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48 Water-Quality Characteristics and Ground-Water Quantity of the Fraser River Watershed, Grand County, Colorado, 1998-2001 for one sample. Variability in radon-222 concentrations was 8.3 percent (±17.3 percent) of the concentration value at the 95-percent confidence level (table 8, Appendix II). Eleven constituents had one or both samples of a replicate pair reported as "less than" concentrations, concentrations undetected at the reporting level (table 7, Appendix II). Of these 11, only bromide, ammonia, nitrite, and MBAS had at least one replicate pair with a "less than" concentration for one sample and a concentration greater than the reporting level for the other sample, indicating some inconsistency in detecting bromide, ammonia, nitrite, and MBAS in the ground-water samples. The most significant of these was for ammonia. One-half (four of eight) of the ammonia replicate pairs with a "less than" concentration also had a detected concentration (table 7, Appendix II). Most concentrations of bromide, ammonia, nitrite, and MBAS in the ground-water samples were low, near the respective reporting levels. Field-blank samples were used to test for bias, defined as the systematic error inherent in sampling and analytical methods. Blank samples were collected using inorganic- and organic-free water that contained no detectable concentrations of the constituents of interest. The blank water was processed through the sampling equipment, handled, and analyzed in the same manner as the environmental sam- ples.The ground- and surface-water field blanks were analyzed for major ions, nutrients, trace elements, and dissolved organic carbon, with the last two only for ground water. Because constituents in this report were analyzed at low concentrations in the laboratory, field blanks were useful in determining possible contamination of a sample from collection, processing, cleaning, storage, and laboratory analysis pro- cedures. Seventeen field-blank samples for ground water were collected for major-ion and trace-element analysis (table 9, Appendix II). Reporting levels for some constituents changed during the 4-year sampling period. In the field blanks, concentrations of chloride, fluoride, potassium, silica, sodium, sulfate, and manganese were below the laboratory reporting levels. Bromide and iron were detected at the lab- oratory reporting levels in one blank sample each. Calcium was detected in three blank samples and magnesium was detected in one blank sample above the laboratory reporting levels. Estimated concentrations were reported for three calcium, two magnesium, and five sodium blank samples and one blank sample each of sulfate and manganese. The one magnesium detection with a concentration greater than two times the lowest rounding unit of the reporting level had no appreciable effect on the reliability of environmental data because of substan- tially higher magnesium concentrations in environmental samples. Therefore, interpretation of major-ion and trace-element environmental data was not affected by detections of major ions and trace elements in the blank samples. Nineteen field-blank samples for ground water were collected for nutrient and dissolved organic carbon analysis (table 9, Appendix II). All constituents except phosphorus were detected in some field-blank samples. Estimated concentrations of ammonia plus organic nitrogen and dissolved organic carbon were detected in two and seven blank samples, respectively. Nitrite and nitrate were detected at or within one lowest rounding unit of the respective reporting levels. Interpretation of the environmental data for ammonia plus organic nitrogen, nitrite, and nitrate in ground water was not affected by detections of these nutrients in blank samples. Ammonia was detected in seven blank samples, primarily at or within three times the lowest rounding unit (table 9, Appendix 11). Blank samples commonly are contaminated with ammonia when exposed to the atmosphere, especially when analyzed at low concentra- tions-as was done in this study. As discussed above, an inconsistency in the detection of ammonia in ground-water samples was indicated by four of eight replicate pairs with a "less than" concentration for one sample and a detected concentration for the other sample. These two factors, combined with blank samples having a similar concentration range of ammonia as that of the environmental samples, indicate that the exact concentration of ammonia in the ground-water samples collected during this study is not known with certainty. It is known, however, that only small amounts of ammonia were present in the samples, and all concentrations were less than or equal to about 0.04 mg/L. The blank sample with an ammonia detection of 21 times the lowest rounding unit did not affect the interpretation of environ- mental data because the sample collected prior to the blank did not indicate the presence of ammonia, and subsequent samples were not collected during that sampling phase. Almost all detections of orthophosphate and dissolved organic carbon in blank samples were equal to or within two times the lowest rounding unit of the laboratory reporting levels (table 9, Appendix II). Interpretation of orthophosphate and dissolved organic carbon data were not affected by detections in blank samples because concentrations in the blank samples were at least one order magnitude smaller than concentrations in environmental samples. Surface-water sample collection at Fraser River below Crooked Creek at Tabernash was done as part of a larger USGS surface-water sampling network in the Fraser River watershed. During the August 1998-September 2001 time period, no replicate samples were collected on the Fraser River, and no replicate-pair analysis could be performed. Field-blank samples for analysis of major ions and nutrients were collected on the Fraser River at four of the network sites, not including the Fraser River site discussed in this report. For major ions, the blank results indicate that the environmental data for major ions were free of bias during sampling and laboratory analysis. Analysis of the three blank samples for dissolved magnesium and sodium indicate no detections. The two dissolved-silica blank samples also had no detections. Dissolved calcium was detected in one blank sample at a value five times the laboratory minimum reporting level. Interpretation of dissolved-calcium data for surface water was not affected by this blank detection because all values for calcium in the environmental surface-water samples were many orders of magnitude higher than the concentration detected in the field blank. Analysis of the surface-water blank samples for nutrients indicate two detections of ammonia at the reporting level and one detection of orthophosphate at one rounding unit above the laboratory reporting level. These detected concentrations are within the expected instru- ment or method variance, and the interpretation of ammonia and orthophosphate concentrations in surface water was not affected by the detections in the blank samples.