Three distinct oxidation-reduction zones have been identified within the uranium-bearing Oakville aquifer. An oxidizing zone with Eh values greater than 300 mv and with measurable dissolved oxygen occurs in areas of recharge and extends to depths as great as 800 ft (244 m). An intermediate zone with Eh values between 110 and 10 mv occurs downdip from the oxidizing zone. Eh in these waters may be controlled by ferrous-ferric mineral reactions. About half of the intermediate Eh waters contain low but detectable concentrations of hydrogen sulfide. In east Texas the deepest wells sampled (1,600 ft; 488 m) penetrate this intermediate zone. A reducing zone with Eh less than -40 mv and dissolved H₂S occurs in deepest parts of the Oakville in south Texas and also at sh llow depths (300 ft; 91 m) associated with faults, which apparently provided conduits for the discharge of reducing, sulfide-rich waters from deeper formations. Present-day Eh is controlled by either the continued discharge of reducing water or the presence of pyrite formed by previous reduction.

Thermodynamic calculations show that uranium and selenium phases may precipitate within the intermediate zone, but that molybdenum and arsenic require more reducing conditions for precipitation. All four elements should occur together only where the oxidizing and reducing zones are adjacent without an intervening intermediate zone. Such areas can occur (1) where fault discharge has superimposed reducing, sulfidic conditions on an otherwise normal Eh gradient or (2) where a sharp Eh gradient separates highly permeable oxidized sediment from less permeable reduced sediment. Where oxidizing and reducing zones are not adjacent, uranium and selenium only should accumulate within the intermediate zone, and mineralization may be diffuse.

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