Groundwater-borne phosphorus import to eutrophic Lake Arendsee (Germany)

Until recently lake eutrophication was generally associated with phosphorus loads of surface inflows, surface runoff, direct sewage discharge, atmospheric deposition, bathers, waterfowls or other well visible nutrient sources. On the contrary, phosphorus (P) imports by groundwater were systematically disregarded because P was considered immobile in aquifers. This applies also to Lake Arendsee where we started our research about the relevance of lacustrine groundwater discharge (LGD) a decade ago. Lake Arendsee is a 50 m deep, monomictic lake in north-eastern Germany; the lake suffers intense eutrophication problems. We quantified P loads of groundwater to Lake Arendsee using volume fluxes of LGD and near-shore measurements of groundwater P concentrations at a high spatial resolution. Results have shown that LGD accounts for about 50% of the overall external P load, thus fueling the eutrophication of the lake. We assume that both, ancient sewage pits and leaking sewer pipes are major sources of the extremely high P concentrations in the aquifer (above 4000 µg/L). Now, after the renewal of many sewer pipes we present a long-term data set (last decade) of P concentrations close to the groundwater-lake interface. Groundwater is still the major source of the high P concentrations of 180 µg/L in the lake.