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Table 4. Agricultural synoptic surface-water-sampling sites, Uncompahgre River Valley, Upper Colorado River Basin [USGS, U.S. Geological Survey] number (fig. 2) Site name USGS site number Sampling date URV 1 Uncompahgre River at Montrose at Hwy 90 382831107530601 05/21/98 URV2 Happy Canyon Creek at Marine Road, near mouth 382928107541101 05/22/98 URV3 Cedar Creek near mouth 383041107544201 05/21/98 URV4 Spring Creek at Jay-Jay Road 383201107575301 05/20/98 URVS Drain at Blossom Road, near Chipeta 383834108001701 05/19/98 URV6 Loutsenhizer Arroyo below Garnet Canal, at mouth 383953108001701 05/19/98 URV7 Dry Creek near Begonia Road, near Delta 09149480 05/21/98 URV8 Dry Creek at mouth, near Delta 384202108032001 05/21/98 URV9 Garnet Canal above Sweitzer Lake diversion 384247108012401 05/19/98 URV 10 Uncompahgre River at Delta 09149500 05/14/98 URV 11 Currant Creek near Read 09137050 05/15/98 URV 12 Alfalfa Run at Austin 384649107570501 05/15/98 URV13 Peach Valley Arroyo near mouth 384604107570701 05/19/98 URV 14 Unnamed drainage at Highway 92 near Read 384551107591901 05/14/98 URV 15 Tongue Creek at Cory 09144200 05/18/98 URV 16 Cummings Gulch at mouth 384448108070301 05/14/98 URV 17 Seep Creek at G Road, near mouth 384408108091501 05/14/98 URV 18 Roubideau Creek at mouth, near Delta 09150500 05/18/98 URV19 Gunnison River above Escalante Creek, near Delta 384527108152701 05/20/98 URV20 Gunnison River near Grand Junction 09152500 05/04/98 Sampling and Laboratory Methods Water-quality samples for pesticide analysis were collected by equal-width-increment sampling and processed onsite using techniques described by Shelton (1994). Pesticide samples were filtered in the field through a 142-mm-diameter, 0.7-F,tm baked glass- fiber filter and stored on ice for shipment or delivery to the USGS National Water Quality Laboratory (NWQL) in Arvada, Colorado. The dissolved pesticide samples were extracted and analyzed at the NWQL using methods described by Zaugg and others (1995) and Werner and others (1996). The concentrations of 83 pesticides and pesti- cide degradation products (hereinafter known as pesti- cides) were determined either by gas chromatography with mass spectrometry (GC/MS) or high-perfor- mance liquid chromatography (HPLC) with ultraviolet spectroscopy. Forty-seven pesticides were analyzed by GC/MS and 39 by HPLC (table 5). Both methods analyzed for carbaryl, carbofuran, and linuron. Water samples from both the fixed-station network and the synoptic network were analyzed for the same pesti- cides. Atotal of 155 (86 fixed-station, 69 synoptic) water samples, including quality-assurance samples, were analyzed for pesticides at the NWQL for this UCOL pesticide study. Pesticide concentrations are reported in terms of method reporting limits (MBEs). The MRL is the minimum concentration of a pesticide that can be reli- ably reported for an analytical method (Timme, 1995). For this study, MRLs ranged from 0.001 to 1.2 ~,g/L (table 5). Some MRLs for pesticides analyzed with the HPLC method were adjusted upwards on December 15, 1997 (table 5). Pesticide concentrations less than the MRL are reported with a "<" symbol. Concentra- tions for some pesticides are reported as estimated (E) concentrations because of variable analytical perfor- mance, or a measured concentration was greater than the maximum method calibration value for the partic- ular pesticide. For a pesticide with an estimated concentration, the pesticide has passed all qualitative 8 Pesticides in Surface Waters of the Upper Colorado River Basin, Colorado, 1996-98