Skip to main content
U.S. flag

An official website of the United States government

Using age tracers and decadal sampling to discern trends in nitrate, arsenic and uranium in groundwater beneath irrigated cropland

November 21, 2019

Repeat sampling and age tracers were used to examine trends in nitrate, arsenic and uranium concentrations in groundwater beneath irrigated cropland. Much higher nitrate concentrations in shallow modern groundwater were observed at both the Columbia Plateau and High Plains sites (median values of 10.2 and 15.4 mg/L as N, respectively) than in groundwater that recharged prior to the onset of intensive irrigation (median values of <1 and <4 mg/L as N, respectively). Repeat sampling of these well networks indicates that high nitrate concentrations in modern, shallow groundwater have been sustained for decades, posing a future risk to older, deeper groundwater used for drinking water. In fact, nitrate concentrations in older modern water (30-60 years since recharge) at the High Plains site have increased in the last decade. Groundwater irrigated areas in the Columbia Plateau tend to have higher nitrate concentrations than surface-water irrigated areas suggesting that repeated dissolution of land applied fertilizer during recirculation may be an important factor causing high nitrate concentrations in groundwater. Mobilization of uranium and arsenic by land surface activities is suggested by the higher concentrations of these constituents in modern, shallow groundwater than in older, deeper groundwater at the Columbia Plateau site. Bicarbonate concentrations in modern groundwater are positively correlated with uranium (r=0.72, p<0.01), suggesting bicarbonate may mobilize uranium in this system. A positive correlation between arsenic and phosphorus concentrations in modern groundwater (r=0.55, p<0.01) suggests that phosphate from fertilizer outcompetes arsenate for sorption sites, mobilizing sorbed arsenic derived from past pesticide use or other sources.

Publication Year 2019
Title Using age tracers and decadal sampling to discern trends in nitrate, arsenic and uranium in groundwater beneath irrigated cropland
DOI 10.1021/acs.est.9b03459
Authors Anthony J. Tesoriero, Karen R. Burow, Lonna Frans, Jonathan V. Haynes, Christopher M. Hobza, Bruce D. Lindsey, John E. Solder
Publication Type Article
Publication Subtype Journal Article
Series Title Environmental Science and Technology
Index ID 70209051
Record Source USGS Publications Warehouse
USGS Organization Oregon Water Science Center