Summary

Description

Declining populations of the endangered Gila topminnow in the Santa Cruz River prompted a 1997 study to assess contaminant levels in water, sediment, invertebrates, fish, and birds. Samples were collected from two sites upstream from the Nogales International Wastewater Treatment Plant NIWWTP and from five sites downstream from the plant. Water pH ranged from 7.4 to 7.9 and was normal for the area. Unionized ammonia concentrations, up to 0.49 mgL this study and 24 mgL related studies were within the range known to be toxic to invertebrates and fish. Independent laboratory toxicity tests run concurrently with our study demonstrated that plant discharges were highly toxic. In 96hour exposure tests using effluent collected monthly, 100 effluent caused 100 mortality of adult fathead minnows during seven of 12 months. Unionized ammonia was identified as the toxicant responsible for fish mortality. We also documented a high proportion of longfin dace, up to 9. 1, with skin and skeletal anomalies at sites downstream from the NIWWTP. Toxicants in NIWWTP effluent appear to have nearly extirpated populations of invertebrates, amphibians, semiaquatic reptiles, and fish, including the endangered Gila topminnow, at sites closest to the treatment plant outfall. The entire aquatic ecosystem also was contaminated with chromium; concentrations were highest in sediment from Nogales Wash, 149 ugg dry weight, a level considerably higher than the 110 ugg toxic threshold for aquatic invertebrates. Almost onehalf 613 of all invertebrate samples contained chromium at concentrations that could be toxic to upper trophiclevel species. All samples of desert suckers contained elevated concentrations of chromium. The maximum chromium concentration in suckers 13.6 ugg wet weight was more than 11times higher than the 1.2 ugg toxic concern level. Concentrations of chromium in suckers increased with increasing distance downstream from the NIWWTP indicating that the plant was not a source of chromium contamination. There may be a pointsource of chromium contamination between the two most downstream sites of Tubac and Chavez Siding. While sediment, invertebrates, and fish were contaminated with chromium, it did not biomagnify to toxic levels in birds. Nickel was recovered at relatively low levels in desert suckers from all sites except one. All suckers from Chavez Siding contained higher than background levels of nickel. There may be a source of nickel contamination between Tubac and Chavez Siding. Copper exceeded the National Contaminant Biomonitoring Program 8ih percentile in 50 of the dace samples and in 95 of the desert sucker samples. Because of its occurrence at relatively high levels at some sites and its propensity to interact with other compounds and elements, copper remains a contaminant of concern in the lower Santa Cruz River. Current organochlorine compound levels in fish should not present a threat to survival and reproduction nor should current residues present a bioconcentration hazard to upper trophic level species that feed on fish. DDT, DDD, and DDE were not detected in fish from locations upstream from the NIWWTP. Since highest residues of DDT occurred at the site farthest downstream from the NIWWTP, it appears that the source of DDT may be runoffdrainage from contaminated soils downstream of the wastewater treatment plant. Four of eight killdeer carcasses contained 3.4 ugg wet weight DDE, a level associated with impaired reproduction and a level that represents a hazard to predatory birds that feed on killdeer. Concentrations of all metals in killdeer livers were within the normal or background range.