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The Importance of Groundwater Monitoring and Recovery Wells Part 4 Submitted by Wes McCall, P.G. Geoprobe ® Environmental Geologist An Investigation of Uranium in Public Water Supply Wells Just a few years ago, Geoprobe Systems ® was invited by Tom Christopherson at the Nebraska Department of Health & Human Services/Well Protection Division to assist them with an investigation at a small community in central Nebraska. This project provided Geoprobe ® engineers a good site to test new machine and tooling products. The implementation of the U.S. EPA’s (Environmental Protection Agency’s) new maximum con- taminant level (MCL) for uranium of 30µg/L (microgram per liter) had led many small community water supply systems across the U.S. being out of compliance. In this community, new supply wells had been installed at a new well field to replace old wells which were out of compliance with the uranium MCL. Test wells installed in the new field found uranium was nondetect, but shortly after the expensive new supply wells were installed and brought online, they exceeded the uranium MCL and bottled drinking water again had to be provided for community residents. Geoprobe ® first used HPT (hydraulic profiling tool) logs to assess the hydrogeology at the new well field. Using the electrical conductivity and pressure logs provided by the HPT system, it was discovered the approx- imately 115-foot-thick formation had several discontinuous, fine-grained layers which inter-fingered through the coarse-grained aquifer. The logs were used to guide the placement and screened intervals of two sets of five nested wells to assess the groundwater quality and uranium distribution across the local aquifer. The small-diameter wells with 5-foot long screens were installed with a Geoprobe ® 8040DT machine using 2.25-inch OD (outside diameter) casing at depths between 15-100 feet at the two locations. Well development was followed by purging and sampling for major element cations and several trace elements, including of course, uranium. The results of the uranium analyses together with the HPT logs revealed uranium concentrations were elevated in the upper half of the aquifer where most of the fine-grained layers occurred. The original test well had been screened deeper in the for- mation and had not been pumped at a sufficient rate to draw in the water with el- evated uranium. Longer screens and filter packs had been used for the supply wells. Hence, when the high-capacity pumps were turned on, water with elevated ura- nium levels from the shallow part of the aquifer was drawn into the wells. A little site characterization could have saved a lot of money and time, not to mention health risk. An in-depth story of this project is available at: www.geoprobe.com/uranium-in-pws Two members of the Geoprobe ® Team collecting HPT logs near the two public water supply wells in Clarks, Nebraska. 17 WorldWide Drilling Resource ® OCTOBER 2016