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During the synoptic study in May 1998, the monthly mean streamflows for the Colorado River near State Line and Gunnison River near Grand Junc- tion were 23 and 16 percent, respectively, above the long-term (WY 1970-98) May mean monthly stream- flows for the two sites. Above average streamflow at each site may have been due to a greater snowpack or more snowmelt in each basin. As irrigation return flow, streamflow in the drains of the agricultural areas was affected by irrigation practices, which in turn were influenced by precipitation and temperature. Precipitation in the Grand Valley and Uncompahgre River Valley was slightly (less than 1 inch) below normal during the synoptic study, as measured at Grand Junction and Montrose, while the temperature was slightly (less than 1°F) above normal at the two sites (National Oceanic and Atmospheric Administra- tion, 1998b). Precipitation and temperature values greatly different from normal would affect irrigation practices and growing conditions, pesticide applica- tions, and, therefore, pesticides in streams and drains. OCCURRENCE AND DISTRIBUTION OF PESTICIDES The occurrence and distribution of pesticides in surface water and whether pesticides are detected in a water sample or not depend on many factors, such as the time, rate, and location of pesticide application, crop type, precipitation or irrigation events, physical and chemical characteristics of pesticides, and atmo- spheric transport and deposition. The time of pesticide application affects when pesticides are detected in streams. In surface water, pesticide detections typi- cally occur after the first precipitation/irrigation event following pesticide application. Also, small amounts of pesticides applied per acre are less likely to be detected than pesticides applied in large amounts. The spatial distribution of pesticides detected in a stream depends on the spatial distribution of pesticide appli- cation and crop type or agricultural practice. For example, atrazine is a very common herbicide used for corn and is commonly detected in streams near corn fields, whereas it is not used and not commonly detected in fruit-growing areas (U.S. Geological Survey, 1998a; Gianessi and Puffer, 1990). Pesticides applied just before or during a rainstorm may be trans- ported more quickly in surface runoff to streams and drains than pesticides applied during dry conditions. The potential of pesticides to be transported from an agricultural field into runoff water differs among pesti- cides and depends on the physical and chemical char- acteristics of each pesticide. Factors such as water solubility, persistence, and acid/base, ionic, and sorp- tion properties determine the runoff potential of a pesticide (Larson and others, 1997). Pesticides with large runoff potentials, such as atrazine, carbofuran, and pendimethalin, are more likely to be transported out of an agricultural field into surface water through runoff than a pesticide that has a small runoff poten- tial, such as malathion. Malathion has low soil persis- tence due to rapid degradation and, thus, is not readily present or available to be included in runoff. Atrazine, carbofuran, and pendimethalin, in contrast, are moder- ately to highly persistent in soil and are readily avail- able for inclusion in runoff. Depending on the characteristics of individual pesticides, storm runoff can be an important mechanism in the transport of pesticides to receiving waters. Finally, some pesticide detections may not be due to local use of the pesticide at all. It is possible that very low levels of detected pesticides may be related to atmospheric transport and deposition. As Majewski and Capel (1995) reported, pesticides have been detected in the atmosphere throughout the Nation, and pesticides applied in one area may be transported and deposited in another area. In the UCOL study unit, 35 pesticides were detected at least once in 82 of the 100 samples collected during the fixed-station and synoptic sampling periods of October 1996 through January 1998 and May 1998 (table 10), respectively, a detec- tion for this report being defined as a concentration of a pesticide equal to or greater than the MRL of the pesticide. Almost 93 percent (76 of 82) of these samples contained two or more pesticide detections. For the 100 samples, 8,248 individual pesticide anal- yses were performed, and there were 476 detections (5.8 percent of the possible total). Fifty-two additional analyses were unreported because of difficulties in the laboratory determination of concentration. Almost 82 percent (390 of 476) of the detections were for the 11 most frequently occurring pesticides. Of these 11 pesticides, 9 were herbicides, 1 (carbofuran) was an insecticide, and 1 (deethylatrazine) was a degradation product of atrazine. Atrazine and alachlor, used on corn and dry beans, were the most commonly detected herbicides, whereas carbofuran, used on pests in alfalfa, corn, and grains, was the most commonly detected insecticide. Pesticide concentrations iii the OCCURRENCE AND DISTRIBUTION OF PESTICIbES 23