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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