This PhD study presents research results from a eutrophic seepage lake in Denmark (Lake Væng). An integrated approach using results from field surveys and regional to local scale flow modelling showed that aquifer geometry, presence of an impermeable gyttja layer at the western part of the lake bottom and heterogeneities in the hydraulic properties of the lakebed have a significant influence on the groundwater flow patterns and discharge dynamics. Part of the groundwater flowing from the west and south is forced to discharge at wetlands/seepage faces at the western and southern lake shores, while deeper groundwater by-passes the lake by flowing underneath the gyttja sediments and discharges at the eastern sandy shore, where groundwater springs and high discharge zones (HDZ) are observed. Hydrogeochemical tracers were successfully used for estimating the general discharge distribution and tracking groundwater flow paths and, thus, to determine the source of the water. These observations were confirmed and explained by flow models. The results of the 2D and 3D flow modelling showed that groundwater contribution is 75% of the total water input into the lake, out of which 35% discharges at a 25-m-wide sandy lakebed, while surface runoff from the western and southern seepage faces delivers approximately 65%. The simulated seepage rates are an acceptable approximation of the average fluxes measured with seepage meters on the eastern shore. Seepage measurements and the observation of thawing patterns from 2010 to 2013 indicate that the distribution of groundwater discharge at the eastern side of the lake changes over time. This temporal variability in the areas of discharge was successfully monitored with distributed temperature sensing (DTS).
Dissolved inorganic phosphorus (DIP) concentration in groundwater sampled from the Pleistocene aquifer interacting with Lake Væng increases from 9 μg/L in the recharge zone to 127 μg/L near the lake. The correlation between DIP and ferrous iron (Fe2+) concentrations suggests that the majority of DIP entering the lake with discharging groundwater is mobilized in the sediments of the old lake/stream bottom due to reductive dissolution of iron hydroxides by organic matter. The process is triggered by the discharge of anoxic groundwater from the deeper parts of the aquifer to the near shore environment. High groundwater seepage rates do not leave enough time for diffusion of oxygen into the aquifer and prevent the re-precipitation of iron hydroxides and DIP immobilization. From the ecological perspective, the continuous, external loading of geogenic DIP in high concentrations (on average 60 µg/L) results in natural lake eutrophication and explains the failure of the two previous lake restoration attempts. The low DIP release ratio in the aquifer (0.14 µg/L/d) indicates that the loading of phosphorus (1.6 g/yr per m2 of the lake area) into the lake with discharging groundwater can maintain the lake eutrophication for over a period of thousands of years.