Using Age Tracers and Decadal Sampling to Discern Trends in Nitrate, Arsenic, and Uranium in Groundwater Beneath Irrigated Cropland

Abstract

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 past decade. Groundwater irrigated areas in the Columbia Plateau tend to have higher nitrate concentrations in groundwater than continued...

Soil and Aquifer Properties Combine as Predictors of Groundwater Uranium Concentrations within the Central Valley, California

“…Groundwater Ca concentration was the most influential variable in the RFR model, followed by nitrate concentrations. These findings agree with past work that identified bicarbonate alkalinity, nitrate, and dissolved Ca, sometimes represented within total dissolved solids (TDSs), as major factors controlling elevated U concentrations in groundwater. ,,, …”

“…Nitrate is commonly formed from nitrification of added ammonium-based or organic fertilizers, or from animal yard effluent, and is highly leachable. Thus, although nitrate is an oxidant of U­(IV), the co-occurrence of U and nitrate in groundwater may result from similar mobilities and transport with (irrigation) recharge water. ,, In two agriculturally influenced aquifers, the Central Valley (CV) and the High Plains (HP), Nolan and Weber identified a moderate positive correlation ( P -value <0.001; Spearman’s ρ = 0.324 (CV) and 0.421 (HP)) between groundwater nitrate and U concentrations. Within the CV, groundwater U weakly correlated with nitrate ( P -value <0.05; Pearson’s r = 0.27) in the eastern San Joaquin Valley, but the correlation was stronger in multiple individual fan areas .…”

“…Uranium is a ubiquitous groundwater contaminant, having a maximum contaminant level (MCL) of 30 μg L –1 in drinking water regulated by the U.S. Environmental Protection Agency (U.S. EPA) and consistent with the World Health Organization’s guideline value. , Uranium is both a radiogenic and metal toxin; the primary threat from ingestion of U-contaminated drinking water results from its chemical toxicity, which can lead to impaired kidney function and have osteotoxic effects. , Historically, U contamination of soils/sediments and water has been linked to mining, milling, nuclear testing and disposal sites, coal and fuel combustion, and phosphate fertilizer application; , however, high dissolved U levels in groundwater are widespread in areas unaffected by or upstream of anthropogenic U activity, such as in arid regions with irrigation-dependent agriculture and influenced by geologic U sources. − Groundwater U contamination has also been reported in humid regions, such as the glacial and bedrock aquifers of Northeastern U.S., , albeit at concentrations typically lower than for arid and semi-arid regions …”

“…Importantly, more than 85% of the dissolved U measurements were calculated to be uranyl–Ca–carbonato species. The U–bicarbonate link has since been studied in aquifers across the United States, Germany, and India. − ,, …”

“…The U−bicarbonate link has since been studied in aquifers across the United States, Germany, and India. [8][9][10]36,40 Beyond bicarbonate alkalinity, Nolan and Weber 9 statistically demonstrated a moderate link between nitrate and U concentrations in the Central Valley (CV) and High Plain (HP) Aquifers, United States. Nitrate is commonly applied to soil as a fertilizer and can convert U(IV) to U(VI) during oxidative dissolution or by a variety of indirect mechanisms.…”