DDE mercury, and selenium in Biota, sediments, and water of the Rio Grande-Rio Bravo Basin, 1965-1995.

An assessment of contaminant stressors on biota of the Rio Grande was conducted to identify relevant contaminant issues, assess exposure and ecological effects, identify data gaps, and determine potential risks. Most contaminant data were from studies conducted during 1965-1995 in the Lower Rio Grande, on the Texas side of the river, within a 100-km boundary from Falcon Dam to the mouth. Contaminants most frequently reported were organochlorine compounds (OCs) and trace elements. The number of records for OCs and trace elements was at least twofold greater for fish than for birds, mammals, or reptiles. Of the OCs, p,p'-DDE was the most commonly reported. Among the trace elements, Hg was one of the most frequently reported; however, Se, As, Pb, Cu, and Zn were also common. The highest concentrations of OCs and trace elements were reported predominantly from Lower Rio Grande Valley locations, with approximately 68% of the highest values detected from Falcon Dam to the mouth of the river. Twenty-six (20%) of the locations with maximum concentrations corresponded to portions of Llano Grande Lake and the Arroyo Colorado. Recent analyses of birds and fish indicate that levels of DDE are currently much lower than in the 1970s or 1980s in Rio Grande wildlife. This apparent decline does not apply to Hg and Se levels in birds and fish, which have remained more or less constant, but may have increased over the years in some locations. Hg was of particular concern because of high levels found recently in addled eggs of aplomado falcons and in their potential prey. Hg was elevated in fish from the Big Bend area. Also, Se in fish sampled in 1993 and 1994 was near or above the threshold for potential effects in fish-eating wildlife. Future investigations should evaluate the potential impacts of Hg and Se on aquatic and terrestrial species from selected sites of concern.