NEW GEOCHEMICAL AND GEOGRAPHIC INFORMATION SYSTEM METHODOLOGIES TO ASSESS ELEMENT MOBILITY IN GROUNDWATER

A new geochemical and geographic information system (GIS) methodology was developed to aid in assessing potential groundwater vulnerability in southeast Texas related to future mining of undiscovered roll-type, Tertiary sandstone-hosted uranium resources in the Texas Coastal Plain sedimentary strata. Element maps indicating the potential for element mobility were constructed using a stepwise approach. Historical geochemical data were culled from the National Uranium Resource Evaluation (NURE; n = 94,000 samples) and USGS National Water Information System (NWIS; 1980-present; n = 168,000 samples) databases. NURE and NWIS data analyzed in GIS determined the geographic distribution of samples meeting geochemical mobility criteria which include oxidation-reduction potential (pE), pH, [H₂S], [Mn], sulfide species, and iron substrates of the groundwater. Twelve-digit hydrologic unit code (HUC) boundaries were used in GIS as analysis areas (average 112 km²) to determine the most commonly occurring mobility category (statistical mode) in each HUC. Modeling of empirical data in PHREEQC and Geochemists Workbench were used to constrain chemical indicators of iron saturation and phase equilibria at varying pE to establish cutoff values to apply to the geochemical mobility model. Modeling results show that iron solids such as iron sulfides and iron oxides can reach saturation at pH conditions ranging from 5 to 9, pE conditions of 2 and 6, and iron concentrations of 10 µg/L and 100 µg/L, which represent conditions observed in the aquifers studied. These iron solids could function as iron substrates important for element sorption. Resulting maps identify areas having varying potential element mobility. Statistical distributions of element concentrations were determined for each mobility category to identify potential elements of concern including uranium, molybdenum and selenium and assess the range of concentrations resulting from the model. Combining other datasets such as sedimentary facies distribution, extent and continuity of confining layers, faults, stream gain/loss, and existing mine site density, along with the groundwater mobility map may help to predict and constrain areas of element mobility in future groundwater vulnerability assessments.