The water balance of a small lake using stable isotopes and tritium

Abstract

A small maar lake, known as the Blue Lake, set in a karstic region of southeastern Australia, provides the municipal water supply for a population of ∼ 20,000. The lake has a volume of 3.6·10 7m 3, of which 10–15% is pumped from it each year. The lake is recharged almost entirely from groundwater and the main objective of this study was to estimate the total groundwater inflow and outflow rates. Estimation of groundwater throughflow in a lake is difficult to assess using classical hydrological techniques and an alternative method involving measurement of the concentrations of the environmental isotopes 3H, 18O, 2H and 14C in the lake water and in the recharging groundwater was used to establish the lake-water balance. The water-balance calculations indicated a total groundwater inflow to Blue Lake of between 5.0·10 6 and 6.5·10 6m 3yr. −1, corresponding to a residence time of water in the lake of ∼ 6 yr. It was not possible to derive the relative proportions of inflow to the lake from the two possible source aquifers, using these isotopes, because their concentrations did not show a sufficiently large contrast to distinguish the two water sources.