INTRODUCTION

Prudhoe Bay, on the Beaufort Sea coast in Arctic Alaska, is the site of the largest oil field ever discovered in the United States or Canada (Figure 1). The Prudhoe Bay area serves as the center for North Slope oil and gas exploration as well as a focal point for crude oil production. Associated with oil production activities is the possibility of environmental pollution. One source of contaminants is drilling fluids from reserve pits,

also known as sumps.

Reserve pits associated with oil production on the North Slope are generally above-grade basins within gravel drilling pads. The pits are designed to hold drill muds, cuttings, and wastewaters from production drilling. They may also contain contaminated or uncontaminated snow, material from local oil spills, and other drilling wastes.

Reserve pit contents vary considerably, since more than 600 trade name materials, including thousands of components, are available for use (Dames & Moore 1978; Wright and Dudley 1982). Only a few components are typically used, however, in any particular oil field or individual well. The various components serve as weighting agents; viscosifiers, thinners, pH and ion controls, dispersants, corrosion inhibitors, lubricants, emulsifiers, foamers and defoamers, and as flocculants.

The geological formation being

drilled also influences pit chemistry, as do the kinetics of the drill fluida given the local pl, temperature, and other variables. Rapid chemical reactions that occur during drilling at high pressure and temperature are followed by slower changes in the pit, such as those resulting from decomposition by photochemical or other oxidation processes (National Research Council 1975; Karrick 1977), emulsification (Berridge et al. 1969), and microbial degradation (ZoBell 1973; Atlas 1975, 1981). In addition, the phase and location of different chemical species may vary depending on polymerization, sedimentation, freeze exclusion, flocculation, scavenging by and complexing with other compounds, resuspension by wind nizing, and entrainment in ice slurries or other substances.

National standards for permissible onshore discharges have not been developed, nor has a consensus on water quality for discharge of pit fluids boen achieved by the states, due to the complexities of reserve pit chemistry, both within and between oil fields. Before 1983, pit fluide regularly entered nearby wetland habitats in the Prudhoe Bay area by seeping through reserve pit walls and bottoms, overtopping of pits, breaching (failure) of the berms, road dust control with the fluids, and deliberate discharge from the pits (dewatering) to adjacent ponds or wetlands. Beginning in 1983, the Alaska Department of Invironmental Conservation (ADEC) began regulating discharges and granted a variance for disposal based on certain restrictions. Dewatering was prohibited if there was a visible oil sheen on the water surface in the pit or "if toxic substances or salt concentrations exceed those expected to cause damage to vegetation, fish and wildlife, or could affect public health." ADEC further stipulated that "in

surface water receiving these discharges” State Water Quality Standards described in 18 ACC 70 not be violated. Subsequently, oil industry data on all discharges were requested. In June 1984, additional, more specific water quality standards for direct tundra discharge on the North Slope were promulgated in a General Permit, including a total settleable solids limit of 0.2 mL/L and a salinity limit of 3000. In addition, ADEC required that pits not be discharged until one year after the last input of drilling effluent or other wastes.

During summer 1983, the U.S. Fish and Wildlife Service began a study of the effects of pit fluid discharges on water quality and the macroinvertebrate community of tundra ponds. Water quality variables included pH, conductivity, dissolved oxygen, hardness, alkalinity, and turbidity. Water samples were also analyzed for arsenic and metal concentrations (aluminum, barium, cadmium, chromium, copper, lead, mercury, nickel, selenium, and zinc), as well as for hydrocarbon concentrations. Samples were collected from reserve pits, from ponds initially receiving reserve pit fluid discharges, and from more distant connected ponds. Three remote ponds (controls) were also examined. Aquatic macroinvertebrates were studied because they are resident species, sensitive to local environmental changes, and widely recognized as sensitive indicator organisms for a variety of environmental pollutants (Hilsenhoff 1977; Buikema and Cairns 1980; American Public Health Association (APHA) et al. 1981).

The North Slope serves as an important nesting, rearing, molting, and feeding ground for about 150 species of sea birds, shorebirds, waterfowl,

raptors, and passerines (Norton et al. 1975). The study was undertaken because aquatic macroinvertebrates are an important food of any of the water birds in the Arctic (Pitelka 1959; Holmes 1966; Holmes and Pitelka 1968; Hilden and Vuolanto 1972; Bergman et al. 1977; Derksen et al. 1981; Connore 1983). The overall availability of invertebrates for food sa an important consideration in rating suitability of wetland habitat for birds. Female ducks require a high protein diet during the nesting season (Moyle 1961; Bengston 1971) and their young have similar requirements for fast growth (Chura 1961; Collins and Collins 1963; Bartonok 1972). This high protein diet is generally supplied by aquatic macroinvertebrates in early oummer before the emergence of adult insects.

We compared mater quality and contaminent concentrations with aquatic invertebrate richness and abundance patterns to identify habitat

We soomed that any chronic detrimental effects en bird populations any not observable for a number of years and that the degree of these effects will depend on decline of food organions as well as the bioaccumulation and blomagnification of contaminants, if these processes are occurring.

STUDY AREA

The Prudhoe Bay area is characterised by flat open expanses of wet and, noist tundra vegetation dominated by the sedge Carex aquatilis and the grass Arctophila fulva (Walker 1981). Shallow lakes and countless shallow ponda cover the area, which is entirely underlain with permafrost. Much of the area is flooded when the snow melts at breakup, usually in June. Surface sheet flow is supplemented by unter transport through systems of interconnected polygon troughs that delineate the patterned ground. Drier areas are found mostly along shores of drained lake basins and on raised pukygon rims, but even those areas may be flooded at breakup if the ccumulation of heavy on if melting is rapid.

The entire Prudhoe Bay area is traversed by a read system and pipelines in the mid 1900s included more than 70 drill sites (~ pada), as well as development centers and related facilities, all built on a layer of compacted gravel. Three north-south river systems occur in the Prudhoe Bay area--the Sagavanickick, Putuligayak, end Emparuk rivers.

We selected sample pond ettes on the basis of: (1) their proximity to discharge site where direct or indirect discharges had occurred during