Organochlorine compounds in fish tissue and bed sediment in the upper Snake River basin, Idaho and western Wyoming, 1992-94

Fish-tissue and bed-sediment samples were
collected from 20 sites in the upper Snake River
Basin in Idaho and western Wyoming as part of the
National Water-Quality Assessment Program to
determine the occurrence and distribution of organochlorine compounds. During 1992-94, 41 samples were analyzed for 28 different organochlorine
compounds in whole-fish tissue and 32 compounds
in bed sediment. Sites sampled were third- through
seventh-order streams that represented three environmental settings: reference conditions, agricultural land use, and mixed (agricultural and urban)
land use. Fourteen organochlorine compounds
were detected in fish tissue and nine in bed sediment. All compounds detected in bed sediment also
were detected in fish tissue. Fish-tissue and bed-
sediment samples from agricultural and mixed
land-use sites contained one or more organochlorine compounds. The most frequently detected
compound at all sites was p,p'DDE, which was
present in 80 percent of the fish-tissue and 30 percent of the bed-sediment samples. A maximum of
three compounds were detected, all in fish-tissue
samples from reference (forest and rangeland)
sites. The highest number of compounds was
detected in fish-tissue (nine) and bed-sediment
(eight) samples from mixed land-use sites. No clear
relation was apparent between the occurrence of
external anomalies and fish-tissue contaminant
concentrations or land use.

The distribution of organochlorine compounds in the basin was related to land use. Total
DDT was detected at sites in all land uses; total
PCB was detected at only agricultural and mixed
land-use sites. Total chlordane was detected in fish-
tissue samples from primarily mixed land-use sites;
samples from six of the eight sites contained
detectable concentrations. Median concentrations
of p,p'DDE in fish-tissue samples from mixed land-
use sites were significantly higher (p<0.05) than
from reference and agricultural sites. Significant
positive relations between percent agricultural land
and concentrations of total DDT (r² = 0.41) and
lipid-normalized total DDT (r² =0.48) were
observed.

Concentrations of p,p'DDE, total PCB, total
DDT, and toxaphene in fish-tissue samples from
three mixed land-use sites equaled or exceeded
national guidelines established for protection of
fish-eating wildlife: Portneuf River at Pocatello,
total PCB; Rock Creek at Twin Falls, p,p'DDE,
total DDT, and toxaphene; and Snake River near
Buhl, p,p'DDE and total DDT. Concentrations of
total DDT and p,p'DDE in 32 and 34 percent of
fish-tissue samples, respectively, analyzed during
this study exceeded the 1980-81 U.S. Fish and
Wildlife Service/National Contaminant Biomonitoring Program (USFWS/NCBP) geometric mean
concentrations. Concentrations of total PCB in
samples from the Portneuf River at Pocatello
and concentrations of toxaphene in samples from
Rock Creek at Twin Falls also exceeded the
USFWS/NCBP geometric mean concentrations.
Comparisons of 1970-84 USFWS/NCBP concentrations of total DDT and total PCB in fish-tissue
samples from the Snake River near Hagerman with
concentrations measured during this study indicated a decreasing trend. Concentrations of
p,p'DDE in all sediment samples from Rock Creek
at Twin Falls exceeded the Canadian Probable
Effect Level guideline. Total PCB was detected in
sediment from only one site, Portneuf River at Pocatello, which was also the only site where concentrations of total PCB in fish tissue were elevated.
Because organochlorine compounds are lipophilic
and tend to bioaccumulate in tissue, fish are a better
indicator of organochlorine contaminant occurrence and distribution than are bed sediment or
water in the upper Snake River Basin.

Some of the highest concentrations of organochlorine contaminants in tissue and sediment in
the basin were detected at sites receiving irrigation-
return flows. Results of this study support the
importance of controlling sediment erosion on irrigated land to reduce the quantity of contaminants
entering streams that receive irrigation-return
flows.