Characterizing contaminant concentrations with depth by using the USGS well profiler in Oklahoma, 2003-9

Abstract

Since 2003, the U.S. Geological Survey (USGS) Oklahoma Water Science Center has been using the USGS well profiler to characterize changes in water contribution and contaminant concentrations with depth in pumping public-supply wells in selected aquifers. The tools and methods associated with the well profiler, which were first developed by the USGS California Water Science Center, have been used to investigate common problems such as saline water intrusion in high-yield irrigation wells and metals contamination in high-yield public-supply wells. The USGS well profiler is a slim (less than 1 inch in diameter), high-pressure hose that can be raised and lowered between the production pipe and casing (or borehole) of a well by using a motorized hose reel. Use of this tool is considerably less expensive than use of standard methods of depth-dependent sampling, and the USGS well profiler generally requires less downtime of the well. In terms of data quality, the greatest advantage of the USGS well profiler is that all data collection is performed under production pumping rates. In Oklahoma, the USGS well profiler has been modified and adapted for use in low-yield (150?350 gallons per minute) wells of various construction types common in Oklahoma. This tool has been used in selected public-supply wells in Hinton, Moore, and Norman to identify which producing zones are contaminated by naturally occurring arsenic. The tool and method also can be used to investigate other nonvolatile contaminants of interest, including uranium, radium, barium, boron, lead, selenium, sulfate, chloride, fluoride, nitrate, and chromium. In 2007, the USGS well profiler was used to investigate saline water intrusion in a deep public-supply well completed in the Ozark (Roubidoux) aquifer. In northeast Oklahoma, where the Ozark aquifer is known to be susceptible to contamination from mining activities, the well profiler also could be used to investigate sources (depths) of metals contamination and to identify routes of entry of metals to production wells.Water suppliers can consider well rehabilitation as a potential remediation strategy because of the ability to identify changes in contaminant concentrations with depth in individual wells with the USGS well profiler. Well rehabilitation methods, which are relatively inexpensive compared to drilling and completing new wells, involve modifying the construction or operation of a well to enhance the production of water from zones with lesser concentrations of a contaminant or to limit the production of water from zones with greater concentrations of a contaminant. One of the most effective well rehabilitation methods is zonal isolation, in which water from contaminated zones is excluded from production through installation of cement plugs or packers. By using relatively simple and inexpensive well rehabilitation methods, water suppliers may be able to decrease exposure of customers to contaminants and avoid costly installation of additional wells, conveyance infrastructure, and treatment technologies.