High dissolved-iron concentrations have been noted in water in wells used to monitor effluent that percolates from wastewater disposal lagoons near Old Faithful in Yellowstone National Park. The concentration of dissolved iron in water in a well increased from 80 μg/L (micrograms per liter) before a nearby lagoon was .used for disposal of effluent to 17000 μg/L after the lagoon was used. The effluent contained 180 μg/L of dissolved iron, and nearby Iron Spring Creek contained 30 μg/L or less of dissolved iron above and below the lagoons. Organic carbon, nitrogen, and sulfur in the effluent as methane, ammonia, and hydrogen sulfide are oxidized to carbon dioxide, nitrate, and sulfate, respectively, in the unsaturated zone and possibly in the saturated zone as ground water moves through sand and gravel toward Iron Spring Creek. This oxidation results in simultaneous reduction of iron in the sand and gravel from the insoluble ferric phase to the soluble ferrous phase. As ground water high in dissolved iron discharges at land surface near the stream, oxygen from the atmosphere oxidizes the iron back to the insoluble ferric phase, and ferric hydroxide precipitates. Ferric hydroxide also precipitates in some of the monitoring wells. Iron bacteria and other organisms are associated with the precipitates.
