Water table recovery in a reclaimed surface lignite mine, Grimes County, Texas

Abstract

Water table recovery in four reclaimed mine blocks containing replaced overburden has been monitored at Gibbons Creek Lignite Mine in Grimes County, Texas since 1986. Recovery analysis was conducted based on data recorded at 27 wells installed in the reclaimed land and 23 wells installed in adjacent unmined land. It was found that water table recovery in reclaimed mine blocks is predictable: recovery is an exponential function of time and may be described by the following equation: Ew = RC log (t) + Eo where Ew equals any water table elevation above the mine floor to which recovery has occurred over the time, t, transpired between the time recovery began to the time Ew is attained. The constant Eo is the y-intercept which approximates the water table elevation at the beginning time of recovery, to referenced from the time of spoil replacement. The Recovery Coefficient (RC) is the average slope of the recovery curve. RC is proportional to inflow rate and the magnitude (potential saturated thickness) of water table recovery. As RC increases, recovery rate and/or magnitude increases. If recovery is uniform with respect to mine floor elevation, RC distributions for wells in a mine block can be standardized with respect to the mine block dimensions such that one RC value is attained for each mine block. RC is controlled by the complex interrelationships of several factors which may be described by the following factorial equation: RC= f (MD, HS, HP, MB, S 99 where MD = Mine block Dimensions, HS = Hydrostratigraphic Setting, HP = Hydraulic Properties of the spoil, MB = Moisture Balance for the mine area, and SW = Surface Water contribution to spoil resaturation. Based on the analyses the following conclusions were made pertaining to water table recovery at Gibbons Creek Lignite Mine: 1) rate of recovery does not appear to be controlled by the amount of sand in the pre-mine overburden, 2) surface water impoundments do not significantly recharge the mine blocks, 3) water table drawdown during mining can impact the local water table down-gradient of the mined land, 4) mining in several locations over an area composed of fluvial-deltaic sediments forces hydraulic connection of many of the stratigraphic units producing an unconfined water table aquifer from the pre-mine confined ground-water systems.