Development and Description of a Composite Hydrogeologic Framework for Inclusion in a Geoenvironmental Assessment of Undiscovered Uranium Resources in Pliocene- to Pleistocene-Age Geologic Units of the Texas Coastal Plain

Abstract

A previously completed mineral resources assessment of the Texas Coastal Plain indicated the potential for the future discovery of uranium resources. Geoenvironmental assessments that include the hydrogeologic framework can be used as a tool to understand the potential effects of mining operations. The hydrogeologic framework for this study focused on the composite hydrogeologic unit of the tract permissive for the occurrence of uranium consisting of the upper part of the Miocene-age Fleming Formation/Lagarto Clay, Pliocene-age Goliad and Pleistocene-age Willis Sands, Pleistocene-age Lissie and Beaumont Formations, and Holocene-age alluvial sediments (fluvial alluvium and eolian sand deposits). This composite hydrogeologic unit, which contains the Chicot and Evangeline aquifers of the Gulf Coast aquifer system, is intended for inclusion in a regional-scale geoenvironmental assessment of as yet undiscovered uranium resources. This article provides (1) a brief literature review describing the geologic and hydrogeologic settings, (2) the methodology used to develop a composite hydrogeologic framework, and (3) descriptions and maps of the land-surface altitude, composite hydrogeologic unit base and midpoint depth, water-level altitude, depth of water, unsaturated and saturated zone thickness, and transmissivity and hydraulic conductivity. A composite hydrogeologic unit, created by combining geologic and hydrogeologic data and maps for individual geologic and hydrogeologic units, is intended for use as a tool in a geoenvironmental assessment to evaluate potential contaminant migration through various avenues. Potential applications include using the hydrogeologic framework as an input into a geoenvironmental assessment to help estimate the potential for (1) runoff of contaminants into surface water, (2) infiltration of contaminants into the groundwater (aquifers), or (3) movement of contaminants from the mining area through wind, groundwater-flow, or streamflow in a given permissive tract. The procedures outlined in this paper also provide a method for developing hydrogeologic frameworks that can be applied in other areas where mining may occur.

A Novel Method for Conducting a Geoenvironmental Assessment of Undiscovered ISR-Amenable Uranium Resources: Proof-of-Concept in the Texas Coastal Plain

“…Together, the geographic areal extent of the permissive tract and the vertical extent of these formations and associated aquifers delineate the full extents of the study area. The geoenvironmental assessment results below highlight the input from the conceptual geoenvironmental model, the historical wastes and disturbances from historical mining, and the hydrogeologic, geochemical, and climatic frameworks in companion studies [6,7,9,12,13].…”

“…A hydrogeologic framework of the Goliad/Willis/Lissie geoenvironmental assessment unit has been described in detail elsewhere [9]. In brief, in the geoenvironmental assessment unit, there are three contiguous aquifer units consisting of the alluvium, Chicot, and Evangeline aquifer units (Table 2).…”

“…These aquifers/formations are not separated by an aquitard and, for the purposes of this assessment, were combined to reflect the possibility that water can flow between units. Although the Chicot and Evangeline are mostly sand, there are extensive clay lenses throughout that may act as confining units that are necessary for ISR [9]. Table 2.…”

“…Hydraulic conductivity describes the ease with which a fluid (usually water) can move through pore spaces. Teeple et al [9,10] derived an equivalent horizontal hydraulic conductivity (Figure 4a) and equivalent transmissivity (Figure 4b) for the composite aquifer system (Alluvial, Chicot, Evangeline) from the U.S. Geological Survey (USGS) Source Water Assessment Program (SWAP) [33,34] datasets. The saturated thickness maps were calculated from water-table altitudes (2007-2016) within the composite aquifer system in the geoenvironmental assessment unit [9,10].…”

“…Teeple et al [9,10] derived an equivalent horizontal hydraulic conductivity (Figure 4a) and equivalent transmissivity (Figure 4b) for the composite aquifer system (Alluvial, Chicot, Evangeline) from the U.S. Geological Survey (USGS) Source Water Assessment Program (SWAP) [33,34] datasets. The saturated thickness maps were calculated from water-table altitudes (2007-2016) within the composite aquifer system in the geoenvironmental assessment unit [9,10]. Transmissivity and hydraulic conductivity are important indicators of the potential for mine-impacted groundwater to flow from the mining zone; however, the SWAP dataset does not cover all of the permissive tract.…”