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Pesticides were identified through solvent extraction followed by gel permeation chromatog- raphy, followed by additional alumina-over-silica column chromatography with quantification through dual capillary-column gas chromatography with electron-capture detection (Furlong and others, 1996). PCBs in bottom sediment were identified through dual capillary-column gas chromatography with electron capture detection (Foreman and others, 1995). Pesti- cide and PCB analysis was done at the USGS National Water Quality Laboratory. PAHs were identified through solvent extraction followed by gel permeation chromatography. The concentration of each compound was determined by capillary-column gas chromatography/mass spectrom- etry (Furlong and others, 1996) at the USGS National Water Quality Laboratory in Arvada, Colorado. Trace elements were analyzed at the USGS National Research Program Laboratory in Reston, Virginia. All trace-element concentrations, except chromium, lead, zinc, and mercury, were determined through inductively coupled plasma-atomic emission spectrometry. Chromium, lead, and zinc were analyzed by graphite furnace atomic absorption and mercury by cold vapor atomic absorption (Lickte and others, 1987; Fishman, 1993). Only core DLN (fig. 2) was representative of sediment deposited throughout the existence of the reservoir. Determination of the approximate age of each core segment was based on measured porosity (volume of the voids divided by the volume of the solids) and the assumption of a constant sedimentation rate from the pre-reservoir (1963) surface near the bottom of the core to the top of the core (representing 1997). The mass of each segment was determined using measured porosity for that segment, thus adjusting for the effects of compaction over time. The length of time to deposit each segment was determined by dividing the segment mass by the average sedimen- tation rate (mass per time). Calculation of the age of each segment was done by subtracting the cumulative deposition time of all segments above the segment of interest from the sampling date. Due to compaction, a segment from the bottom of the core had a lower porosity than the sediment deposited at the top of the core. Therefore, a I-centimeter segment from the bottom of the core spanned a longer time period a than I-centimeter segment from the top of the core. Streambed-Sediment Samples Streambed-sediment samples were collected near the reservoir on each of the main three tributaries (fig. 2) and analyzed for trace elements as part of other studies within the UCOL. Samples on the Snake River and Tenmile Creek were collected in 1996. Protocols and methods used for these samples are described by Deacon and Driver (1999). The sample from the Blue River was collected in 1995, and protocols and methods are described by Apodaca and others (2000). Water Samples Stream-water samples were collected monthly at the three tributary sites and downstream from the dam on the Blue River (fig. 2) over a 13-month period beginning in May 1997. Samples were collected using protocols described by Shelton (1994). Equal-width- increment samples were processed onsite and shipped to the USGS National Water Quality Laboratory in Arvada, Colorado. Samples were analyzed for trace- element concentrations using methods described by Faires (1993) and Hoffman and others (1996). Stream- water analysis data are available in the USGS National Water Information System (NWIS) data base. Reservoir-water samples were collected at discrete depths at four locations in June, August, and November of 1997. Water from a particular depth was pumped to a boat and processed for shipment to the USGS National Water Quality Laboratory. The same laboratory methods used for the stream-water samples Collecting water-quality samples on the Blue River upstream from Dillon Reservoir. STUDY METHODS 7