Chemical mass budget of two dredged lakes embedded in shallow Quaternary aquifers in southern Austria

Determination of the chemical mass budget is of major importance in evaluat ing the resulting hydrogeochemical and biochemical changes due to the forma tion of dredged lakes. This is not only very useful to enable precautionary measures to be taken to protect shallow aquifers from pollution, it also p rovides a basis fur decisions regarding the location of future projects for sand and gravel mining in shallow groundwater fields. The Schwarzl and the Weizelsdorfer Lakes are typical through-flow systems, whose water and solu te budgets are primarily controlled by transient groundwater. Through-flow rates, estimated with the help of stable isotopes, deuterium and oxygen-18, together with the hydrochemical data from 1991 allowed the quantification of the chemical mass budget of the major chemical species within 20-30% of uncertainty. The hydrochemical composition of studied lakes is considerably altered by dilution and in-lake source/sink processes. The dilution effect (due to precipitation falling direct on the surface of the lakes) was appr oximated at. 26 and 14% of the inputs to the Schwarzl Lake and the Weizelsd orfer Lake respectively, using chloride concentration data. The change in c oncentration of the major dissolved solutes resulting from in-lake source/s ink function (net annual budget - dilution effect) was variable. The genera l change pattern was a decrease of Na+, Ca2+, HCO3-, NO3- and SiO2 componen ts and an increase in Mg2+, SO42- and K+ concentrations in both lakes. SIca lcite and SIdolomite together with P-CO2 and the evaporative signature on d elta H-2 and delta O-18 prove to be very useful tools in determining the im pact of dredged lakes on the mixing zones, i.e. in identifying the wells co ntaining lake water and to quantify the mixing ratio. From chemical data, o nly the wells that contain a large fraction of lake water (> 60-70%) could be identified. This fact emphasizes the advantage of the stable isotopes as conservative tracers over chemical parameters in determining the impact of evaporated lakes on the down-gradient groundwater fields.