In-stream nutrient uptake; Point source nutrient loads.