Estimating Lithium Concentrations in Groundwater Used as Drinking Water for the Conterminous United States

Lithium (Li) concentrations in drinking-water supplies are not regulated in the United States; however, Li is included in the 2022 U.S. Environmental Protection Agency list of unregulated contaminants for monitoring by public water systems. Li is used pharmaceutically to treat bipolar disorder, and studies have linked its occurrence in drinking water to human-health outcomes. An extreme gradient boosting model was developed to estimate geogenic Li in drinking-water supply wells throughout the conterminous United States. The model was trained using Li measurements from ∼13,500 wells and predictor variables related to its natural occurrence in groundwater. The model predicts the probability of Li in four concentration classifications, ≤4 μg/L, >4 to ≤10 μg/L, >10 to ≤30 μg/L, and >30 μg/L. Model predictions were evaluated using wells held out from model training and with new data and have an accuracy of 47-65%. Important predictor variables include average annual precipitation, well depth, and soil geochemistry. Model predictions were mapped at a spatial resolution of 1 km2 and represent well depths associated with public- and private-supply wells. This model was developed by hydrologists and public-health researchers to estimate Li exposure from drinking water and compare to national-scale human-health data for a better understanding of dose-response to low (<30 μg/L) concentrations of Li.

Predictions of Arsenic in Domestic Well Water Sourced from Alluvial Aquifers of the Western Great Basin, USA

Chronic exposure to high levels of arsenic in drinking water can have wide-ranging health effects and is a global health concern. The domestic well population of the western Great Basin (WGB) is at increased risk of exposure to arsenic due to the hydrologic, geologic, and climatic setting of the region. A logistic regression (LR) model was developed to predict the probability of elevated arsenic (≥5 μg/L) in alluvial aquifers and assess the potential geologic hazard level posed to domestic well populations. Alluvial aquifers are susceptible to arsenic contamination, which is a concern because they are the primary source of water for domestic well users of the WGB. The probability of elevated arsenic at a domestic well is strongly influenced by tectonic and geothermal variables, including the total Quaternary fault length in the hydrographic basin and the distance between the sampled well and a geothermal system. The model had an overall accuracy of 81%, sensitivity of 92%, and specificity of 55%. Results show a >50% probability of elevated arsenic in untreated well water for approximately 49 thousand (64%) alluvial-aquifer domestic well users in northern Nevada, northeastern California, and western Utah.