A toxic contaminants study was performed in the lower Rio Grande Valley in February, 1994. The purpose was to screen selected sites for the occurrence and impact of toxic chemicals.
A total of eight stations were sampled, including two on the Rio Grande, two on the Arroyo Colorado, one on the North Floodway, one at Brazos Santiago Pass, one in San Martin Lake, and one in the Gulf of Mexico. The scope of evaluation included determinations of toxic chemical concentrations in water (8 sites), sediment (7 sites), and fish tissue (8 sites); toxicity testing of water (8 sites) and sediment (7 sites); and biosurvey of fish and benthic macroinvertebrate communities (4 sites).
A total of 26 toxic chemicals were detected, 17 of which exceeded screening level concentrations. Four toxicants exceeded screening levels in water, one in sediment, and 15 in tissue. In the toxicity tests, significant adverse effects occurred in seven of 60 determinations, involving water samples from stations 4 (Brazos Santiago Pass), 6 (Arroyo Colorado near the Port of Harlingen), and 7 (Arroyo Colorado near the mouth), and water and sediment samples from station 8 (North Floodway near the mouth). Fish and benthic macroinvertebrate communities generally were healthy, and the degree of biotic integrity indicated that if toxic impacts were occurring, the effects were relatively slight. No instances of severe aquatic life use impairment were observed during the study period.
Toxic chemical concentrations in fish tissue did not exceed U.S. Food and Drug Administration action levels. However, low-level human health criteria were exceeded in water and/or edible fish tissue at all eight sites, indicating that long-term human health risks could result from regular consumption of untreated water and/or fish over the course of a human lifetime.
All available information was used to identify sites and chemicals of potential concern, to facilitate water quality management decisions and future monitoring efforts. The 17 chemicals that exceeded screening levels in the present study were considered to be of potential concern, and were assigned an approximate level of importance based on occurrence. A high priority group included arsenic, chromium, selenium, and zinc (although the inclusion of zinc is questionable); a medium priority group, copper, mercury, chlordane, and p,p' DDE; and a low priority group, cadmium, lead, nickel, silver, thallium, chlorpyrifos, gamma-bhc (lindane), arochlor 1254, and arochlor 1260.
Regarding sampling sites, the stations were ranked in the following order with respect to potential for toxic chemical impact: 6>8>7>5>4>1>3>2. Categorically, there were no stations that exhibited a high potential for toxic chemical impact. The potential was moderate for station 6 (Arroyo Colorado near the Port of Harlingen); slight for station 7 (Arroyo Colorado near the mouth) and station 8 (North Floodway near the mouth); very slight for station 1 (Rio Grande near Brownsville), station 3 (Gulf of Mexico near mouth of Rio Grande), station 4 (Brazos Santiago Pass), and station 5 (San Martin Lake outlet); and negligible for station 2 (Rio Grande near the mouth).
The scope of toxic chemical contamination was of lesser magnitude than in a recent study of the Rio Grande basin, which found 30 toxic chemicals of potential concern, and 13.3% of the 45 sampling sites to have a high potential for toxic chemical impact. The difference was attributed to sampling sites in the present study being further removed from point source wastewater discharges, and influenced to a greater extent by dilution and dispersion.