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

Abstract

Determination of the chemical mass budget is of major importance in evaluating the resulting hydrogeochemical and biochemical changes due to the formation of dredged lakes. This is not only very useful to enable precautionary measures to be taken to protect shallow aquifers from pollution, it also provides a basis for 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 solute 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 approximated at 26 and 14% of the inputs to the Schwarzl Lake and the Weizelsdorfer Lake respectively, using chloride concentration data. The change in concentration of the major dissolved solutes resulting from in-lake source/sink function (net annual budget – dilution effect) was variable. The general change pattern was a decrease of Na+, Ca2+, HCO3 –, NO3 – and SiO2 components and an increase in Mg2+, SO4 2– and K+ concentrations in both lakes. SIcalcite and SIdolomite together with PCO2 and the evaporative signature on δ2H and δ18O prove to be very useful tools in determining the impact of dredged lakes on the mixing zones, i.e. in identifying the wells containing lake water and to quantify the mixing ratio. From chemical data, only 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.