01

10,000

DENSITY OF 2.90

DEPTH

salinity increases in the water in the lower Calcasieu River (a phenomenon termed "salting out").

Other organic compounds, such as naturally occurring humic and fulvic compounds, show similar "salting out" tendencies. This is shown in the decrease in DOC from the freshwater reach of the river, where DOC values ranged from 5 to 6

mg/L, to the brackish and saltwater reaches of the river, where DOC values decreased to 3 to 4 mg/L. This decrease in DOC occurred in the fraction that has an affinity for suspended sediment, while the fraction that has an affinity for water remained constant. In the process of "salting out," the sediment-loving fraction of the DOC may also facilitate the removal

the information necessary to assess permit applications and formulate restoration schemes for this complex aquatic environment.

Arsenic Contamination

of the Cheyenne River System, Western South Dakota

By Kimball E. Goddard

The 1960's brought an awareness of the serious environmental hazards and economic damage that could result if our Nation's rivers and streams continued to be used for disposal of waste. Resulting legislation, such as the Federal Water Pollution Control Act Amendments of 1972 (P.L. 92-500), the Toxic Substances Control Act of 1976 (P.L. 94-469), and the National Pollutant Discharge Elimination System (NPDES) Permit Program, provided the impetus for a nationwide cleanup. As a result, the quality of the Nation's rivers and streams has improved substantially in several respects, reflected by increased dissolved-oxygen concentrations and decreased bacterial contamination. These improvements are largely attributable to measures for controlling point-source discharges and the construction or improvement of wastewater-treatment facilities. Numerous other water-quality issues remain, however, and the focus of concern is now shifting from sewage disposal to the control of potentially more hazardous wastes such as toxic metals and synthetic organic compounds.

Recent studies of the geochemical behavior of toxic metals and synthetic organic compounds in river and stream systems have demonstrated that these constituents are commonly associated with river or stream sediments. The extremely low solubilities of some metals, such as mercury and lead, in stream water and the affinity of many synthetic organic compounds to adhere to or adsorb to sediments generally result in undetectable dissolved concentrations of these constituents in sur

face water. At the same time, the geochemical behavior of these constituents may cause large enough concentrations to accumulate in bottom sediments that the constituents can be absorbed and (or) ingested by aquatic plants, benthic organisms, or bottom-feeding fish. Although toxic constituents concentrated and buried in bottom sediments may be isolated from the environment for long periods, they can be resuspended into the water column and transported during floods. The adsorption of hazardous constituents onto channelbottom sediments exacerbates the difficulty of understanding the processes responsible for the movement and fate of these constituents in river systems.

In 1985, the USGS began to investigate how hazardous substances associated with river sediments react in surface-water systems. The Cheyenne River System in western South Dakota was one area selected for detailed study (fig. 9). Whitewood Creek and downstream reaches of the Belle Fourche and Cheyenne Rivers have been extensively contaminated by mine tailings and mill wastes from gold mining operations located in the northern Black Hills of South Dakota.

Gold was discovered in the Black Hills in 1874 and, by 1876, full-scale hardrock mining was underway. Although mining and milling technology have changed over the last 100 years, the basic approach remains the same: ore is pulverized to fine sand- and silt-size particles and then treated with elemental mercury or sodium cyanide solutions to remove the gold. The remaining tailings slurry, along with wastewater from the mills and water pumped from mines, is discharged to a stream. The practice of discharging mine and mill wastes to Whitewood Creek or its tributaries in the Lead and Deadwood area continued until 1977, when a tailings dam was completed. No tailings solids have been discharged to the streams since that time.

About 100 million tons of mill tailings are estimated to have been discharged to Whitewood Creek. The mill tailings were transported by natural surface-water flow down Whitewood Creek to the Belle Fourche River, to the Cheyenne River, and finally to the Missouri River. Alluvium in the channel and along the floodplains of Whitewood Creek and the downstream reaches of the Belle Fourche and Cheyenne

1100

mineralogic data indicate that a substantial proportion of the existing arsenic coprecipitated with or adsorbed onto iron oxide and other metallic oxides. The transformation from a reduced form (arsenopyrite) to an oxidized form (sorbed onto ferric hydroxide) determines the stability of arsenic in the environment.

Arsenopyrite is relatively unstable in an oxidizing environment, such as in active stream sediments or in shallow floodplain soils or sediments. In these environments, arsenopyrite will oxidize and form ferric hydroxide and arsenic ions. Arsenic ions produced by the oxidation of arsenopyrite will always be precipitated out or be sorbed by the ferric hydroxide and will not be available for solute transport. If deeply buried in floodplain deposits, however, arsenopyrite can remain unaltered for long periods.

Ferric hydroxide is quite stable in an oxidizing environment. If ferric hydroxide is buried in a reducing environment such as a thick, organic-rich floodplain deposit, however, it will dissolve and release sorbed arsenic into the ground water. Reducing conditions commonly exist in large deposits of contaminated sediment, suggesting that the reduction of ferric hydroxides, rather than the oxidation of arsenopyrite, causes the large dissolved-arsenic concentrations present in water in some alluvium.

Dissolution of ferric hydroxides in alluvium forms a pathway for arsenic migration. Instead of being adsorbed on solids,