Estimation of groundwater exchange in alpine lakes using non-steady mass-balance methods

The role of groundwater in regulating the hydrology and chemistry of cirque lakes in the Cabinet Mountains Wilderness, Montana was investigated to assess the potential for adverse impacts from groundwater drainage stresses produced by proposed underground mining. Non-steady water budget and chemical mass-balances for Ca2+, K+, Mg2+, Na+, Cl−, δ2H and δ18O in conjunction with the Penman combination method to estimate evaporation were used to estimate groundwater inflow over 2–4 week time intervals during the ice-free seasons of 1999, 2000, 2001 and 2002. Extreme seasonal fluctuations in solute and isotope content require the use of a non-steady approach. The degree of groundwater inflow differs widely between lakes ranging from negligible to direct communication with the local groundwater system. For lakes with groundwater inflow, a major portion of the solute load is contributed by groundwater compared to other inputs. The solute mass-balance method proved robust, yielding consistent groundwater inflow values for all lakes while success with stable isotopes was more limited. Mining induced disruptions in groundwater exchange could compromise the hydrologic and chemical balance of lakes with important subsurface water balance components.