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UCOL study unit generally were low. Including all samples and analyses, only atrazine and alachlor had median concentrations (50th percentile in table 10) greater than their MRLs, and the median concentra- tions were only slightly greater than the respective MRLs (table 10). Carbofuran and 2,4-D were the only pesticides having detections at consistently higher concentrations than the other pesticides studied (table 10). Carbofuran concentrations exceeded 0.447 µg/L in 10 percent of the samples. Concentra- tions of 2,4-D exceeded 0.17 µg/L in 25 percent of the samples and 0.385 µg/L in 10 percent of the samples. Pesticides were detected at 41 of the 44 pesticide- sampling sites in the UCOL, a114 sites of the fixed- station network, and 40 of the 43 synoptic sites. Pesticides at Fixed-Station Network Fifty-seven water samples collected from October 1996 through January 1998 from the four fixed-station network sites in the UCOL study unit were analyzed for 83 pesticides. Twenty-three pesti- cides were detected at least once (table 11) in 42 samples. Forty samples contained two or more pesticides. For the fixed-station samples, 4,687 indi- vidual pesticide analyses were performed. Detections occurred in 253 analyses, or 5.4 percent of the total. The 10 most frequently occurring pesticides accounted for almost 85 percent of the 253 detections. The frequency of detection for a1123 pesticides and their measured detected concentrations are shown in figure 4, in descending order of frequency. Sixteen herbicides and seven insecticides were detected, with only the herbicides atrazine (70 percent) and alachlor (54 percent) being detected in more than 50 percent of the samples. Carbofuran and chlorpyrifos, detected in 18 percent of the samples, were the most frequently detected insecticides. Concentrations of the detected pesticides in the fixed-station network generally were low, ranging from 0.001 µg/L to 2.60 µg/L. More than 36 percent of the detections were greater than or equal to 0.01 µg/L, and almost 22 percent were greater than or equal to 0.05 µg/L. Carbofuran and 2,4-D accounted for 47 percent of the detections that were greater than or equal to 0.05 µg/L. Statistical summa- ries of the detected pesticide concentrations for each of the four fixed-station sites individually and combined are in table 11. For the samples collected at the fixed-station sites, no pesticide concentrations exceeded U.S. Envi- ronmental Protection Agency (USEPA) drinking- water standards or health-advisory guidelines; maximum pesticide concentrations were much lower than these water-quality criteria. The criterion for the protection of freshwater aquatic life, however, was exceeded for one pesticide at Reed Wash on May 21, 1997. The estimated azinphos-methyl concentration of 0.062 µg/L was greater than the USEPA freshwater- chronic criterion for aquatic-life protection of 0.01 µg2 (table 5). Interpretation of water-quality data based on standards and guidelines has to be done with qualifica- tions. The description of water quality in an area has to be tempered by the knowledge that water-quality criteria have not been set for many pesticides, pesti- cide mixtures and degradation products have not been evaluated, and not all pesticide effects have been studied. Temporal Distribution For the fixed-station sampling, pesticides were detected throughout most of the October 1996 through January 1998 sampling period (fig. 5). The mean number of pesticide detections per sample by month was greatest for April through August 1997 (fig. 6). Detections of herbicides were more common than insecticides. Herbicides were detected in every month that a detection occurred, while insecticides were detected only from April through August 1997 (fig. 6). Atrazine was the only herbicide detected every month in which a detection occurred, and it was present in 40 of the 42 samples with detections. Atra- zine is used as a season-long weed control in corn, so its detection during much of the year is common. Carbofuran and chlorpyrifos, the most commonly detected insecticides, were found in 10 of the 42 samples with detections. A1110 detections occurred in the spring and summer months, April through August 1997. Because of their use as insecticides, Carbofuran and chlorpyrifos would not be expected to be detected year round but would be more common during the times of insect activity. The concentrations of detected pesticides in the fixed-station study were elevated from April through August or September 1997, which corresponded to the time of higher streamflows because of irrigation and irrigation return flows and also snowmelt in May and 26 Pesticides in Surface Waters of the Upper Colorado River Basin, Colorado, 1996-98