Prediction of postmine ground-water quality at a Texas surface lignite mine

Abstract

The prediction Of postmine ground-water quality is encumbered with many complications resulting from the complex hydrologic system found in mine spoils. Current analytical methods such as acid/base accounting have only had limited success. The predominant factors which affect spoil water quality have not been completely identified to date. Therefore, the Gibbons Creek Lignite Mine in Grimes County, Texas was chosen as a test site to evaluate the potential factors that can affect the geochemical evolution of ground water in spoil. Water quality was monitored in two mine blocks and combined with data accumulated since 1986. Monitoring results were used to identify geochemical trends and were coupled with chemical modeling of the data using MINTEQA2, an equilibrium speciation model. Mine spoil was also sampled and analyzed in the lab for both water content and water quality using batch-mixing tests. Results of the analyses led to the formation of a ground-water quality factorial equation: GWQ = f(RR, SR, AR, OL, SP, SM, ED, RT, SA) where GWQ is the ground water quality, RR is the rate of recharge, SR is the source of recharge, AR is the amount of recharge, OL is the overburden lithology, SP is the spoil permeability, SM is the spoil mineralogy, ED is the environment of deposition of the overburden, RT is available reclamation techniques, and SA is the spoil age. The factorial equation qualitatively describes the factors which influence spoil water quality. The first three factors are controlled by the fluid medium and play a principal role in the rate and degree of oxidation of the spoil material. The following four factors are properties of the spoil medium and primarily control the mineral chemistry. The remaining two factors are the result of mining activities. Results appear to indicate that the factors governed by the fluid medium have the most influence upon the evolution of the ground-water quality. Spoil water quality at Gibbons Creek is predominantly controlled by the rate of groundwater recharge and the lithology of the overburden. Preceded by a premine evaluation of the geologic and hydrologic character of the mine site, the conceptual model proposed can be used to qualitatively predict the ground-water quality at a surface lignite mine.