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The distribution of replicate samples is listed in table 2. The type of replicate sample is based on design considerations. As data collection started, concentration ranges for general chemical constitu- ents, nutrients, organic carbon, and trace elements were determined for each site. Sites within the network were compared and then categorized as low, medium, or high based on measured concentrations. Discharge ranges for each site were defined as low (base flow), medium (flows above base to about 50 percent of the peak), and high (flows greater than about 50 percent of the peak). The distribution of quality-control samples was monitored and adjusted during the data-collection phase of the project, producing a dynamic program design. Additional samples of medium- and high-flow conditions were needed to balance the overall distribution. The distribution of field-blank samples and information for the site sampled before the blank was processed are listed in table 3. Some field blanks were processed using equipment that had been cleaned in the office laboratory. To test the efficiency of the field cleaning protocol, additional field-blank samples should be collected following field cleaning rather than laboratory cleaning. Results of the quality-control sampling listed in tables 2 and 3 are presented in the following sections of this report. Replicate Samples Replicate samples provide an indication of the magnitude and source of variability in sample results. Each type of replicate sample addresses Table 2. Distribution of replicate samples [--, no sample] Concentration range 1 Discharge2 Type of Site Land General Dissolved Suspended range for replicate Trace name use chemical Nutrients organic organic sample sample constituents carbon carbon elements State Line Mixed m m m h I Concurrent and post-processing split Cameo Mixed m m m h Concurrent and post-processing split Dotsero Mixed m m m I Sequential Gunnison Mixed m m m m I Pre-processing split near Grand Junction Gunnison Mixed m h Post-processing split Tunnel reference3 Gunnison at Mixed m m h Concurrent and post-processing split 32 Road East Rural urban, I Concurrent and post-processing split recreation h Concurrent and post-processing split Gore Rural urban, m m m I Pre-processing split recreation I Concurrent and post-processing split Uncompahgre Mining m I I m I I Pre-processing split French Mining I I I m h I Pre-processing split Reed Agriculture h h h h m Sequential Dry Agriculture h h h h m Pre-processing split Dry Fork Reference, h I h m Pre-processing split cp4 Baker Reference, m Concurrent and post-processing split srm4 Sequential m Pre-processing split 1 Concentration ranges with respect to all sites within the surface-water network, I = low, m = medium, h = high. 2Discharge ranges, I = base flow, m = flows above base to about 50 percent of the peak, h = flows greater than about 50 percent of the peak. 3Downstream from major reservoir system. 4Physiographic province, cp = Colorado Plateau, SrIll = Southern Rocky Mountain. 4 Interim Results of Quality-Control Sampling of Surface Water for the Upper Colorado River National Water-Quality Assessment Study Unit, Water Years 1995-96