of HOC's from the water column to bottom material by providing additional organic material for the HOC to attach to in the bottom material. Therefore, owing to the decrease in solubility of HOC's in saltwater and the presence of natural organic carbon, bottom material in the lower Calcasieu River serves as a major trap for these compounds.

The "salting-out" effect in the lower Calcasieu River also moderately enhances the concentration of the organic compounds into biota and bottom material. Concentrations of HOC's in biota and water were well below their equilibrium values relative to concentrations in bottom material. This lack of equilibrium between HOC concentrations in bottom material and the water column suggests a limited amount of exchange that may result from the slow diffusion of the sorbed contaminants from bottom sediments. In contrast, HOC concentrations in water, biota, and suspended sediments were much closer to equilibrium values. Concentration factors of HOC's in four fish and shellfish species (Atlantic croakers, spotted sea trout, blue catfish, and blue crabs) indicated that these compounds passively entered the tissues of the fish and shellfish, owing to diffusion of the compounds throughout their habitat. More information is needed, however, to determine if the compounds enter the tissues by

other routes, such as actively through the digestive tract of the species.

The molecular configuration of haloarenes and hexachlorobutadiene also provides an explanation for the longitudinal differences in concentrations of compounds observed in bottom material collected from the lower Calcasieu River. The higher weight molecular compounds, such as hexachlorobenzene, are more insoluble in water, and readily adhere to particulate matter compared with the more soluble lighter weight molecular compounds such as 1,3-dichlorobenzene. Therefore, the more chlorinated compounds (hexachlorobutadiene and hexachlorobenzene) accumulate in bottom material closer to the source than the less chlorinated compounds (dichlorobenzenes and trichlorobenzenes) do. In both cases, however, solubilities are low and transport is localized, so distribution of these compounds in the lower Calcasieu River bottom material is not widespread.

Current (1987) work with organic compounds includes studies of possible mechanisms for remobilization of these compounds into the water column from bottom material under different environmental conditions and of the long-term uptake of organic contaminants by aquatic organisms to determine the extent of passive uptake in these organisms. Results from the lower Calcasieu River project will provide State and Federal environmental agencies with

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

Rivers is contaminated by the mill tailings. Some alluvium is nearly 100 percent mill tailings. Although the tailings are composed largely of silicate minerals, they also contain the metallic sulfide minerals pyrite, pyrrhotite, and arsenopyrite. The primary contaminant of interest is arsenic, derived from arsenopyrite, a matrix mineral common in the gold-bearing ore. Owing to the substantial concentrations of arsenopyrite present in the mill tailings, alluvium in the channels and along the floodplains of Whitewood Creek and the downstream reaches of the Belle Fourche and Cheyenne Rivers contains large concentrations of arsenic, as much as 11,000 micrograms per gram (μg/g). This contrasts with arsenic concentrations measured in uncontaminated sediments in the Cheyenne River basin that average about 10 μg/g. Arsenic concentrations measured in sediment samples randomly collected from the floodplain of the downstream reach of the Belle Fourche River are shown in figure 10, and the marked contrast between the uncontaminated (above Whitewood Creek) and contaminated reaches is apparent.

The large variation in arsenic concentrations within the contaminated reach of the Belle Fourche River floodplain demonstrates the difficulty of studying the distribution of contaminants associated with alluvium. That the collection of a few samples provides little information on how the contaminant (in this case arsenic) is distrib

uted in the system is clear. These variations are a cumulative result of physical dispersion of the mill tailings and their uneven intermixing with uncontaminated alluvium, geochemical geochemical transformations between various solid and aqueous phases of arsenic, bias caused by variations in grain-size distributions in individual samples, and analytical imprecision resulting from differences in mineral composition of individual samples.

Physical dispersion of the mill tailings is considered to be the primary cause of contaminant variation. During downstream transport, the mill tailings are intermixed with natural alluvium to varying degrees depending on flow regime and the tributary contribution of uncontaminated sediments. Deposits of intermixed mill tailings and natural sediments formed during periods of low discharge, when the concentration of naturally occurring suspended sediment is minimal, consist almost entirely of tailings. Conversely, deposits formed during flood flow, when huge quantities of natural suspended sediment are available for intermixing, contain only a small amount of tailings.

Geochemical transformations, between various solid forms of arsenic and between its solid and aqueous forms, create additional pathways for the transport of arsenic. Although the entire arsenic mass originally was present in the mineral arsenopyrite, onsite observations and

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,

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the released arsenic moves with the ground water and eventually discharges to rivers and streams. The dissolved arsenic released to the rivers and streams is again adsorbed, this time onto sediments present in the active channel. The release of arsenic from floodplain deposits, arsenic transport through the ground-water system, and arsenic discharge to rivers and streams are currently (1987) under intensive investigation (fig. 11).

The occurrence and magnitude of arsenic contamination in Whitewood Creek and the Belle Fourche and Cheyenne Rivers are now controlled by natural processes acting on the tailings-contaminated sediments. The contamination of alluvium affects the surface-water system by direct introduction of arsenic-rich sediments from bank collapse, from overland runoff, or from channel scouring. Transformations between solid and aqueous forms in the alluvial aquifer also contribute arsenic to rivers and streams and to active channel sediments. Although the contamination was caused originally by the direct discharge of mill tailings to Whitewood Creek and its tributaries, which ceased 10 years ago, continued contamination of the rivers and streams cannot be completely mitigated. Unlike many surface-water contamination problems of the past that have been mitigated or eliminated by controlling contaminant discharge and construction of new treatment facilities, widespread contamination of river and stream sediments by