CONTROL OF IRON AND TRACE-METAL MOBILITY IN AN ALLUVIAL AQUIFER AFFECTED BY ACIDIC ROCK DRAINAGE

Shallow ground water in a floodplain contaminated by acidic rock drainage (ARD) derived from fluvial mine-tailings deposits near Leadville, Colorado, was studied to delineate the processes controlling metal mobility in the aquifer. Dissolved metals associated with the ARD included copper, iron, lead, manganese, and zinc. Elevated concentrations of copper (from 0.05 to greater than 1 milligram per liter [mg/L]) and lead (from 0.05 to greater than 1 mg/L) were spatially associated with acidic ground water that occurred underneath deposits of fluvial mine tailings. In contrast, elevated concentrations of iron (greater than 10 mg/L) and manganese (greater than 1 mg/L) were spatially associated with acidic conditions in the vicinity of the fluvial-tailings deposits but also occurred in “background” areas in water having near-neutral pH, dissolved organic carbon and sulfide, and high proportions of ferrous to ferric iron. The sources for iron and manganese in the background area were naturally occurring minerals in the aquifer that are unrelated to mining activity. Reducing conditions in the background area mobilized the iron and manganese. Thus, elevated concentrations of iron (and manganese) in this alluvial aquifer are related both to acid-producing source material and to reducing conditions in “background” areas. In addition, colloidal iron in portions of the aquifer affected by ARD enhanced the mobility of copper and lead in the aquifer. At these locations, filtered concentrations of copper, iron, and lead were as much as 100 percent less than total concentrations. Trace-metal mobility in the aquifer is controlled by (1) proximity of source material for the metals, (2) maintenance of pH and redox conditions that promote metal mobility, and (3) the presence of mobile iron colloids in the aquifer that enhance transport of the metals.