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Table 7. Summary of differences between environmental and pre-processing split-replicate samples for nutrients and organic carbon [Lowest rounding unit is equal to the magnitude of the least significant figure] Constituent Percent of samples with no difference Ammonia Dissolved ammonia plus organic Total ammonia plus organic Nitrite Nitrite plus nitrate Dissolved organic carbon Suspended organic carbon Dissolved phosphorus Total phosphorus Orthophosphate 71 71 57 86 57 14 29 86 71 100 Percent of samples within plus or minus 1 lowest rounding unit 86 100 86 100 100 100 71 86 86 100 Percent of samples within plus or minus 2 lowest rounding units 100 100 86 100 100 100 71 86 86 100 Comparison of Trace Elements Trace-element samples are routinely collected at only two of the network sites, therefore only two pre-processing split replicates were collected for trace-element constituents. The environmental and pre-processing split-replicate results for these samples are listed in table 8. Because of the low number of pairs of samples, percentage of samples within ranges of rounding units are not provided. Many of the trace-element concentrations are less than the laboratory reporting limit (indicated by a less-than value). To determine the differences between the environmental and pre-processing split-replicate sample concentrations, a value of one-half of the reporting limit was substituted for the values desig- nated as a "less than." None of the differences were greater than 1 uglL (microgram per liter) or 1 lowest rounding unit. 12 Interim Results of Quality-Control Sampling of Surface Water for the Upper Colorado River National Water-Quality Assessment Study Unit, Water Years 1995-96