An Evaluation of Sediments in the Middle Rio Grande, Elephant Butte Reservoir and Caballo Reservoir as Potential Sources for Toxic Materials

The distribution of a large number of priority pollutant trace metal and organic species in water and sediments in surface waters in the Middle Rio Grande region of New Mexico has been surveyed. Study sites include Socorro and San Marcial along the Rio Grande to determine delivery of trace substances and Elephant Butte and Caballo Reservoirs. Elephant Butte Reservoir is the repository for suspended sediments collected by the Rio Grande while Caballo Reservoir provides a control site. In addition to sediments and water, limnological data was collected on the reservoirs, radionuclide and particle size analysis was performed on the sediments and a limited number of fish were surveyed for trace metals and organics.

The sediments carry elevated levels of the metals Hg, Cd, As, Se, and U and fish may be biomagnifying Hg, Pb, and V through the food chain. Mercury levels in fish are high enough to warrant concern from a health standpoint.

Detectable levels of 18 different chlorinated organic pesticides were found in samples of water and bottom sediments. These pesticides were found predominantly in association with suspended particulates or with fine particles in the bottom sediments. Pesticides were detected more frequently in the Fall than in the Spring. Because of heavy sediment inputs contaminated by adsorbed pesticides, the calculated annual input to Elephant Butte Reservoir lies between 900-11,000 kg for each of the seven most frequently detected pesticides.