|Flow Contribution and Water Quality with Depth in a Test Hole and Public-supply Wells: Implications for Arsenic Remediation through Well Modification, Norman, Oklahoma, 2003-2006 (EPA/600/R-09/036) April 2009|
The City of Norman, Oklahoma, is one municipality affected by a change in the Environmental Protection Agency's National Primary Drinking Water Regulation for arsenic. In 2006, the maximum contaminant level for arsenic in drinking-water was lowered from 50 to 10 micrograms per liter. Arsenic concentrations in water produced by 32 Norman public-supply wells ranged from less than 1 to 232 micrograms per liter. Some Norman wells with arsenic concentrations marginally exceeding 10 micrograms per liter are suspected of producing water from zones with acceptably low arsenic concentrations and zones with unacceptably high arsenic concentrations. If water with high arsenic concentrations can be limited or excluded from production without causing an excessive decrease in well yield, these wells may be rehabilitated to comply with the new regulation.
The flow contribution and water quality of each producing zone was measured in 11 City of Norman wells to determine which wells were potential candidates for arsenic remediation by well rehabilitation. Depth-dependent flow-contribution and water-quality data were collected under normal production conditions using the U.S. Geological Survey combined well-bore flow and depth-dependent water sampler (U.S. Geological Survey well profiler). The depth-dependent water-quality data collected by the U.S. Geological Survey well profiler were extremely useful as a qualitative tool for identification of zones that degrade water quality in the Norman wells. The depth-dependent water-quality data, even without flow-contribution data, showed the depth at which the water mixture in the well bore was unsuitable for public supply.
Eleven Norman wells were investigated for remediation potential. Most of the selected wells (Wells 06, 07, 13, 15, 18, 23, and 31) showed elevated (greater than 10 micrograms per liter) or near-elevated arsenic concentrations at all depths in the well. For these wells, well-modification techniques would be ineffective in lowering well-head arsenic concentrations to less than 10 micrograms per liter. Wells 02, 05, 33, and 36 showed potential for successful application of well modification techniques for arsenic remediation because greater differences in arsenic concentrations between depths were observed.
Two of the eleven selected wells (05 and 36) were selected for repeated sampling to determine the effects of pump intake relocation on well yield and water quality. Both wells had elevated arsenic concentrations in water from the deepest zone and arsenic concentrations less than 10 micrograms per liter in water from shallower zones. Both wells showed short-term improvements in water quality as the pump was moved to higher locations in the well. In Well 05, arsenic concentration at the well head decreased by about 32 percent and well yield decreased by 12 percent. In Well 36, arsenic concentration at the well head decreased by 84 percent and well yield increased by 13 percent. However, additional samples collected a few months later in Well 36 revealed that improvements in well-head water quality were only temporary.
An alternate remedial approach of zonal isolation was implemented in Well 36. Only the deepest zone in Well 36 (648-658 feet below land surface) was suspected of contributing elevated arsenic concentrations to the well. A retrievable bridge plug was installed at a depth of 640 feet in Well 36 to isolate the suspect zone from production. Unfortunately, the installation of the bridge plug had little effect on well-head water quality. Compared to well-head samples collected prior to the installation of the bridge plug, specific conductance and concentrations of arsenic and chromium at the well head each decreased by only 2 percent after installation of the bridge plug. However, the bridge plug may have been placed too deep to exclude the arsenic-contaminated water from production.
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