Hydrogeology of an Alkaline Fly Ash Landfill in Eastern Iowa

The hydrogeology and chemical quality of the shallow ground‐water regime at a coal fly ash landfill was investigated near Montpelier, Iowa. An embankment dam retains the two‐hectare deposit of silt‐sized alkaline coal fly ash in an upland ravine. The fly ash landfill was operated between 1964–;1973 and was subsequently capped with a thin loess layer and seeded to pasture. The ash is underlain by loess, over clay‐rich till, over sandstone. Ground water now saturates the lower one‐half of the ash. Nineteen shallow monitoring wells were installed around and within the landfill. Sampling in 1983–;1984 was conducted for water levels, temperature, pH, alkalinity, specific conductance, calcium (Ca 2+ ), magnesium (Mg 2+ ), sodium (Na + ), potassium (K + ), sulfate (SO4 2− ), chloride (Cl − ), arsenic (As), and selenium (Se). Calculated ion balances for 30 analyses had an error of less than 4%. Native loess‐derived ground water of a calcium bicarbonate type enters the landfill, and then shifts to a calcium sulfate type. A distinct leachate plume presently extends at least 46 meters downgradient from the landfill, passing under and through the dam, then discharging into a small pond. SO 4 2− and Se concentrations in the plume exceed EPA drinking‐water standards, and trace As was detected. Although ground‐water pH increased after entering the landfill, bicarbonate alkalinity declined. Results of equilibrium solubility calculations suggest that this condition evolved from calcite supersaturation within the landfill, precipitating calcium carbonate. Dissolution of calcium and magnesium oxides on the glassy fly ash spheres sustains the highly alkaline leachate strength.