In 1986, the Department of the Interior conducted reconnaissance investigations in nine areas of the western conterminous United States to determine whether irrigation drainage has caused or has the potential to cause harmful effects to human health, fish, and wildlife, or may adversely affect the suitability of water for beneficial uses. Data collected in the lower Rio Grande valley and Laguna Atascosa National Wildlife Refuge reconnaissance investigation indicate that concentrations of dissolved minor elements in water are small. The maximum dissolved concentrations of arsenic, cadmium, mercury, chromium, selenium, and zinc exceed the 75th-percentile baseline values developed for the study; however, maximum dissolved concentrations of cadmium, mercury, and selenium exceeded the 75th-percentile baseline values by 1 microgram per liter or less. Concentrations of dissolved boron increased significantly from west to east. The smallest concentration of dissolved boron, 220 micrograms per liter, was detected in International Falcon Reservoir. The largest concentration of dissolved boron, 11,000 micrograms per liter, was detected on the refuge in Athel Pond.

No chlorophenoxy herbicides were detected in water during the June 1986 sampling. Simazine, prometone, and atrazine were the only triazine herbicides detected, and concentrations of these herbicides did not exceed 0.8 microgram per liter. DDE, the only organochlorine Insecticide detected in water, was detected at two locations at concentrations of 0.01 micrograms per liter. Methyl parathion, malathion, and diazinon were the only organophosphorus compounds detected in the June 1986 sampling, and the maximum concentrations of these compounds were 0.75, 0.71, and 0.26 micrograms per liter, respectively. The analysis of three samples collected in August 1986 indicate that the types of pesticides collected during runoff were similiar to those detected during the June 1986 sampling. The exception was that the herbicide 2,4-D was detected during runoff.

Concentrations of dissolved cadmium exceeded the chronic criteria for freshwater aquatic life in the Cayo Atascoso in the Laguna Atascosa National Wildlife Refuge. Chromium exceeded the acute and chronic freshwater criteria at four locations in the refuge and in the Laguna Madre. Chromium also exceeded the chronic saltwater criteria in Athel Pond. Concentrations of dissolved copper exceeded the acute and chronic criteria for saltwater aquatic life at 13 locations. Mercury exceeded the chronic criteria for freshwater and saltwater aquatic life at three locations, and dissolved nickel concentrations exceeded the chronic criteria for saltwater aquatic life in the Rio Grande at Anzalduas Dam and in the Resaca de los Frenos near Russeltown.

No organophosphorus insecticides, polychlorinated napthalenes, or polychlorinated biphenyl compounds were detected in four bed-sediment samples. DDE, an organochlorine insecticide, was detected in all four samples at concentrations ranging from 0.2 to 34 micrograms per kilogram. Chlordane, ODD, DDE, DOT, and dieldrin were all detected in the Resaca de los Fresnos at U.S. Highway 77 at San Benito with concentrations of 4.0, 9.7, 9.3, 7.3, and 0.1 micrograms per kilogram, respectively. Data collected by U.S. Fish and Wildlife Service in 1985 indicate that DDE was detected in approximately 75 percent of the bed sediment samples analyzed. The maximum concentration detected in that study was 6.0 micrograms per gram; the median concentration was 0.01 micrograms per gram.

Minor-element data from 22 fish samples indicate that the maximum concentrations of arsenic, copper, mercury, selenium, and zinc exceeded the 85thpercentile baseline concentrations established by the U.S. Fish and Wildlife Service for the National Contaminant Biomonitoring Program. None of the median concentrations of these minor elements exceeded the baseline concentrations. The maximum concentrations of aluminum, barium, iron, manganese, and tin were detected in fish collected from International Falcon Reservoir. This reservoir stratifies in the summer, and minor elements may be released from the bed sediments in the deep parts of the reservoir and incorporated into the food chain.

Toxaphene was detected in 11 fish samples; detectable concentrations ranged from 0.98 to 5.1 micrograms per gram, wet weight. DOT also was detected in 11 fish samples with concentrations ranging from 0.021 to 0.066 micrograms per gram, wet weight. ODD was detected in 21 fish samples; concentrations ranged from 0.015 to 0.16 micrograms per gram, wet weight. DDE was detected in all 22 fish samples, and concentrations ranged from 0.36 to 9.9 micrograms per gram, wet weight. The maximum concentrations of DOT and ODD exceeded the 1980-81 baseline concentrations. The median and maximum concentrations of toxaphene and DDE exceeded the 1980-81 baseline concentrations. The largest concentrations of toxaphene, ODD, and DDE in fish were all measured in samples collected at the Main Floodway near Progreso.