Summary

Description

Levels and potential effects of pesticides and metals on fish and wildlife of the lower Gila River and associated agricultural drainage canals in Maricopa County, Arizona, were investigated in 199495. Residues of DDT, an insecticide that has been suspended from use in Arizona for 25 years, are still present in fish, turtles, and birds. DDT was detected in 86, 62, and 50 of the fish collected from Buckeye Canal, Gillespie Dam, and Allenville, respectively. DDT was recovered in all turtles from Gillespie Dam, and in all blackcrowned nightherons from Painted Rock. DDE residues in fish from the lower Gila River drainage were the highest in the United States associated with agriculturally applied pesticides. Although DDE residues declined over the past decade; current levels remain extremely high. DDE was present at concentrations known to impact biotic resources. Fish collected from agricultural drainage canals generally contained higher DDE residues than fish from the river. DDE residues were highest in common carp Cyprinus carpio from Buckeye Canal 11.17 ugg wet weight, an agricultural drain and tributary to the Gila River. The overall frequency of occurrence of organochlorine compounds is declining, however, as only 6 compounds were detected in samples collected in 199495 versus 16 recovered in samples collected from the same sites in 1985. Residues of all other pesticides and PCBs were below levels associated with adverse effects on fish and wildlife. Concentrations of 11 potentially toxic metals were detected in fish. Carp collected near Allenville had the second highest aluminum concentration ever recorded in Arizona. Copper exceeded the national 85th percentile in 31 of 48 samples. Because of its occurrence at relatively high levels and its propensity to interact with other compounds and elements, copper remains a contaminant of concern. Concentrations of most metals remained unchanged from 1985 to 199495. Spiny softshell turtles Trionyx spiniferus generally contained higher organochlorine and metal concentrations than fish. DDE and most metals were highest in turtles from the middle river sampling sites. Residues of DDE and chlordane declined by about onehalf from 1985 to 199495. PCB, dieldrin, and DDT levels remained relatively constant over the past decade while copper, nickel, vanadium, and zinc concentrations declined. Levels of arsenic, mercury, and selenium in turtles collected in 199495 were statistically similar to concentrations in turtles collected in 1985. Whiptail lizards Cnemodophorus spp. collected from areas adjacent to the lower Gila River contained higher organochlorine residues than lizards collected from other Arizona locations. DDE and selenium exceeded toxic threshold levels that could be hazardous to avian predators that consume a large proportion of lizards in their diet. Mean mercury concentrations increased from 1985 to 199495 and were about 3 to 10times higher in lizards collected in the Gila River basin than in lizards from other Arizona locations. DDE, mercury, and selenium pose a significant environmental challenge to blackcrowned nightherons Nycticorax nycticorax and possibly to other avian species nesting and wintering on the lower Gila River. All nightheron and two of four redwinged blackbird Agelaius phoeniceus carcasses contained 3.4 ugg wet weight DDE; the level associated with impaired reproduction. Onehalf of the blackbird carcasses contained sufficiently high DDE residues to represent a hazard to predatory birds that regularly feed on blackbirds. Mercury concentrations in five of six nightheron livers approached or exceeded the toxic threshold and the maximum concentration, 28.07 ugg dry weight, was 4.5times higher than the toxic threshold. Selenium concentrations in nightheron livers 13 18 ugg dry weight indicate that adults are not at risk of selenium intoxication, but selenium impacts on reproduction are possible.