Tracing calcite precipitation with specific conductance in a hard water alpine lake (Lake Bourget)

The authors discuss the reliability of a new method used to depict precisely the authigenic calcite precipitation process in the water column of a mesotrophic alpine lake (Lake Bourget, Savoie, France). Complete chemical analysis of dissolved major elements shows that, in the epilimnion, calcium concentration and alkalinity are the only chemical species presenting large variations. The behaviour of these two parameters is only related to calcite precipitation. The method is based on in situ specific conductance measurements (C₂₅). Experimental values were compared with theoretical calculations. The results demonstrate clearly that dissolved calcium concentration can be directly related to measured specific conductance. Hence, a mathematical relationship was developed to compute calcium concentration profiles in the water column with a centimetre spatial resolution through the whole water column. Finally, authigenic calcite production can be observed and quantified on a time scale covering two years of experimental data. Calcite production is estimated to 4.8 mol m^-2 yr^-1. Extrapolation to the whole lake surface leads to 16,000±2000 tons yr^-1 as CaCO₃. Specific conductance is an easy-to-measure in situ parameter which could be used to trace authigenic calcite production in other hard water alpine lakes.