Effects of seasonal operation on the quality of water produced by public-supply wells.
Bottom Line: Seasonal variability in groundwater pumping is common in many places, but resulting effects of seasonal pumping stress on the quality of water produced by public-supply wells are not thoroughly understood.Seasonal variability in contaminant concentrations in Modesto is influenced primarily by effects of summer pumping on vertical hydraulic gradients that drive migration of shallow groundwater through the aquifer to supply wells.Results suggest that even in aquifers dominated by old groundwater, changes to seasonal pumping patterns and/or to depth of well completion can help reduce vulnerability to selected contaminants of either natural or anthropogenic origin.
Affiliation: U.S. Geological Survey, Placer Hall, 6000 J St., Sacramento, CA 95819.Show MeSH
Mentions: Continuous hydraulic-head and specific-conductance data from the monitoring-well nest located within 30 m of the MSW (Figure 6) show that seasonal changes in the quality of water from the MSW are as a result of wellbore flow that occurs during prolonged periods of nonoperation. The steep vertical hydraulic gradient across the screened interval of the MSW causes shallow groundwater to enter the well or annular space and move downward through the wellbore when the well is not pumping; this flow exits the wellbore near the bottom of the well and enters the deep part of the aquifer (Jurgens et al. 2008). Movement of shallow groundwater into the deep part of the aquifer is reflected by the rise in hydraulic head and specific conductance in the deep monitoring well when the MSW has been idle for long periods (Figure 6). The head and conductance data begin to fall during the summer, when long periods of pumping cause the stored shallow groundwater to be evacuated from the deep part of the aquifer.
Affiliation: U.S. Geological Survey, Placer Hall, 6000 J St., Sacramento, CA 95819.