No abstract

The heterogeneous nature of both groundwater discharge to a lake (inflow) and nitrate concentrations in groundwater can lead to significant errors in calculations of nutrient loading. Therefore, an integrated approach, combining groundwater flow and transport modelling with observed nitrate and ammonium groundwater concentrations, was used to estimate nitrate loading from a catchment via groundwater to an oligotrophic flow-through lake (Lake Hampen, Denmark). The transport model was calibrated against three vertical nitrate profiles from multi-level wells and 17 shallow wells bordering a crop field near the lake. Nitrate concentrations in groundwater discharging to the lake from the crop field were on average 70 times higher than in groundwater from forested areas. The crop field was responsible for 96% of the total nitrate loading (16.2 t NO3 /year) to the lake even though the field only covered 4.5% of the catchment area. Consequently, a small change in land use in the catchment will have a large effect on the lake nutrient balance and possible lake restoration. The study is the first known attempt to estimate the decrease of nitrate loading via groundwater to a seepage lake when an identified catchment source (a crop field) is removed.

An ecohydrological study was carried out at an oligotrophic seepage lake, Lake Hampen, Denmark, to determine the seepage in and out of the lake and determine the mass budgets for nutrients. The lake is primarily surrounded by forest, although there are agricultural fields bordering a small portion of the lake perimeter. The water and mass balances indicate that there is a groundwater input of 1643 × 103 m3 year−1 into the lake (∼70% of total input) and a groundwater output of 1997 × 103 m3 year−1. The groundwater input carries with it 2253 kg N year−1 (∼67% of total input) and 40 kg P year−1 (∼85% of total input). The majority of the nitrogen is leached from the agricultural fields bordering the lake. Concentrations as high as 1750 µM nitrate were measured in the rhizosphere of the littoral zone at this location. It is estimated that the macrophytes are able to take up 1695 kg N year−1 (∼50% of the input). The phosphorus uptake by plants was 310% of the input, indicating that plants are highly dependent on accumulated and remineralized phosphorus in the lake. The nutrients lost through recharge was much lower than input through groundwater discharge entailing a high retention of nutrients in the lake (92–96%). A significant part of the retained nitrogen may be lost through denitrification. Although the plant uptake may have slowed the eutrophication of the lake, the excess nutrients may over time give rise to eutrophication and a shift towards pelagic production. Copyright © 2011 John Wiley & Sons, Ltd.