Evaluation of mean residence time in subsurface waters using oxygen-18 fluctuations during drought conditions in the mid-Appalachians

Seasonal variations of the stable isotope, oxygen-18 (O), were used to estimate mean residence times of water in two Valley and Ridge Province watersheds in central Pennsylvania. Mean residence time was estimated by applying O input precipitation signatures to response function models that describe flow conditions in the subsurface system. Precipitation O is usually seasonally periodic; however, during this study period (3-98 to 6-99), unusual weather conditions and severe drought caused an abnormal seasonal signature in precipitation O. The atypical input precipitation amounts and O signatures required that adjustments be made using recharge factors and an extended past input function to represent the varying fraction of input water that contributed to turnover within the watershed during the study period. Oxygen18 variations were investigated in output waters from tension and zero-tension soil water lysimeters, shallow wells, and streamflow. Soil water O signatures were more responsive to precipitation O variations than streamflow O signatures. Theoretical models based on exponential piston-flow and dispersion flow processes fit data better than did other ground water age distribution models (i.e., pure piston flow or pure exponential models). Analysis suggested that during drought conditions, larger portions of older water dominated streamflow O composition, and that the current year’s precipitation O signature became more attenuated in the subsurface flow system. Mean residence times obtained for streamflow at each site were 9.5 and 4.8 months for a 14 and 100 ha watershed, respectively, and soil water mean residence time at 100 cm depth was on the order of a couple months; indicating a relatively rapid response of shallow ground water to precipitation.