Symbol picture of the River Spree in Berlin. | Photo: Morgengry on pixabay
Electrical conductivity measurement is based on the fact that dissolved ions in a solution have electrical charges and increase the flow of electric current. The method is used in water research, for example, to determine the natural salinity of water or the salinity caused by human discharges. The team of researchers tested this method in a field study on the River Spree between Lake Müggelsee and the mouth of River Erpe to investigate the dynamics of the reverse flows and the effect on the chemical water quality.
Water from the River Erpe flows into the upstream sections of the River Spree
The results confirm that during the summer months, water in the studied section of the River Spree flows in reverse direction, allowing water from the River Erpe to reach the upper reaches of the River Spree.
The issue is that 60 to 80 percent of the River Erpe consists of treated wastewater which contains trace organic contaminants. These are human-made chemical compounds in very low concentrations, such as those found in pharmaceuticals, detergents, pesticides, paints and varnishes. They can potentially have adverse effects on ecosystems or human health. Many of these substances are persistent in the environment and cannot or only partly be removed in conventional wastewater treatment plants.
The fact that the River Spree stagnates or even flows in reverse direction in this section, especially during the hot summer months when there is little rainfall, is nothing new. And the reasons are well known: low discharge of River Spree from the catchment, bank filtration for drinking water production and high evaporation at Lake Müggelsee and permanent inflow of wastewater treatment plant effluents. With phase-out of lignite mining in the Lusatian district, which means less drainage water reaching the Spree, the annual number of days with reverse flow in River Spree will likely increase further: The Berlin Senate's water action plan predicts an increase from 64 in 2019 to 117 days a year by 2050.
"Our case study from River Spree shows that the interactions between water quantity and water quality during reverse flows are complex and highly variable over time. Therefore, it is a first success that we have now a relatively simple method that can capture changes in high temporal resolution", said IGB doctoral candidate Christoph Reith, first author of the study.
