Estimating surface/ground-water mixing using stable environmental isotopes

Abstract

The stable isotopes of oxygen, hydrogen and sulfur were used to establish the mixing relationships between ash-pond waters and ground water at the Utah Power & Light, Company Power Plant in Lincoln County, Wyoming. This coal-fired plant discharges ash-laden water which is isotopically distinct from the local ground water. These environmental isotopes function well as tracers of pond leakage and mixing of discharged water with ground water. The plant is within the southwestern part of the Wyoming thrust belt and lies on west-dipping (10-20 0 ) beds of the Late Cretaceous Hilliard Shale. This marine shale contains abundant gypsum along partings and fissures. Coal for the plant is mined from the Late Cretaceous Adaville Formation, which lies stratigraphically above the Hilliard Shale. The local ground water is non-potable, containing up to 20,000 mg/1 of total dissolved solids (TDS). Sulfate accounts for over 60 percent of the TDS in this predominantly calcium-sulfate water. Ash-pond waters are lower in TDS, generally not exceeding 1000 mg/1, of which sulfate is about 50 percent of the dissolved solids. The plant-process water is obtained by pipeline from the Ham's Fork drainage, ten miles to the north. This water yields discretely different oxygen and hydrogen isotopic values compared to the local ground water. This difference is further enhanced by evaporative enrichment occurring within the discharge ponds. Two isotopically distinct sources of sulfur are observed in this system: (1) sulfate from the combustion of coal, and (2) sulfate from the distribution of gypsum in the Hilliard Shale. Estimates of mixing between discharge water and ground water were calculated from the isotope data. These calculations indicated that, although the plant is contributing substantial volumes of water to the aquifer, ground water is diluted by the mixing of less saline discharge water. This study demonstrates that stable environmental isotopes can be used as estimators of surface/ground-water mixing, especially when isotopically distinguishable sources can be identified.