University of California

Presentations 2016

Burow, Karen

Presentation Title
Decadal-scale changes in uranium and bicarbonate concentrations in groundwater in the U.S.: Effects of irrigation on the mobilization of uranium
U.S. Geological Survey
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Uranium has been shown to be mobilized by downward-moving irrigation water in the eastern San Joaquin Valley (SJV) of California (Jurgens and others, 2010). Development of the landscape for crop production has caused increased bicarbonate concentrations in shallow groundwater, and increased rates of recharge. The downward moving, bicarbonate-enriched groundwater mobilizes naturally-occurring uranium in the sediments. Consequently, uranium concentrations at depth are increasing with time, and are affecting wells used for drinking water. It is hypothesized that the process observed in the SJV will be operative in other arid and semi-arid areas of the U.S., but not in humid areas. Water-quality data from 1,105 wells from across the U.S. were compiled from the U.S. Geological Survey National Water-Quality Assessment project. Most of the wells were first sampled during 1993-2002 and then subsequently sampled during 2001-2012. Some wells used in the analysis were sampled in 2001-2005 and subsequently sampled during 2012-2014. Uranium concentrations in groundwater were highest in the arid to semi-arid climate zones in the western U.S, where uranium in surficial sediments and rocks are abundant. Sixty-four wells (6 percent) sampled in the second decade were above the U.S. EPA MCL of 30 ug/L; all but one are in the arid west. Areas with low uranium concentrations in surficial rocks and sediments such as the Coastal Plain in the southeastern U.S., the upper Midwest, and northeast parts of Oregon and Washington generally have low concentrations of uranium in groundwater. Large uranium and bicarbonate increases (differences are greater than the uncertainty in concentrations) occur in 109 wells between decade 1 and decade 2. Similarly, large uranium and bicarbonate decreases occur in 76 wells between decade 1 and decade 2. Significantly more wells are concordant (uranium and bicarbonate are both going the same direction) than discordant (uranium and bicarbonate are going opposite directions) (p<0.001; Chi-square test). The largest percent difference in uranium concentrations occur in wells where uranium is increasing and bicarbonate is also increasing, These large differences occur mostly in the arid to semi-arid western U.S., consistent with the process of the mobilization of uranium by the increasing bicarbonate concentrations in irrigated areas. Uranium was not detected in either decade in 53 percent of the wells in the dataset. Ninety percent of these wells with no detections occur in the humid or dry sub-humid climate zones of the eastern U.S. Speciation calculations on water from wells with a detection in at least one decade indicate that the uranium in groundwater occurs predominantly in the form of ternary complexes of uranyl carbonate with calcium (94 percent of samples). The concordant change in uranium and bicarbonate concentrations and the dominantly neutral pH ranges (6-8) are consistent with the speciation calculations, and consistent with the processes found by Jurgens and others (2010). References: Jurgens, B.C., Fram, M.S., Belitz, Kenneth, Burow, K.R., and Landon, M.K., 2010, Effects of groundwater development on uranium: Central Valley, California, USA: Ground Water, v. 48, no. 6, pp. 913-928

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