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ur4_anochiorine Contaminants in Fish of the Grand Valley (Grand Junction Area of Colorado) Introduction The Grand Valley is a large agricultural and residential valley in western Colorado through which flows the Colorado River. Irrigation laterals withdraw and return substantial volumes of water in this reach of the Colorado River. There have been numerous catches of the Colorado squawfish (a Federally endangered species) and razorback sucker (a Colorado state endangered species) in the Grand Valley reach of the Colorado River. Most of the fish were captured in gravel pits or irrigation return flows. where they were probably most susceptible to sampling equipment. The•area is significant to endangered species, as indicated by their year-around presence. There has been speculation that domestic, agricultural, and industrial pollution impede extensive natural reproduction and proliferation of these species-in the area (CR%P, 1982, Par t 1) . Reports of Colorado squawfish using washes have.persisted, but such use has never been documented (Grand Valley Report. 1982). Although squawfish have been caught in the vicinity of where washes enter the Colorado River, the attractiveness and importance of washes relative to other habitats is unknown (Chuck McAda. CRFP, personal communication). High total dissolved solids (TDS) levels in irrigation returns and ditches may reach levels that Colorado squawfish would avoid (CRFP, 1962, Part 1). Aauatic sampling for the Grand Valley Salinity Project Wildlife Monitoring Study raised the possibility of pesticide contamination in the Grand Valley. Sampling in 1981 and 1982 detected low fish populations in several streams and a scarcity of aquatic insects during the irrigation season. High pH and . conductivity contribute to the harsh conditions, but were not sufficient to explain the complete absence of aquatic insects in some sampleg during the irrigation season. Water quality measurements do not explain the rebound of invertebrate levels,in almost all samples during the post-irrigation season (November through March). It was hypothesized that pesticides were entering the water system either by direct application or by irrigation runoff during the summer season when pesticides are used. Areas outside the Grand Valley Salinity Project study boundary could also be contributing pesticides. These areas all drain into the Colorado River. The effect of toxavhene and other pollutants carried by these waters to the river ecosystem is unknown. In 1982, composite visceral fat samples from fish taken at several locations within the Grand Valley Salinity Control Project study area were submitted to the Colorado Eoidemiologic Pesticide Studies Center, CSU, for pesticide analysis. Toxaphene, at levels from 0.2 ppm to 61.5 ppm, was the only organochlorine compound identified. Laboratory results indicated traces of other organochlorine compounds, but aualitation and quantitation of these residues was not possible.