Use of pesticides and other organic compounds in many parts of the Nation poses a potential for serious nonpoint source contamination of streams and ground water. More than 1.1 billion pounds of pesticides is used each year in the United States [19]. In addition, new pesticides are continually being formulated and introduced to improve safety and minimize adverse environmental effects. As part of the NAWQA study, samples from streams, bed sediment, fish tissue, and ground water were collected and analyzed for pesticides, semivolatile organic compounds (SVOCs), and volatile organic compounds (VOCs). The results indicate that many pesticides and other organic compounds are reaching the water resources of the basin and that some compounds have accumulated in fish tissue and bed sediment. Although most of the compounds detected were at low concentrations, some compounds were detected at concentrations in excess of drinking-water standards or national guidelines for the protection of fish-eating wildlife.
Crop yields have improved in the upper Snake River Basin as a result of widespread pesticide applications. Although many of the pesticides currently in use break down rapidly in the environment following application, some pesticides are reaching the ground- and surface-water supplies of the basin (photograph by M.G. Rupert, U.S. Geological Survey).
In basinwide stream sampling conducted in May and June 1994, EPTC, or Eptam (used on potatoes, beans, and sugar beets); atrazine (used on corn) and its breakdown product desethylatrazine; metolachlor (used on potatoes and beans); and alachlor (used on beans and corn) were the most commonly detected pesticides, accounting for about 75 percent of all detections [20]. Only four different pesticides were detected in samples collected upstream from American Falls Reservoir. No pesticides were detected in the Snake River or its tributaries upstream from its confluence with the Henrys Fork. Seventeen different pesticides were detected downstream from American Falls Reservoir; the largest number was detected in agriculturally degraded tributaries to the Snake River between Twin Falls and Hagerman [20]. During followup sampling in 1995, two samples from agricultural drains near Twin Falls contained concentrations of the insecticides chlorpyrifos and methylazinphos that exceeded USEPA criteria for the protection of aquatic life (see Summary of Compound Detections and Concentration, page 26). However, none of the stream samples collected during 1994 and 1995 contained concentrations of pesticides that exceeded USEPA drinking-water standards. Rock Creek near Twin Falls has long been recognized as one of the most severely degraded streams in the State of Idaho [21]. During 2 years of monitoring at Rock Creek, 19 different pesticides were detected in stream samples, and 42 of 43 samples contained at least 1 pesticide, most of which were herbicides [22]. The number of pesticides and the pesticide concentrations were largest in Rock Creek in the spring and early summer following early season crop applications [4]. This pattern is typical in many parts of the United States [19].
Similar to basinwide sampling results, the most commonly detected compounds in Rock Creek were EPTC, atrazine, desethylatrazine, metolachlor, and alachlor. Concen- trations of the pesticides in Rock Creek were well below USEPA drinking-water standards and aquatic-life criteria [22]. Even though most of the pesticides in Rock Creek were detected only during the growing season, atrazine and desethylatrazine, because of their mobility and relative resistance to breakdown, were detected year round at concen-trations ranging from 0.01 to 0.03 microgram per liter.
Tributaries, such as Rock Creek near Twin Falls, discharge irrigation wastewater and associated pesticides to the Snake River. The number and concentrations of pesticides in water, bed sediment, and fish tissue are largest in the downstream parts of the upper Snake River Basin (photograph by G.M. Clark, U.S. Geological Survey).
Streambed sediment and bottom-feeding fish, primarily suckers and carp, were collected and analyzed for organochlorine compounds such as DDT and PCBs. Because most of these compounds are not water soluble and are not readily broken down, many can remain in relatively large concentrations in bed sediment and in the fat tissue of fish for long periods of time. Although many of the organochlorine compounds have been banned from use in the United States since the early 1970's, studies conducted in the late 1980's documented the presence of DDT in 98 percent of fish-tissue samples collected at 388 stream sites across the Nation [23]. However, comparison of fish-tissue data collected during the NAWQA study with data collected during the early 1970's indicates that the bans on use have been effective in reducing the environmental concentrations of organochlorine compounds in the Snake River Basin. Samples of tissue from fish collected from the Snake River near Twin Falls indicate that concentrations of DDT and other organochlorine compounds, although still present, have decreased substantially, some by as much as 10 times [10].
Of the 41 fish-tissue and 43 bed-sediment samples collected at 22 sites during the upper Snake River Basin NAWQA, 14 different organochlorine compounds were detected in fish tissue, and 9 organochlorine compounds and 47 SVOCs were detected in bed sediment [10]. The most frequently detected organochlorine compounds were DDT and its breakdown products, DDD and DDE, present in fish tissue at 83 percent of the sites and in bed sediment at 29 percent of the sites. At least one SVOC was detected in all of the bed-sediment samples. The most frequently detected SVOCs in bed sediment were phthalates (98 percent of the samples), phenol (86 percent), and 2,6-dimethylnaphthalene (83 percent). Sources of these compounds can be industrial and recreational activities. Currently, however, no human health advisories for fish consumption are in force for streams in the upper Snake River Basin.
The positive results of the 1972 ban on the production and distribution of DDT are evident in tissue concentrations in bottom-feeding fish in the Snake River near Twin Falls. Data were collected for the U.S. Fish and Wildlife Service National Contaminant Biomonitoring Program (1970-84) and the upper Snake River Basin NAWQA study (1992-94). Although concentrations of DDT have declined substantially since the early 1970's, because of their persistence, DDT and its breakdown products are still present in fish tissue and bed sediment.
In headwater streams where degradation of stream-water quality is minimal, tissue of fish, primarily suckers, from four of eight sites contained small concentrations of DDT and its breakdown products. Although no obvious sources exist for DDT in headwater areas of the upper Snake River Basin, other studies in the Nation also have documented the presence of DDT in fish tissue and bed sediments in pristine watersheds [25]. Streams draining areas influenced by agricultural and industrial activities contained the largest number of organochlorine compounds and SVOCs and in the largest concentrations [10]. Concentrations of organochlorine compounds in tissue of fish from the Portneuf River at Pocatello, Rock Creek near Twin Falls, and Snake River near Twin Falls equaled or exceeded national (NAS/NAE) guidelines for the protection of fish-eating wildlife [10].
Water samples from domestic, irrigation, stock, and public supply wells were collected and analyzed for pesticides and VOCs during a basinwide study conducted during the summers of 1994 and 1995 [26]. Results showed that water from 39 percent of 195 wells sampled contained at least 1 detectable pesticide (M.G. Rupert, U.S. Geological Survey, written communication, 1997). Water from 13 percent of the wells contained 3 or more pesticides. Fourteen different pesticides and 11 different VOCs were detected in the 195 well samples. Atrazine (in 27 percent of the samples), desethyl-atrazine (in 27 percent), metribuzin (in 7 percent), simazine (in 7 percent), and prometon (in 7 percent) were the most commonly detected pesticides. None of these pesticides, however, exceeded USEPA drinking-water standards.
Results from the basinwide sampling, in conjunction with intensive sampling in selected areas of the upper Snake River Basin, showed no detectable pesticides in ground water in some areas; however, in other areas, ground water contained numerous pesticides. Samples collected from wells in the Jackson, Wyoming, area; in the mountainous northern part of the basin; and along the southern boundary of the basin contained few, if any, pesticides. The largest number and concentrations of pesticides were in samples from wells that also contained large nitrate concentrations and were located in agricultural areas adjacent to the Snake River. Intensive sampling of 105 domestic and irrigation wells in agricultural areas north of the Snake River between Burley and Hagerman showed that water from 73 percent of the wells contained at least 1 pesticide and that 41 percent contained 3 or more pesticides (M.G. Rupert, U.S. Geological Survey, written communication, 1997). The VOC 1,2-dichloropropane was the only organic compound detected in ground water that exceeded USEPA drinking-water standards. However, it was detected in water from only one shallow domestic well at a concentration of 6.6 micrograms per liter [26].
Because the toxicological effects of drinking water that contains multiple pesticides or other organic compounds are not well understood [19], health risks associated with drinking ground water in some areas of the basin cannot be determined. More research is necessary to establish the health risks associated with drinking water containing small concentrations of more than one pesticide or other organic compound.
Basinwide, EPTC, atrazine, and the atrazine breakdown product desethylatrazine were the most commonly detected pesticides in surface water. In ground water, atrazine, desethylatrazine, and metribuzin were the most commonly detected pesticides. The large percentage of detections of atrazine and desethylatrazine is surprising because atrazine is not widely used on crops in the upper Snake River Basin.
