Summary of major issues and findings
Issue: Ground water is the main source of drinking water in the Rio Grande Valley Study Unit; its quality is a major concern.
A variety of chemicals used in human activities, including pesticides, volatile organic compounds (VOCs), and nitrate, were detected in ground-water samples from shallow wells (within the top 10-15 feet of the water table). Samples from deeper ground water underlying the Rio Grande flood plain, which is more typically used as a drinking-water source, contained one pesticide, no VOCs, and nitrates (
pages 10
,
13
, and
14
).
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Nitrate concentrations exceeded the U.S. Environmental Protection Agency (EPA) drinking-water standard in 31 percent of shallow wells in the San Luis Valley and 17 percent of shallow wells in the Rincon Valley. Both of these areas are predominantly agricultural land use.
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· Pesticides were detected in both agricultural and urban land-use areas, with 29 percent of shallow wells containing at least one pesticide. Prometon and metolachlor were the most frequently detected, yet neither compound exceeded EPA drinking-water standards, although standards do not exist for all pesticides detected Only one pesticide was detected in the deeper ground water.
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Six VOCs were detected in shallow ground water from 11 percent of the wells sampled, most commonly in an urban land-use area. No VOCs were found in deeper ground water. No concentrations exceeded EPA drinking-water standards; however, standards exist for only three of the six compounds detected.
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Radon, a naturally occurring radionuclide in the Rio Grande Valley, was detected in all ground-water samples in concentrations that ranged from 190 to 2,300 picocuries per liter. The highest median concentrations occurred in shallow wells in the San Luis Valley and in the deeper ground water. About 57 percent of the samples exceeded 300 picocuries per liter, the proposed EPA drinking-water standard that has recently been withdrawn for further evaluation.
Issue: Pesticides are present in surface water, bed sediment, and whole-body fish at sites sampled in the Rio Grande and its tributaries and drains.
No pesticide concentration detected in surface water exceeded EPA drinking-water standards or applicable Federal or State ambient criterion or guideline. One or more pesticides were detected at 94 percent of the sites sampled in the Rio Grande, its tributaries, or drains; most concentrations, however, were at or only slightly above the laboratory level of detection (
pages 10
,
11
, and
12
).
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In the Mesilla Valley, there were more detections of more different pesticides during the nonirrigation season than during the irrigation season; as many as 27 percent of the pesticide detections attributed to pesticide use in the Mesilla Valley may be from urban sources.
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In the Rincon Valley, more pesticide compounds were detected more frequently in agricultural drains in April than in October or January; some individual pesticides detected in agricultural drains were not detected in shallow ground water.
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Concentrations of DDE were detected in composited samples of fish collected at 10 of 11 sites; these concentrations, however, are below the national median reported by the U.S. Fish and Wildlife Service (FWS) National Contaminant Biomonitoring Program.
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The presence of DDT and its metabolites, DDE and DDD, in bed sediment and whole-body fish confirms the persistence of this pesticide in the environment.
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Cis
-chlordane,
trans
-chlordane, and
trans
-nonachlor were detected in whole-body fish samples; concentrations, however, were below the national median reported by the FWS National Contaminant Biomonitoring Program.
Issue: Have elevated trace-element concentrations impaired reaches of the Rio Grande and its tributaries and, if so, can the sources be identified?
The water quality in reaches of the Rio Grande and some of its tributaries has been impaired by elevated concentrations of trace elements; however, data indicate that the concentrations tend to decrease downstream from the source. A combination of natural conditions and human activities appears to be associated with elevated trace-element concentrations (
page 19
).
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Highly elevated concentrations of antimony, arsenic, cadmium, copper, lead, mercury, silver, and zinc were detected in bed sediment from Willow Creek and the Rio Grande, downstream from the Creede, Colorado, Mining District.
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Elevated concentrations of arsenic, cadmium, copper, lead, and zinc were detected in fish tissue (liver) in the Rio Grande downstream from the Creede, Colorado, Mining District.
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Analysis of data collected using transplanted bryophytes (moss) indicates that concentrations of cadmium, copper, lead, and zinc in bryophytes increased at sites on the Rio Grande immediately downstream from tributaries that drain mining districts but decreased with distance from the tributaries. Concentrations of most trace elements in bryophytes were lower in a tributary stream downstream from urban land use than at sites near mining or agricultural land use.
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Dissolved concentrations of beryllium, cadmium, cobalt, lithium, manganese, molybdenum, nickel, and zinc were moderately higher at a site on the lower Red River than in the Rio Grande downstream of its confluence with the Red River.
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Highly elevated concentrations of dissolved cadmium, cobalt, copper, iron, manganese, nickel, strontium, uranium, and vanadium were detected in the Alamosa River where it enters the San Luis Valley.
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Maximum total recoverable concentrations of most trace elements in bed sediment were detected near the mouth of the Rio Puerco, an indication of the importance of sediment in trace-element transport.
Issue: Is the fish community structure in the Rio Grande Valley an indication that sites are environmentally stressed?
Based on the number of introduced, omnivorous, pollution-tolerant fish and the number of fish with external anomalies, six of the ten sites sampled appear to show some indications of environmental perturbation (
page 20
).
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Fish community-structure data indicate that introduced species predominate at four of the six sites sampled during 1994. Stocking introduced species has probably resulted in the displacement of native species through competition or predation.
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The number of introduced, omnivorous, pollution-tolerant, and anomalous fish at sites sampled in the Rio Grande indicate that sites are environmentally stressed.
Issue: Is significant habitat degradation occurring in the Rio Grande Valley?
Six of the 10 sites sampled appear to have significant habitat degradation based on stream modification, bank erosion, bank vegetation stability, and riparian vegetation density (
page 21
).
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Rio Grande near Del Norte, Colorado; Conejos River near Lasauses, Colorado; Rio Chama near Chamita, New Mexico; Santa Fe River above Cochiti Lake, New Mexico; Rio Grande at Isleta, New Mexico; and Rio Grande at El Paso, Texas, appear to have significant habitat degradation.
