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Table 1. Water-quality monitoring sites in the Upper Colorad River study unit Site number in figure 1 1 2 3 4 5 6 7 8 9 10 11 12 13 14 Site name Colorado River near Colorado-Utah State Colorado River near Cameo Colorado River near Dotsero Gunnison River near Grand Junction Gunnison River below Gunnison Tunnel Gunnison River at County Road 32 East River below Cement Creek Gore Creek at mouth Uncompahgre River near Ridgway French Gulch near Breckenridge Reed Wash near Mack Dry Creek at Begonia Road Dry Fork at upper station near DeBeque Colorado River below Baker Gulch Site name used in this report State Line Cameo Dotsero Gunnison near Grand Junction Gunnison Tunnel Gunnison at 32 Road East Gore Uncompahgre French Reed Dry Dry Fork Baker USGS station identifier 09163500 09095500 09070500 09152500 09128000 383103106594200 09112200 09066510 09146200 09046530 09153290 09149480 09095300 09010500 Purpose and Scope The objectives of this report are to (1) describe what factors were considered in the initial quality- assurance sample design; (2) present and describe quality-control data for water years 1995-96; (3) analyze the quality-control data for systematic problems that can be resolved in water year 1997; and (4) provide insight into operation of the network for the remainder of the data-collection phase of the UCOL study unit. Several types of quality-control samples were collected during water years 1995-96 and included pre-processing split replicates, concurrent replicates, sequential replicates, post-processing split replicates, and field blanks. Pre-processing split-replicate samples are samples in which twice the normal volume of water is collected and then split into two unique samples before processing. Concurrent- replicate samples are samples that are collected using two sampling teams or two sets of sampling equip- ment at the same time. Sequential-replicate samples are collected sequentially: first one sample is collected and then a replicate is collected. Post-processing split replicates are samples that are split after all processing has been completed but prior to shipment to the labo- ratory for analysis. A field blank is a sample prepared using blank water (water that is free of the analytes of interest), which is passed through all of the sampling and processing equipment. Acknowledgments The authors would like to express thanks to Jeff Foster, Dave Hartle, and Dennis Smits for data collection and processing. Dave Mueller and Terry Schertz provided exceptional technical review of the manuscript. Thanks also are extended to Joy Monson (manuscript and layout), Sharon Powers (illustrations), and Hyla Strickland (editorial review). QUALITY -CONTROL SAMPLING PROGRAM When surface-water sampling began in the UCOL network, limited information was available to design the quality-control sampling program. In the initial and continuously developing quality- control design, several factors were considered, which included streamflow, expected constituent concentra- tions, basin characteristics, land use, and whether vari- ability or bias would be important at a site. Variability is the degree of random error in independent measure- ments of the same quantity. Replicate samples test for variability, which could be important at sites that have high constituent concentrations. Bias is the systematic error inherent in a method. Blank samples test for bias, which could be more important at sites having low constituent concentrations. QUALITY-CONTROL SAMPLING PROGRAM 3