Groundwater – the disregarded component in lake nutrient budgets

Eutrophication is one of the most important threats to lakes in temperate climatic zones. It is necessary to determine the relevance of different nutrient sources to conduct effective management measures, to understand in-lake processes and to model future scenarios. A prerequisite for nutrient balances are water balances. Surface inflows from streams, rivers and ditches can be precisely quantified and based on local weather data precipitation and evaporation can be calculated. Quantifications of groundwater infiltration and exfiltration are more difficult. Sometimes they are determined as residual in the water balance equation but that method is error prone and only net groundwater flow can be determined. Alternative approaches used by us are seepage meters, temperature depth profiles, ion depth profiles, stable isotopes, piezometer nests, distributed temperature sensing or groundwater flow models. For nutrient balances some additional input paths have to be taken into account, for example, dry deposition, waterfowl, swimmer and anglers. Furthermore, concentration fluctuations of the different inflows have to be considered. The quantification of nutrient imports by groundwater is quite complex and therefore often disregarded in nutrient balances although being an important nutrient source in several lakes. Our studies aim at determining general exfiltration patterns and identifying the relevant hydrological and biogeochemical mechanisms at the interface aquifer-lake. For that purpose we conduct several case studies at Lake Arendsee, Lake Schulzensee, Small Lake Gollin, Lake Stechlin, Lake Scharmützel, Lake Fürstensee, Lake Concordia, and Mining Lake 77 in the Lusatian mining area.