Many trace elements are essential nutrients; however, certain trace elements such as arsenic, cadmium, and mercury are known to be persistent environmental contaminants and toxic to most forms of life (Schmitt and Brumbaugh, 1990). Trace elements are generally present in small concentrations in natural water systems. Their occurrence in ground and surface water can be due to natural sources such as dissolution of naturally occurring minerals containing trace elements in the soil zone or the aquifer material or to human activities such as mining, application of pesticides, burning of fossil fuels, smelting of ores, and improper disposal of industrial wastes.
The EPA has established primary and secondary MCLs, HAs, and issued guidelines for the establishment of aquatic-life criteria as they apply to trace elements. The States of Colorado, New Mexico, and Texas have also established individual aquatic-life criteria for selected trace elements that apply to specific river reaches within the Study Unit. Acute exposure (short term, high concentration) to certain metals can kill organisms directly, whereas chronic exposure (long term, low concentration) can result in either mortality or nonlethal effects such as stunted growth, reduced reproductive success, deformities, or lesions.
During 1992-95, ground-water samples were collected for analysis of trace elements during three land-use studies and the aquifer subunit survey. Most trace-element concentrations in ground water ranged from less than 1 to 10 micrograms per liter (µg/L). Only four trace elements exceeded EPA MCLs, SMCLs, or HAs (table 10). Uranium concentrations in water from 15 of 119 wells exceeded the EPA-proposed MCL of 20 µg/L (U.S. Environmental Protection Agency, 1996a). Water from two wells exceeded the EPA HA of 40 µg/L for molybdenum. The relatively low number of detections and small concentrations of most trace elements indicate that they are influenced more by natural processes and less by land use.
Two trace elements that consistently exceeded SMCLs were manganese and iron, which can stain plumbing fixtures or impart a metallic taste to the water. Manganese concentrations in water from 62 wells and iron concentrations in water from 14 wells exceeded EPA SMCLs. Large manganese and iron concentrations are often associated with small dissolved-oxygen concentrations. Where oxygen has been consumed, microorganisms will reduce manganese and iron, resulting in large concentrations. This commonly occurs in areas of septic-system effluent discharge.
On the basis of analysis of the spatial distribution and range of trace- element concentrations (table 10), human activities have not caused widespread trace-element contamination in ground water. The majority of trace elements detected are likely the result of dissolution of naturally occurring minerals in the aquifer.
Since the late 1800's, mining activities have been prevalent in the northern part of the Study Unit in Colorado and New Mexico. Most mining activities have ceased, and several areas are in various stages of remediation. To document the surface-water quality in and immediately adjacent to the Rio Grande, a synoptic study focusing on trace elements in the water column, suspended sediment, and bed material was conducted in June and September 1994. In January 1995, selected trace elements were analyzed in samples collected in the Mesilla and Rincon Valleys as part of a pesticide and nutrient synoptic study.
Analysis of trace-element data collected in the northern part of the study area confirmed previous study results. Moderately to highly elevated trace-element concentrations were detected in the Creede, Colorado, and Red River, New Mexico, mining areas and in the Alamosa River, which drains a mining area and also contains several areas of naturally occurring minerals that contribute trace elements to the water column.
For the entire reach of the Rio Grande sampled in 1994, the largest concentrations of dissolved antimony, beryllium, cadmium, cobalt, copper, iron, lead, mercury, nickel, silver, and zinc were detected at sites downstream from areas of known mineral extraction. 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. At the site on the Alamosa River where it enters the San Luis Valley, highly elevated concentrations of dissolved cadmium, cobalt, copper, iron, manganese, nickel, strontium, uranium, and vanadium were detected.
The largest concentrations of dissolved arsenic, barium, boron, chromium, lithium, molybdenum, and uranium were detected at sites in the Mesilla and Rincon Valleys. The elevated concentrations of these trace elements may be related to discharges from geothermal springs, the return flow of irrigation water, or urban land-use discharges.
Concentrations of dissolved cadmium, copper, manganese, iron, silver, and zinc in the water column exceeded the following applicable standards at one or more sites. Dissolved concentrations of cadmium, copper, iron, and zinc exceeded the Colorado chronic aquatic-life standard (Colorado Water Quality Control Commission, 1989) at the site on the Alamosa River and dissolved silver concentrations exceeded the Colorado chronic aquatic-life standard for trout at four sites in Colorado . The copper concentration at one site on the Red River exceeded the New Mexico coldwater fishery chronic standard (New Mexico Water Quality Control Commission, 1991).
Bed-sediment samples were collected to characterize the occurrence and distribution of trace elements in the Study Unit. Because of the large areal extent of naturally occurring mineralized areas in the Study Unit, particularly in the northern one-third, the source of trace-element concentrations detected in bed-sediment samples may be natural rather than anthropogenic.
Trace-element concentrations in bed sediment closely reflect the same pattern as water-column concentrations. Highly elevated concentrations of antimony, arsenic, cadmium, copper, lead, mercury, silver, and zinc were detected in bed-sediment material from Willow Creek and the Rio Grande downstream from the Creede mining area compared to the rest of the study area. Concentrations of these elements in samples from the Alamosa River and Red River areas were elevated compared with the rest of the study area. At present, no Federal or State guidelines have been established for trace-element concentrations in bed sediment.
As part of the fish-tissue contaminant study, fish were collected at 12 sites and their livers analyzed for trace-elements. The bioavailability of trace elements is an important factor in assessing threats to aquatic organisms, ecosystems, and public health. As with water-column and bed-sediment samples, the sample from the Rio Grande near Creede contained elevated concentrations of arsenic, cadmium, copper, lead, and zinc. Some trace elements were detected at higher concentrations in liver samples than in bed-sediment samples from the same site.
Accumulation of trace elements by fish appears to be species dependent. Zinc concentrations in liver samples of common carp liver at certain sites were three to four times those in samples of white sucker at other sites. A brown trout sample contained significantly higher concentrations of arsenic, copper, mercury, and selenium than all other fish samples. To differentiate between environmental and species-dependent factors, however, the collection of multiple species at a number of individual sites would be necessary.
Transplanted bryophytes (moss) were used to determine the spatial distribution of trace elements in relation to land-use practices, compare accumulation rates of trace elements in bryophytes at sites known to be contaminated by trace elements, and evaluate transplanted bryophytes as a tool for examining the bioavailability of trace elements in relation to concentrations in water and bed sediment.
Thirteen sites on the Rio Grande and tributary streams in southern Colorado and northern New Mexico were sampled for determination of 12 trace elements in transplanted bryophytes. Analysis of the data indicates that (1) concentrations of cadmium, copper, lead, and zinc in bryophytes increased at sites on the Rio Grande immediately downstream from tributaries that drain the mining districts, (2) concentrations of these metals in bryophytes decreased with distance from the tributaries, and (3) concentrations of most trace elements in bryophytes were lower in a tributary stream downstream from an urban area than at sites near mining or agricultural land use.