Stable isotope hydrology of a polymictic lake: Capturing transience of groundwater interactions

Although there have been many studies of groundwater inflow to small lakes, no systematic attention has been paid to the role of the time interval in the reliability of transient flow analysis. We addressed this issue in a two-year study of the isotope hydrology and water budget of a small lake in eastern Washington State (USA) that has been subject to limited management over several decades. The weighted local meteoric water line is δ²H = 7.14 δ¹⁸O – 5.22, reflecting the impact of convective recycling in this semi-arid region of inland northwestern North America. Groundwater inflow to the lake was quantified over two years using a short-interval isotopic transient mass balance approach. Calculated inflow was less than a fifth of the lake’s total water budget. Unrealistic temporal fluctuation of the calculated inflow was apparently correlated with fluctuation of the observed isotopic ratio of lake water (${\delta }_L$). We obtained realistic lake fluctuation and groundwater–lake exchange estimates by experimenting with the time interval of the isotope mass balance. It is crucial to acknowledge that each lake possesses unique characteristics that influence hydrologic and isotopic variations. Therefore, the optimal time interval for sampling and calculating mass balance may vary among different small lakes. Our findings have implications for long-term studies with frequent interval sampling.