Spatiotemporal variations of surface water quality in a medium-sized river catchment (Northwestern Germany) with agricultural and urban land use over a five-year period with extremely dry summers

Buss J, Achten C

Abstract

Medium-sized rivers, which are used for intensive agriculture and urban infrastructure, are subject to manifoldhydrochemical stressors. Identifying and monitoring these stressors is important for river basin management anda functioning ecosystem. To understand the spatiotemporal variation of surface water quality in a highly modifiedlowland river, the Münstersche Aa River (Northwestern Germany) with 62% of land used for agriculture and 26%urban/residential area, was exemplarily studied. A total of 519 samples were collected using two automatedhigh-frequency samplers and five catchment-wide sampling campaigns. They covered the five-year period2015–2020 and included two extremely dry summers. TheMünstersche Aa catchment is dominated by low permeablestrata resulting in surface water runoff (BaseflowIndex: 0.41)which leads to a high amplitude of discharge variation(mean discharge: 0.7 m3/s) with high flow conditions in winter/spring, and low discharge during summer/fall. In wintertime, maximum nitrate concentrations (up to 73 mg NO3/L) and loads (up to 1300 t NO3/a; up to98% in winter) correlate with high-flowconditions. δ18Oandδ15Nisotopic analysis indicated manure fromfarmlandas the major source of nitrate whereas the impact of municipal wastewater treatment plants was neglectable. Increasednitrate concentrations are linked to the higher proportion of farmland in the upper catchment (77%) comparedwith the lower catchment (47%). In summertime, at extremely low flow conditions, surface water consistedof up to 100% of treated wastewater, resulting in the highestmeasured chloride, sodium and potassium concentrations.The river is impacted by strongly seasonal and different stressors, which can be expected to intensifywith ongoingclimate change. Results from this study may help to adapt monitoring schemes for the Münstersche Aa butalso for other lowland streams with comparable land-use targeting the goals of theWater Framework Directive.