U.S. Geological Survey

By Kathleen A. McCarthy, William D. McFarland, James M. Wilkinson, and L.D. White

Journal of Hydrology, vol. 135, no. 1/4

Abstract

Part of the water extracted from wells constructed in highly transmissive aquifers located adjacent to streams may be derived from induced infiltration of the nearby surface water. Knowledge of the fraction of well water derived from the stream is important for identifying potential sources of contaminants, understanding problems of scaling and corrosion, and predicting the effects of pumping on groundwater levels. Deuterium and oxygen-18 were used as natural tracers to investigate the hydraulic relationship between the Columbia River and the Blue Lake gravel aquifer near Portland, Oregon. A time series of stable-isotope data collected from surface and groundwater during a March 1990 aquifer test confirmed that the river and aquifer are hydraulically connected. Water from the Columbia River was found to be significantly depleted in deuterium and oxygen-18 as compared with other water in the Blue Lake area. Calculations based on simple mixing show that the river contributed 40-50% of the yield from three of the five production wells after 5-6 days of pumping. Data collected during August 1990 show that the river contributed 65-80% of the yield of one well after 22 days of pumping and indicate that the contribution of the river was still increasing. It was not possible to calculate the equilibrium composition of well yields without a longer time series of data, but it is apparent that the Columbia River contributes substantially to the yields of wells completed in the Blue Lake gravel aquifer during the winter and summer months.