A MULTI-SCALE INVESTIGATION OF GROUNDWATER-CONTROLLED PHOSPHOROUS LOADING TO A EUTROPHIC LAKE: A FIELD STUDY USING HYDROCHEMICAL TRACERS AND NUMERICAL ANALYSIS

Lake Vaeng in Denmark is a eutrophic lake with high total phosphorous (TP) levels. Lake restoration has failed as the lake rebounded to its previous poor ecological state. Focus is now on understanding how the lake connects with groundwater and the lake catchment and in particular investigating how external processes and P sources in the catchment may control P loading to the lake.

The study combines multi-scale hydrogeological (including UAVs, DTS), geophysical (off-shore and on-land), and hydrochemical field studies with regional (Modflow) and local (Feflow) groundwater flow and flow path modeling.

The dominant source of water to the lake is groundwater, derived mainly from the west as the lake connects to a large catchment. Discharge to the lake happens in several flow convergence zones on both sides of the lake, i.e., through seepage faces on both sides and through the lake bottom, but here mainly in a very narrow zone only a few meters wide, a so-called High-Discharge Zone, located on the eastern side of the lake. A major part of the western inflow thus flows underneath the lake and discharges on the eastern side with fluxes as high as 10-100 cm/d.

Shallow oxic and deeper anoxic groundwater mix near the flow convergence zones on the western side. Dissolved inorganic P (DIP) concentrations are much higher in wells near the lake shores than in the catchment suggesting that the P source is near the lake. Here increased DIP and Fe2+ concentrations show that P may be released by reductive dissolution of P-containing iron oxides by sedimentary organic matter, although part of the P may be released from organic matter itself. The P source is therefore likely of geologic origin from near-shore P sedimentary deposits in terraces of the lake (old lake bottom).The mobilized DIP will be transported beneath the lake in anoxic groundwater due to the underflow. Chemical profiles in the eastern lake bed show anoxic conditions and almost uniform concentration profiles (including DIP). The effect is a loading of DIP into the lake.

In summary, the special groundwater-lake interactions, the near-shore P reservoir, and geochemical conditions may continue to pollute the lake. Geology and groundwater is so to speak bleeding P to the lake and the lake will be natural eutrophic.