The occurrence and distribution of contaminants in aquatic
systems are major components of the National Water-Quality
Assessment (NAWQA) Program. Bed-sediment samples were collected
at 18 sites in the Rio Grande Valley study unit between September
1992 and March 1993 to characterize the geographic distribution
of organic compounds, including chlorinated insecticides,
polychlorinated biphenyls (PCB's), and other chlorinated
hydrocarbons, and also trace elements. Two-millimeter-size-
fraction sediment was analyzed for organic compounds and less
than 63-micron-size-fraction sediment was analyzed for trace
elements. Concentrations of p,p'-DDE were detected in 33 percent
of the bed-sediment samples. With the exception of DDT-related
compounds, no other organochlorine insecticides or
polychlorinated biphenyls were detected in samples of bed
sediment. Whole-body fish samples were collected at 11 of the bed-
sediment sites and analyzed for organic compounds. Organic
compounds were reported more frequently in samples of fish, and
more types of organic compounds were found in whole-body fish
samples than in bed-sediment samples. Concentrations of p,p'-DDE
were detected in 91 percent of whole-body fish samples.
Polychlorinated biphenyls, cis-chlordane, trans-chlordane, trans-
nonachlor, and hexachlorobenzene were other organic compounds
detected in whole-body samples of fish from at least one site.
Because of the extent of mineralized areas in the Rio Grande Basin
arsenic, cadmium, copper, lead, mercury, selenium, and zinc
concentrations in bed-sediment samples could represent natural
conditions at most sites. However, a combination of natural
conditions and human activities appears to be associated with
elevated trace-element concentrations in the bed-sediment sample
from the site Rio Grande near Creede, Colorado, because this
sample exceeded the background trace-element concentrations
calculated for this study. Fish-liver samples were collected at
12 of the bed-sediment sites and analyzed for trace elements.
Certain trace elements were detected at higher concentrations in
fish-liver samples than in bed-sediment samples from the same
site. Both bed-sediment and fish-tissue samples are necessary for
a complete environmental assessment of the occurrence and
distribution of trace elements.
