Hydrogeochemistry of the Vadose Zone in Unmined and Reclaimed Deposits at Big Brown Lignite Mine, East Texas

In the subsurface and at the outcrop, waters in argillaceous deposits of the Calvert Bluff Formation (lower Eocene, Wilcox Group) are brackish to saline. Samples of vadose water from the outcrop of argillaceous beds at Big Brown lignite mine in Freestone County, Texas, have chloride concentrations of as much as 3,500 mg/L and total dissolved solids of as much as 8,000 mg/L. The composition of brackish ground water was probably modified from Eocene seawater by sevenfold to ninefold dilution with rain water. Ion exchange, pyrite oxidation, and calcite solution further modified water composition.Recharge rates through the vadose zone vary with lithology. Outcropping sands annually transmit about 10 cm of recharging vadose water to the water table, two to five times the amount that unmined argillaceous beds transmit. Chemical composition of vadose water in outcropping sand deposits is affected by weathering of feldspar and other silicate minerals in the soil zone. The recharge rate through reclaimed mud deposits is probably about the same as that through unmined sand deposits. The chemical composition of vadose water in reclaimed land is inherited from formation water in unmined mud deposits but is changed by dilution and water-rock reactions.Argillaceous deposits of low hydraulic conductivity in the Calvert Bluff Formation in the EastTexas Basin most likely function as local confining beds. At their outcrop at Big Brown mine, the confining beds can retard movement of vadose water from reclaimed land to juxtaposed aquifers, limiting the impact of mining on ground-water quality. The lateral extent of the argillaceous deposits varies among lignite mines in East Texas.