Acid-generating salts and their relationship to the chemistry of ground water and storm runoff at a pyritic coal refuse deposit in southwestern Indiana

Ground- and surface-water data were collected from a small watershed at the Friar Tuck Abandoned Mine Site between April 1988 and November 1989 in an effort to understand the physical and chemical processes creating acid mine drainage problems in nearby streams. These data revealed that storm runoff was largely produced by Hortonian over-land flow and contained concentrations of total dissolved solids that as great as 8260 mg/L. Ground water did not contribute to channel flow but was discharged through a remote zone of seeps located downgradient from the study watershed. Hydrated iron sulfate minerals that occurred abundantly on the refuse-pile subface were hypothesized as the source of solutes and acidity in storm runoff.;This thesis describes the results of field and laboratory analyses that establish quantitative relationships between the hydrated iron sulfate minerals and the chemistry of the ground water and storm runoff. In August 1989, twenty samples of hydrated iron-sulfate minerals were collected from the surface of the study watershed. Bulk mineralogy was determined with x-ray diffractometry and the chemistry of dissolved salt solutions was determined using ion chromatography and inductively coupled plasma atomic emission spectroscopy. A computer algorithm was developed to quantitate the salts mineralogy based on the x-ray diffraction patterns and the chemical analyses. Cation distributions in dissolved salt solutions were compared with those in samples of ground water and storm runoff in an effort to identify source-product relationships between the solid and aqueous phases.;The analyses showed that two mineral suites, one primarily composed of melanterite, rozenite, and szomolnokite, and the other composed almost entirely copiapite, were most abundant at the study site. Iron accounted for 92 mole percent of the cations in the melanterite-rozenite-szomolnokite mineral suite. The copiapite-rich suite had larger concentrations of aluminum, calcium, and zinc and only about 81 mole percent iron.;Comparisons were made between the cation distributions in solutions of dissolved salts and samples of ground- and surface-water and evaluated in light of other field-generated information that include soil moisture profiles, streamflow, overland flow, and precipitation data, and complete chemical analyses of ground- and surface-water samples. These comparisons indicate that the copiapite-rich suite precipitated from vadose zone ground water that was brought to the surface by evaporative forcing. During rainfall events the copiapite-rich suite served as the primary source of solutes in storm runoff. Monitoring instream chemistry during a short summer thunderstorm showed that solute inputs from dissolution of the copiapite-rich suite varied little with time or with distance downstream.;Although the melanterite-rozenite-szomolnokite mineral suite appeared abundantly in the watershed, it showed no similarity to cation distributions in surface water samples and only appeared to effect water chemistry in the unsaturated zone during the early part of the recharge season.