Integrating multiple methods for simulating lake catchment water balance in northern Europe

The potential expansion of an oil shale quarry near environmentally protected Lake Uljaste in northern Estonia has ignited a public discourse on its potential impact on the lake's water level. This study, conducted between 2021 and 2023, aimed to elucidate the hydrogeological dynamics of the area, understand the functioning of groundwater flow systems, and predict the consequences of quarry expansion on the lake's water balance over the next 15 years.

Lake Uljaste is a small (area of 0,64 km²) closed-basin lake with an average depth of 3,4 m. As the lake does not have any channelized surface water inflow or outflow, most of the water budget components have to be estimated indirectly. For this purpose, two water balance models were developed, incorporating hydrological/meteorological and stable isotope data, respectively, serving as inputs for a transient groundwater model.

In addition, several different methods were applied including geophysical mapping, coring surveys, monitoring well construction, electrometry, and groundwater/surface water monitoring to characterize the local geological conditions and groundwater-surface water interaction. Seismo-acoustic profiling provided insights into the depth of lake bottom sediments, while electrical conductivity and isotopic composition of surface water and groundwater aided in conceptualizing the local groundwater flow system.

The study revealed that, the lake water balance is mainly controlled by precipitation and evaporation and is very sensitive to changes in climatic conditions. Despite the fact, that the amount of direct groundwater inflow is small, the groundwater level beneath the lake significantly influences its water level. Predictions from the groundwater model indicate a notable lake level decline due to the potential water abstraction of the planned quarry. Thus, without preventive measures against groundwater level decline beneath the lake, the expansion of the quarry to its planned position would result in a significant decline in lake water level leading probably to severe environmental problems.

Several uncertainties remain regarding the conceptual understanding of the lake water balance and catchment hydrology which need to be addressed to improve the existing models. The most important of these concerns need for a more precise determination of hydrodynamic properties of the lake bottom sediments, which are needed to estimate the subsurface outflow from the lake. In addition, the installation of a weather station for characterizing the microclimatic conditions near the lake and refining of the stable isotope mass-balance model through improved characterization of vertical and horizontal mixing in the lake, are needed for more accurate water balance calculations. These refinements are crucial for assessing the extent of lake water seepage into the groundwater, a vital parameter for groundwater modeling and an important pre-requisite for sustainable groundwater resource management in the area.