Real and perceived sources of gradient calculation error in the Savannah River Site groundwater monitoring program

Understanding and predicting contaminant migration trajectories is a major concern at the Savannah River Site. Migration of contaminated groundwater is controlled by hydraulic gradients and local hydrogeologic conditions. Because errors in estimating the hydraulic gradient may result in substantial risks to downgradient water bodies, a program was initiated to quantify the sources and magnitude of potential contributors to hydraulic gradient uncertainty. The primary objective was to develop a management program that better incorporates theoretical and local information to minimize and estimate uncertainties in the hydraulic gradient calculation. Hydraulic and statistical tools were coupled to provide computational resources for evaluating alternative management actions. Primary sources of gradient uncertainty identified at SRS were (1) water level variation due to diurnal variation in barometric pressure mediated by geologic effects on barometric efficiency; (2) intermittent variations in streamflow that locally alter water levels in adjacent aquifers; and (3) intermittent rainfall events that temporarily increase water levels in confined aquifers due to loading effects.