A five-year study of water quality in the Rio Grande Valley, from its headwaters in Colorado to near El Paso, Texas, has been completed by the USGS. The study focused on groundwater in the flood plain of the Rio Grande and surface water in the Rio Grande and selected tributaries.
Adverse effects on water quality were observed in shallow groundwater, which is most susceptible to contamination, by the detection of anthropogenic chemicals, such as nitrates, pesticides, and volatile organic compounds; naturally occurring trace elements, such as iron, manganese, molybdenum, and uranium; and radon.
Compared to established human health standards, nitrate concentrations in the San Luis and Rincon valleys exceeded the U.S. EPA maximum contaminant level (MCL) for drinking water in 31% and 17% of the wells, respectively. Shallow groundwater generally is not used as a drinking water source in these areas.
Pesticides were detected in shallow groundwater in both agricultural and urban land-use areas; 29% of the wells contained at least one pesticide. The detected concentrations were very small compared to established MCLs, although not all pesticides have MCLs. Multiple pesticides were detected in several wells; the effect that combinations of pesticides may have on human health is unknown. Volatile organic compounds were detected in shallow groundwater from 11% of the wells sampled. No concentrations of pesticides and volatile organic compounds exceeded EPA drinking water standards.
Only two trace elements exceed EPA MCLs or Health Advisories for drinking water. Uranium concentrations exceeded the EPA-proposed MCL in 13% of the wells and molybdenum exceeded the EPA Health Advisory in 2% of the wells.
Radon was detected in all groundwater samples. Water from shallow wells in the San Luis Valley and deeper wells in the Rio Grande flood plain contained the highest median concentrations. At present no EPA drinking water standard exists for radon.
Water quality in reaches of the Rio Grande and some of its tributaries has been impaired by pesticides, elevated concentrations of trace elements, and anthropogenic disturbance. Pesticides were detected in surface water, bed sediment, and fish samples collected at sites in the Rio Grande and its tributaries and drains. However, no pesticide concentration exceeded EPA drinking water standards or applicable federal or state ambient criteria or guidelines. One or more pesticides were detected at 94% of the sites sampled; most concentrations were at or only slightly above the laboratory level of detection.
Analysis of fish community-structure data collected at ten sites indicates that six of the sites show indications of environmental perturbation. This analysis is based on the number of introduced, omnivorous, and pollution-tolerant fish and those with external anomalies counted at the sites sampled. On the basis of habitat data collected on stream modification, bank erosion, bank vegetation stability, and riparian vegetation density, six of ten sites sampled have significant habitat degradation.
A 39-page color report summarizes the results of the study. For technical information, contact Gary Levings at (505) 262-5335 or send email to glevings@usgs.gov.