SIR 06-5265: Abstract
Frohner Meadows lies in a low-topographic-gradient glaciated terrane near the headwaters of Lump Gulch, about 15 miles west of the town of Clancy, Mont., in the Helena National Forest. During the last 120 years, ore was extracted at two sites (the Frohner mine and the Nellie Grant mine) upstream from Frohner Meadows. Lead-zinc-silver veins in granitic rocks of the Boulder batholith were mined, and the ore was treated nearby. As a result, accumulations of mine waste and mill tailings were distributed downslope and downstream by anthropogenic and by natural processes.
This report presents the results of an investigation of the geochemistry of the wetlands, streams, and unconsolidated sediment deposits and of the hydrology, hydrogeology, and water quality of the area affected by these sources of ore-related metals. Ground-water samples from mostly shallow wells in the meadows contained high concentrations of arsenic, which exceeded the Montana numeric water-quality standard for human health. Transport of cadmium and zinc in ground water was indicated at one site near Nellie Grant creek (this informal name and Frohner Meadows creek, below, are applied for convenience to unnamed tributaries) on the basis of water-quality data from one well near the creek. Mill tailings deposited in upper Frohner Meadows contribute large arsenic loads to Frohner Meadows creek; Nellie Grant creek contributes large arsenic, cadmium, and zinc loads to upper Frohner Meadows. Concentrations of total recoverable cadmium, copper, lead, and zinc in most surface-water sites downstream from the Nellie Grant mine exceeded Montana aquatic-life standards and consensus-based probable effect concentrations in the streambed sediments. The pH of nearly all samples of surface water and ground water was neutral to slightly alkaline.