Focused groundwater discharge of phosphorus to a eutrophic seepage lake (Lake Væng, Denmark): implications for lake ecological state and restoration Jacob Kidmose & Bertel Nilsson & Peter Engesgaard & Mette Frandsen & Sachin Karan & Frank Landkildehus & Martin Søndergaard & Erik Jeppesen Abstract A study on Lake Væng in Denmark demon- strates a high potential for loading of phosphorous via groundwater to seepage lakes. Groundwater discharges are displayed as an important source of phosphorous to a lake due to: (1) high concentrations in the aquifer just below the lake, and (2) the main flow paths through the aquifer– lakebed interface either being overland flow through a seepage face, or focused in zones with very high discharge rates. In-lake springs have measured discharge of up to 7.45 m 3 per m 2 of lakebed per day. These findings were based on seepage meter measurements at 18 locations, stable isotope (δ 18 O) analyses, temperature profiles and mapping of ice cover distribution. Groundwater–lake interaction was modelled with a 2D conceptual flow model (MODFLOW) with hydrogeology interpreted from catchment multi electrode profiling, on-lake ground- penetrating radar, well logging and borehole data. Discharge was found to be much focused and opposite to expected increase away from the shoreline. The average total phosphorus concentration in discharging groundwa- ter sampled just beneath the lakebed was 0.162 mg TP/l and thereby well over freshwater ecological thresholds (0.043–0.612, median = 0.117 mg TP/l). The study illustrates a direct link between groundwater and lake chemistry. Keywords Groundwater/surface-water relations . Phosphorous . Numerical modelling . Stable isotopes . Denmark Introduction Groundwater–lake interactions may exert an important control on lake ecology in multiple ways (Shaw et al. 1990). Groundwater can be an important constituent of the lake water balance, and the residence time in a lake will be a function of the inflow (hereafter referred to as discharge) or/and outflow (hereafter referred to as recharge) of groundwater (Winter 1999). This groundwater discharge/ recharge may ultimately determine in-lake biogeochemical and ecological processes. Numerous studies have shown that groundwater may also contribute importantly to the nutrient budget of lakes (e.g. Winter 1978; Frape and Patterson 1981; Brock et al. 1982; Belanger et al. 1985; Shaw et al. 1990; Robertson et al. 2005; Özen et al. 2010) and affect macrophyte growth (Lillie and Barko 1990; Sebestyen and Schneider 2004; Frandsen et al. 2012) and benthic algal biomass (Hagerthey and Kerfoot 1998). Several studies (e.g. Burkart et al. 2004; Holman et al. 2008, 2010; Tomer et al. 2010) have shown that groundwater bodies may have phosphorous (P) concen- trations high enough to create eutrophic conditions in surface waters. Although a challenge, quantification of the distribution and magnitude of groundwater–lake exchange rates at whole-lake scale is important for estimating, for example, the residence time of water in a lake. During the last 40 years, several techniques have been developed using, for instance, seepage meters (McBride and Pfannkuch 1975; Lee 1977; Cherkauer and Nader 1989; Belanger and Montgomery 1992; Rosenberry et al. 2000; Rosenberry and LaBaugh 2008; Kidmose et al. 2011), environmental Received: 22 April 2013 / Accepted: 20 August 2013 Published online: 17 September 2013 * Springer-Verlag Berlin Heidelberg 2013 J. Kidmose : B. Nilsson : M. Frandsen Geological Survey of Denmark and Greenland, Øster Voldgade 10, 1350 Copenhagen K, Denmark J. Kidmose ()) : P. Engesgaard : S. Karan Department of Geography and Geology, University of Copenhagen, Øster Voldgade 10, 1350 Copenhagen K, Denmark e-mail: jbki@geus.dk M. Frandsen The Freshwater Biological Laboratory, Department of Biology, University of Copenhagen, Helsingørsgade 51, 3400 Hillerød, Denmark F. Landkildehus : M. Søndergaard : E. Jeppesen Department of Bioscience, Aarhus University, Vejlsøvej 25, 8600 Silkeborg, Denmark Hydrogeology Journal (2013) 21: 1787–1802 DOI 10.1007/s10040-013-1043-7