Brackish Water in Unsaturated Confining Beds at a Texas Lignite Minea

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Abstract
Deposits of low permeability are hydrologic barriers between reclaimed land and juxtaposed aquifers and should limit the impact of mining on ground‐water quality. Clay‐stone and mudstone in argillaceous facies of the Calvert Bluff Formation (lower Eocene, Wilcox Group) function as confining beds in the East Texas Basin. In the subsurface and at the outcrop, water in argillaceous deposits is brackish to saline. Samples of vadose water from the outcrop of confining beds at the Big Brown lignite mine in Freestone County, Texas, have a chloride concentration of up to 3,500 mg/l and total dissolved solids of up to 8,000 mg/l. Ground‐water composition evolved from Eocene sea water by seven‐ to nine‐fold dilution with rain water. Ion exchange, pyrite oxidation, and calcite dissolution further modified water composition. The amount of recharge through the vadose zone where confining layers crop out is probably negligible over an extremely long time. Meteoric flushing in reclaimed land at the surface mine is many times greater than that in unmined mudstone deposits, and the chemical composition of vadose water in reclaimed land is changed by further dilution and water‐rock reactions.