Geochemistry of ground water in the Miocene Oakville sandstone : a major aquifer and uranium host of the Texas coastal plain

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Henry, Christopher D.

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University of Texas at Austin. Bureau of Economic Geology


The Oakville Sandstone is a major aquifer and a major host of uranium deposits in the Texas Coastal Plain. This report discusses the geochemistry - including major ions, the trace elements uranium, molybdenum, arsenic, and selenium, and oxidation-reduction potential - of Oakville ground water to evaluate the potential effect of uranium mining on water quality. Ground-water chemistry was investigated both regionally and in two major uranium mining districts.

Major ion composition of Oakville ground water shows a regional variation. East Texas water has low to moderate dissolved solids (350 to 1,100 mg/L) and evolves from a calcium-magnesium-bicarbonate water in near-surface recharge areas to a sodium-bicarbonate composition downdip. The composition and evolution result from calcite solution and cation exchange. South Texas water has higher dissolved solids (up to 3,000 mg/ L) dominated by sodium, chloride, and sulfate. The high dissolved solids result from solution of evaporites within the Oakville and from the discharge along faults into the Oakville of high-dissolved-solids water from deeper aquifers.

There are three oxidation-reduction zones within the Oakville: (1) an oxidizing zone with Eh values greater than 300 mV, controlled by the presence of measurable concentrations of dissolved oxygen, (2) an intermediate zone with Eh values between 110 and 10 mV, probably controlled by ferrous-ferric mineral reactions, and (3) a reducing zone with Eh values less than -40 mV, probably controlled by the presence of FeS2.

Uranium, molybdenum, arsenic, and selenium show regional variations in concentrations, with trace element values in South Texas water distinctly higher than in East Texas water. The higher concentrations may result either from secondary dispersion from uranium deposits, which are more abundant in South Texas than in East Texas, or from a residual primary source in South Texas. Uranium closely follows behavior predicted from thermodynamic data; equilibrium with coffinite determines uranium concentrations in intermediate-Eh and reducing water. The other three elements do not follow predicted behavior, probably because no data are available on complexing.

Analysis of water chemistry around the two uranium mining districts gives results similar to those found in the regional study. Very high concentrations of trace elements occur only in a few samples within ore zones. These high concentrations probably result from natural dispersal from the deposits, although this is not a widespread phenomenon.


To obtain a print version of this publication visit: and search for: RI0118. Tx Doc no.: Z, UA220.7, R299, no. 118

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