Temporal and spatial sulfate variability in groundwater at a lignite mine, northeast Texas
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Sulfate (SO₄ ²⁻) concentrations in the groundwater at the Luminant Monticello lignite mine, northeast Texas, vary with both time and space. SO₄ ²⁻ is monitored closely because it is a product of chemical reactions that can lead to acid mine drainage. Although acid mine drainage is not a problem at Monticello, SO₄ ²⁻ concentrations in some areas are high and correspond to high total dissolved solids (TDS), low pH groundwater. At Monticello, chloride, total dissolved iron, and total dissolved manganese concentrations also are variable. To examine this variability, 46 monitoring wells are divided into those: 1) screened in rock underneath the lignite seam (underburden); 2) screened in the rock over the lignite seam (overburden); and 3) screened in the reclaimed spoil. There is a wide range of SO₄ ²⁻ concentration across the mine with statistically significant differences between the SO₄ ²⁻ distributions for each well category. Three hypotheses may explain the spatial SO₄ ²⁻ variability: (1) The heterogeneity of the mined material (2) Flushing of SO₄ ²⁻ from the reclaimed section into the overburden (3) Exposing iron sulfides to oxidizing conditions when the water table is dropped by mine dewatering operations. Possible sources of SO₄ ²⁻ include the oxidation of pyrite and/or the dissolution of gypsum. Temporal SO₄ ²⁻ variability is evident in the SO₄ ²⁻ time series for each well. In the underburden, SO₄ ²⁻ seems to be approaching a steady state; however, some wells in the unmined overburden and reclaimed area have SO₄ ²⁻ trends that are increasing or are highly variable with time. Water table fluctuations or flushing caused by seasonal meteorological changes may control these trends. Correlation analyses show that for most underburden, overburden, and reclaim wells, SO₄ ²⁻ does not correlate with pH, indicating that acidity does not accompany the incorporation of SO₄ ²⁻ and that SO₄ ²⁻ may be advected from another part of the mine. Analysis of water level and SO₄ ²⁻ time series, as well as Cl⁻ and SO₄ ²⁻ time series, show that advection from reclaimed areas cannot account for every high SO₄ ²⁻ value or increasing SO₄ ²⁻ concentration in the overburden and reclaim. Groundwater in most overburden and reclaimed areas are at equilibrium with gypsum, indicating gypsum precipitation if SO₄ ²⁻ is flushed into the area and gypsum dissolution if SO₄ ²⁻ is flushed out. Time series analyses shows some of the wells with increasing SO₄ ²⁻ concentration to be approaching equilibrium. Also, most SO₄ ²⁻ time series have a seasonal component. Flushing from the reclaimed areas is considered to be the dominant process controlling SO₄ ²⁻ variability in the overburden. A combination of flushing and aqueous geochemical processes related to heterogeneity controls the SO₄ ²⁻ concentration in the reclaimed land.