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U.S. Geological Survey Toxic Substances Hydrology Program--Proceedings of the Technical Meeting, Colorado Springs, Colorado, September 20-24, 1993, Water-Resources Investigations Report 94-4015

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An Experiment to Quantify Temporal Variability of Water Samples Obtained from Screened Wells, Cape Cod, Massachusetts

by

Thomas E. Reilly (U.S. Geological Survey, Reston, Va.) and Denis R. LeBlanc (U.S. Geological Survey, Marlborough, Mass.)

Abstract

The chemical composition of water sampled from some screened wells has been observed to exhibit sustained temporal variability. Results from a field experiment, undertaken on August 12, 1992, in the area of the contaminant plume from the Otis Air Base sewage-disposal sand beds, support the recent hypothesis that the temporal variability can be attributed to the flux along the well screen reflecting the spatial heterogeneity in hydraulic conductivity and water chemistry of the aquifer near the well. The experiment consisted of withdrawing water from a well with a screened interval approximately 40 ft long and sampling over time during a 5-hour period. Water samples also were collected from multilevel samplers surrounding the well before, during, and after the pumping of the well. Selected constituent concentrations and properties measured in samples collected from the discharging well showed different temporal trends during the 5-hour test. For example, the ferrous iron concentration decreased, the calcium concentration increased, and specific conductance remained relatively constant. The different trends are due to the different distributions of the constituents around the well as documented by analysis of water samples collected by the set of multilevel samplers. The observed trends are in agreement with the hypothesis and apparently are due not to the purging of the water standing in the well but to the flow and transport of the water and chemical species in the heterogenous aquifer in the immediate vicinity of the well.

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