Electrical Conductivity as a Tracer for Seasonal Reverse Flow and Transport of Trace Organic Contaminants in River Spree

Climate change, population growth, urbanisation and water pollution will exacerbate the closely linked challenges of water quantity and water quality. The River Spree in Berlin, Germany, experiences recurrent low flow conditions in summer with seasonal flow reversals in certain sections of the river. This reverse flow leads to the transport of treated wastewater to upstream sections of River Spree and possibly to the introduction of treated wastewater into Lake Müggelsee, which is located upstream of the city centre and important for drinking water production via bank filtration in Berlin. A better understanding of the flow and contaminant dynamics in River Spree is required, but field data on the reverse flow are still lacking. In 2022 and 2023, we collected surface water samples to quantify major ions and trace organic contaminants. Over a period of nine months in 2023, we also measured the specific electrical conductivity at six locations with a temporal resolution of five minutes. During summer, the specific electrical conductivity increased at the sampling locations in River Spree upstream of the mouth of the wastewater‐impaired River Erpe. The specific electrical conductivity proved to be an indicative parameter for the seasonal dynamics of reverse flow periods. During reverse flow, we observed increased concentrations of wastewater‐derived trace organic contaminants, many of which correlated positively with the specific electrical conductivity. Strong differences in the reverse flow intensity between 2022 and 2023 indicate that both precipitation and discharge of the River Spree upstream of Lake Müggelsee have a strong influence on the reverse flow. This study demonstrates the applicability of easy‐to‐measure specific electrical conductivity as a proxy for hydrological conditions and chemical water quality.