A STATISTICAL COMPARISON OF PRE- AND POST-DISTURBANCE WATER QUALITY AT OAK HILL MINE, A LIGNITE SURFACE MINE IN NORTHEAST TEXAS

Near-surface lignite deposits in Texas are primarily associated with the Eocene-age Wilcox Group and lie within the recharge zone of the prolific Carrizo-Wilcox aquifer. The presence of low pH shallow groundwater and seeps has delayed the release of some surface mined lands from bond. Low pH (acidic) waters can adversely affect water quality because metal cations such as iron, manganese, and aluminum are soluble in toxic amounts at low pH levels. Surface mining can result in depression of water pH when dewatering and stripping of overburden change the redox conditions of the sediments. These new conditions then begin or enhance the oxidation and hydrolysis of reduced iron sulfide minerals, releasing hydrogen cations (acidity). Therefore, state and federal regulations require that surface waters draining reclaimed mines have a stable pH between 6 and 9. However, the Sabine Uplift region in northeast Texas where some lignite surface mines operate has a history of low pH surface water and groundwater seeps that pre-dates lignite mine production. This study evaluates whether surface mining and reclamation degrades key water quality parameters in areas where weathering of iron sulfide-rich geologic units (the Upper Wilcox Group, the Carrizo Sand, and the Reklaw Formation) may have already led to natural deterioration of these same key parameters. Applying the Wilcoxon Rank Sum Test to a portion of my dataset at the Oak Hill Mine in Rusk County, northeast Texas reveals that some areas at the mine have experienced a statistically significant change in ground or surface water pH following mining activities, whereas other sites have not. I hypothesize: 1) that lignite pits crossing the Carrizo Formation (Lower Claiborne Group) have not created a statistically significant drop in pH because the iron-sulfide mineral inclusions in this highly porous formation were already undergoing oxidative dissolution under pre-mine conditions, or alternatively, 2) that these lignite pits may have produced a statistically significant drop in pH, but that the magnitude of the change is small and not ecologically significant. These hypotheses, which are tested by analyzing water quality data collected from the Oak Hill Mine and correlating these data in GIS with geologic maps of the region, are not disproven by this study.