ScholarWorks at University of Montana

Mine tailings deposited by historic floods contaminate large areas of the upper Clark Fork River floodplain. Figure 1 (see below). The metals which are highly enriched over background levels in floodplain sediments include Cu (up to 1800 times) and As, Pb. and Zn (up to 80 times). Based on floodplain mapping of tailings, over 704,000 m3 of mine wastes are spread over 275 ha along a 10 kilometer reach between Warm Springs and Racetrack. Figure 2 (see below). Tailings are up to 1.2 m thick and occur primarily in fine-grained overbank deposits and in point bars as reworked mixtures of tailings and cleaner sediment. Although most flood-contaminated areas have less than 30 cm of tailings, the thickest tailings generally are near the river and have the highest probability of being eroded into the river. Metal concentrations in total, acid, soluble, and water-soluble extracts of floodplain sediments indicate that metals released by oxidation of sulfides in tailings move either to the ground surface and precipitate as hydrated metal sulfates or move downward to be concentrated in acid-extractable phases such as diagenetic sulfides and organic complexes in reduced tailings or pre-mining floodplain deposits Bioavailable (acid-extractable) concentrations of As, Cu, Fe, Pb, and Zn are very high in bank sediments. When eroded into the river, concentrations of these metals can exceed EPA aquatic-life standards. Cattle grazing has a deleterious effect on streambank vegetation and increases the extent of bank erosion and, therefore, the amount of metal-rich sediment in the river. The percentage of actively eroding streambanks increases from 2.5% in ungrazed reaches to 16-21% in grazed reaches.

Crusts of sulfate precipitates on streamside tailings dissolve readily in rainwater and release high concentrations of As, Al, Cd, Cu, Fe, Mn, Zn, and acid. A pollution index is proposed to quantify the average enrichment over aquatic-hazard levels of water-soluble metals, which occur in floodplain surface sediments, Figure 3 (see below).The pollution index correlates strongly with pH because much of the acidity produced during dissolution of surface salts is caused by the natural acidity of the transition metal (Cu, Cd, Mn, and Zn) ions released from metal sulfates. Because the pollution index can be predicted from pH, reconnaissance mapping of metal contamination in surface sediments on the Clark Fork River floodplain can he conducted quickly.

The downstream distribution of Cu, Cd, and Pb in fine-grained sediments and benthic insect larvae of the Clark Fork River, Montana is characterized. This river has been heavily con laminated as a result of past mining and smelling operations near its headwaters. Concentrations of all metals in bed sediments displayed a simple exponential downstream decrease through the upper 181 km of the river. The trend suggested metal contamination originated from source(s) in the headwaters, with physical dilution occurring downstream. Additional data suggested floodplain sediments also were contaminated by the original source(s). Secondary inputs from cutbanks in the floodplains may have extended the downstream influence of the contamination. The exponential model predicted that sediment contamination should extend at least 550 km downstream, a result that was verified with data from a separate, independent study. Metal contamination, as observed in all taxa of insect larvae collected from the upper Clark Fork. Concentrations in the insect larvae were highest in the upper 100 km of the river, but downstream trends were more complex than those of the sediments. Some differences in trends occurred among taxa and metals. Areas in the river of enhanced or reduced metal contamination also were apparent. Metal contamination, however, was still evident at 381 km, the most downstream station sampled. Metal concentrations in sediments and insects decreased at the confluences of uncontaminated tributaries, but the influence of tributaries on metal contamination in the Clark Fork River was localized, extending for only 1-2 kin below the confluences.

A computer flow model of the Missoula aquifer was constructed as part of the initial phase of the EPA Wellhead Protection Program. The goal of the modeling effort was to develop a regional flow model that would; 1) provide detailed information on ground water flow directions and velocities, 2) simulate the effects of recharge to, and discharge from, the aquifer over time, 3) assist in defining key hydrologic parameters of the aquifer, and 4) provide a tool for future miu1ilgement decisions regarding the aquifer.

As part of the project, all available water level data from February 1986 to October, 1989 which were useful to the project were combined in one database. Monthly water level surface maps were then constructed for a one-year period (February 1986 to January 1987) based on observed water levels at about 100 1ocations.

Five pump tests were conducted on municipal supply wells using nearby observation wells and precise water level measuring techniques to obtain time verses draw down data. Values of transmissivity and hydraulic conductivity were obtained which are believed to be accurate within plus or minus 20%.

A computer flow model of the aquifer was constructed which simulated the water level surfaces observed for the period February 1986 to January 1987. The model was successfully calibrated using hydraulic conductivity values obtained from the project pump tests. The model simulated leakage from the Clark Fork River into the aquifer in the eastern portion of the study area, and return flow from the aquifer to the river in the western portion of the study area. Outflow from the aquifer to the Bitterroot River was also simulated. Based on model results, all approximate ground water budget of inflows to the aquifer and outflows from the aquifer was developed.

Under Section 525 of the 1987 amendments to the federal Clean Water Act, Montana initiated an intensive monitoring program to identify and rank the major point and nonpoint sources of nutrients to the Clark Fork River. A 51 station monitoring network was established, including 19 stations on the Clark Fork River, 22 stations on tributary streams, and 10 municipal and industrial wastewater discharges to the river. In the first year of monitoring, samples were collected 15 times and analyzed for total and soluble forms of phosphorus and nitrogen.

Several small tributaries to the upper Clark Fork (Gold, Flint, Lost, Racetrack, and Dempsey creeks and the Mill-Willow Bypass) and all 10 wastewater discharges exhibited elevated nutrient concentrations. The Missoula, Butte, and Deer Lodge municipal wastewater discharges were responsible for the largest nutrient concentrations in the Clark Fork. Inflows from good quality tributaries such as Rock Creek and the Blackfoot, Little Blackfoot, Bitterroot, and Flathead rivers were important in diluting nutrient concentrations in the Clark Fork.

Overall, soluble phosphorus loading to the Clark Fork originated about equally from tributary inflows and wastewater discharges. About two-thirds of the soluble nitrogen loading came from tributaries, with effluents contributing the remaining third. During the summer low flow period, an even greater share of the soluble nutrient loading came from effluents.

Tributary sources of soluble nutrient loading were dominated by the Flathead, Bitterroot, and Blackfoot rivers. Gold and Flint creeks ranked fourth in importance as tributary sources of soluble phosphorus and nitrogen, respectively.

The Missoula, Butte, and Deer Lodge municipal wastewater treatment plants and the Stone Container Corporation kraft mill discharged most of the soluble nutrient loading from effluents. However, the Warm Springs treatment ponds on Silver Bow Creek removed most of the Butte nutrient load prior to reaching the Clark Fork River.