State Water Resources Research Institute Program (WRRI)

Details for Project ID 2014NE265G, 2014

Nitrate Mediated Mobilization of Naturally Occurring Uranium in Groundwater

Institute: Nebraska
USGS Grant Number: G14AP00171
Start Date: 2014-09-01 End Date: 2017-08-31
Total Federal Funds: $230,839 Total Non-Federal Funds: $236,661

Principal Investigators: Karrie Weber, Daniel Snow

Abstract: Soluble uranium (U) is a recognized contaminant in public ground water supplies in various regions throughout the United States. Increasing occurrence of elevated U concentrations in drinking water in both urban and rural communities has led to human health concerns including kidney failure and cancer risk. Mechanisms driving U mobilization in these aquifers remains poorly understood. In order to develop management strategies and prevent further contamination of drinking water sources, it is necessary to gain a fundamental understanding of the mechanisms stimulating U mobilization and improve predictive models. There are two fundamental mechanisms that drive U mobilization, i) desorption of U as a result of increased alkalinity or ground water removal or ii) oxidative dissolution of reduced U minerals. Reduced U minerals are subject to oxidation by available oxidants, such as oxygen, nitrate, and Fe(III) oxides. The oxidation product exists primarily as a dissolved U(VI) species and is thus mobile. It is well recognized that anthropogenic activities involving the application of N onto agricultural fields and urban landscapes has resulted in nitrate contamination of surface and ground waters throughout the United States. The sandy soils above the Ogallala aquifer have made this region particularly susceptible to nitrate contamination. Abiotic and bio-oxidative dissolution of U(IV) coupled to nitrate reduction has been recognized as a potential U oxidative mechanism. Thus, the influx of nitrate as a primary groundwater contaminant can subsequently influence U mobility resulting in a secondary contaminant in groundwater. Not only can nitrate directly oxidize U(IV), but nitrate plays a role in biologically mediated Fe(II) oxidation and precipitation of Fe(III) oxides. Fe(III) oxides are also known to oxidize U(IV). Together these reactions can lead to the mobilization of U in aquifers used for drinking water supplies throughout the U.S. We hypothesize that oxidative dissolution of natural U(IV) minerals by biotic and abiotic pathways is stimulated by nitrate and results in U mobility in groundwater. While the studies proposed here will focus on subsurface environments in Nebraska, these processes could be widespread in the Rocky Mountains or other regions in the United States where U is deposited as a reduced mineral and nitrate is a contaminant. The outcome of this project will result in the development of reactive transport model predicting U mobilization in areas where reduced U is deposited. The objective of this project is to determine the mechanisms governing the oxidative dissolution of U(IV), U mobility, and quantify the rate and extent of these reactions in order to develop a reactive transport model to predict U mobility in ground water.