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Table 10. Summary of differences between environmental and concurrent-replicate samples for general chemical constituents [Lowest rounding unit is equal to the magnitude of the least significant figure] Percent of samples Percent of samples Percent of samples Constituent within plus or minus 1 within plus or minus 2 with no difference lowest rounding unit lowest rounding units Calcium 29 100 100 Magnesium 43 86 100 Sodium 57 71 100 Potassium 57 100 100 Sulfate 71 100 100 Chloride 14 57 71 Fluoride 86 100 100 Laboratory alkalinity 43 71 86 Silica 43 86 86 Iron 43 71 86 Manganese 71 100 100 Lab pH 57 86 86 Laboratory specific conductance 14 43 71 Residue on evaporation 17 34 50 rounding unit. It does not appear that the larger differ- ences are related to sampling site or constituent concentration. Variability associated with sample collection, processing, handling, shipment, and anal- ysis for general chemical constituents is low and will have little effect on interpretation of environmental data. Comparison of Nutrients and Organic Carbon Results of comparison between environmental and concurrent replicates and between environmental and sequential replicates for nutrients, dissolved organic carbon, and suspended organic carbon are listed in table 11. Shaded cells in table 11 represent differences greater than plus or minus I lowest rounding unit. The pattern of shaded cells indicates that the larger differences are not related to sampling site or constituent concentration. A summary of the differences between the environmental and concurrent-replicate samples for the constituents in table 11 is given in table 12. Sequential-replicate differences are not summarized because of the small number of samples available. The majority of the differences are within plus or minus I lowest rounding unit with the exception of suspended organic carbon. The majority of the suspended organic carbon differences are within plus or minus 2 lowest rounding units. Variability associated with sample collection, processing, handling, shipment, and anal- ysis for nutrients and organic carbon is low and will have little effect on interpretation of environmental data. 20 Interim Results of Quality-Control Sampling of Surface Water for the Upper Colorado River National Water-Quality Assessment Study Unit, Water Years 1995-96