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A summary of the differences for the constitu- ents listed in table 4 is given in table 5. Except for specific conductance and residue on evaporation, most of the environmental and pre-processing split- replicate samples agree within plus or minus 1 lowes rounding unit. The differences for conductance agree within plus or minus 2 lowest rounding units. These differences would equal plus or minus 2 IlS/cm (microsiemens per centimeter at 25 degrees Celsius) for all but the Dry sample (table 4) and probably are well within instrument variation. Differences of 2 IlS/cm are not considered excessive. Differences between environmental and split-replicate samples fo residue on evaporation are greater than any other of the constituent differences. It appears that differences of a few lowest rounding units for residue on evapora tion would be common, and lower differences may be limited by the precision of the laboratory method. Results in table 4 that have differences greater than plus or minus 1 lowest rounding unit have been shaded. By observation of the random position of the shaded cells, it appears that the larger differences for each constituent do not seem to be related to the concentration or the sampling site. Variability associ- ated with sample processing, handling, shipment, and analysis for general chemical constituents is low and will have little effect on interpretation of environ- mental data. Comparison of Nutrients and Organic Carbon Results of comparison of the environmental and pre-processing split-replicate samples for nutri- ents, dissolved organic carbon, and suspended organic carbon are listed in table 6. To determine the differ- ences between the environmental and pre-processing split-replicate sample concentrations for values less than the laboratory reporting limit (values that have a less-than symbol), a value of one-half of the reporting limit was substituted for the less-than value. A summary of the differences between environmental and pre-processing split-replicate samples for each constituent is listed in table 7. Most of the differences between the environmental and pre-processing split- replicate samples for all constituents are within plus or minus 1 lowest rounding unit. Variability associated with sample processing, handling, shipment, and analysis for nutrients and organic carbon is low and will have little effect on interpretation of environ- mental data. Table 5. Summary of differences between environmental a d pre-processing split-replicate samples for general chemical constituents [Lowest rounding unit is equal to the magnitude of the least significant fi ure] Constituent Percent of samples with no difference Calcium Magnesium Sodium Potassium Sulfate Chloride Fluoride Laboratory alkalinity Silica Iron Manganese Laboratory pH Laboratory specific conductance Residue on evaporation 57 57 86 29 57 29 86 71 57 71 71 57 14 14 Percent of samples within plus or minus 1 lowest rounding unit 86 100 100 86 100 86 100 71 100 86 86 71 43 14 Percent of samples within plus or minus 2 lowest rounding units 86 100 100 100 100 100 100 86 100 86 86 100 100 28 QUALITY-CONTROL SAMPLING PROGRAM 9