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 ( 18O), were used to estimate mean residence times (MRTs) of water in two Valley and Ridge Province watersheds in central Pennsylvania. MRT was estimated by applying 18O input precipitation signatures to response function models that describe flow conditions in the subsurface system. Precipitation 18O is usually seasonally periodic; however, during this study period (March 1998 to June 1999), unusual weather conditions and severe drought caused an abnormal seasonal signature in precipitation 18O. The atypical input precipitation amounts and 18O 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. Oxygen-18 variations were investigated in output waters from tension and zero-tension soil water lysimeters, shallow wells, and streamflow. Soil water 18O signatures were more responsive to precipitation 18O variations than streamflow 18O signatures. Theoretical models based on exponential piston-flow and dispersion flow processes fit data better than did other groundwater 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 18O composition, and that the current year's precipitation 18O signature became more attenuated in the subsurface flow system. MRTs obtained for streamflow at each site were 9.5 and 4.8 months for a 14 and 100 ha watershed, respectively, and soil water MRT at 100 cm depth was on the order of a couple months; indicating a relatively rapid response of shallow groundwater to precipitation.