Groundwater-borne phosphorus import into Lake Arendsee and its changes over the last 15 years

Lake Arendsee, a 50 m deep, monomictic lake in north-eastern Germany, has been suffering from anthropogenic eutrophication for more than 40 years. Lake eutrophication is generally associated with phosphorus (P) loads from surface inflows, direct sewage discharges, atmospheric deposition, surface runoff, bathers, waterfowl or other highly visible nutrient sources. In the case of Lake Arendsee previous research has shown that more than 50 % of the P load is due to excessive P inputs from groundwater. The aim of the study is to assess the changes in groundwater P concentrations over the last 15 years and to understand how the P load has changed between two measurement campaigns (2012 & 2022). P in groundwater was determined using permanent wells, temporary piezometers and private domestic wells, the latter sampled as part of Citizen Science campaigns. Lacustrine Groundwater Discharge (LGD) rates for 2022 were determined using temperature (T) lances, KSAT tests, Darcy calculations and hydraulic gradients, allowing the total annual P load to Lake Arendsee to be calculated. Despite a lower number of temporary piezometers installed along the lake shore in 2022 compared to 2012, the results showed similar spatial patterns, indicating the reliability of the method. High P concentrations were particularly common in the urban area. Both campaigns also showed similar patterns, despite using different domestic wells, indicating the reliability of the method. Unfortunately, two different methods for calculating LGD rates for 2012 (temperature lances) and 2022 (KSAT tests) were used. The comparability of the methods is limited but revealed that most groundwater discharge took place along the shore of the City of Arendsee. It is unlikely that the spatial LGD pattern changed within 10 years as there is no reason for a change of hydraulic conductivities. Therefore, the calculation of the annual P loads in 2022 was based on the patterns of exfiltration rates determined using T lances in 2012 but using hydraulic gradients of 2022 to adapt total LGD rates on the situation in 2022. The study shows that P in groundwater in the catchment has remained largely unchanged, with the exception of a few monitoring sites. Due to a decrease in both hydraulic gradients (consecutive dry years) and P in some near-shore temporary piezometers, P loads entering the lake in 2022 are lower than in 2012. Further studies are needed to determine whether the reduced P loads in 2022 will increase to levels similar to those observed in 2012 due to the increase in groundwater and lake water levels in 2024, or whether improvements (e.g. replacement of sewers) in the subsurface catchment of Lake Arendsee are the reason for the reduced P loads. Groundwater is still the major source of the high P concentrations of 180 μg/L in the lake.