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Intersite-comparison studies have been used since November 1978 to assess the accuracy and precision of pH and specific-conductance measurements made by National Atmospheric Deposition Program and National Trends Network site operators. Between November 1978 and June 1980, the first four intersitecomparison studies were administered by the Illinois State Water Survey Central Analytical Laboratory, Champaign, Illinois. The U.S. Geological Survey assumed responsibility for the program in October 1981. Twenty-four intersite-comparison studies were completed as of November 1989. The pH and specific-conductance measurements made during intersite-comparison studies were primarily evaluated using two methods of analysis. The first method of analysis evaluated site-operator reported values in terms of the measurementaccuracy criteria established by the Network Operations Subcommittee. The measurement-accuracy criteria for pH are ±0.10 unit if the most probable pH value is 5.0 or less or ±0.30 unit if the most probable pH value exceeds 5.0. In every intersite-comparison study except study 22, the median pH was less than 5.0. The measurement-accuracy criteria for specific conductance are ±4.0 microsiemens per centimeter of the most probable value. The second method of analysis evaluated the differences between site-operator reported values and network median values. The percentage of site operators reporting pH values within the measurement-accuracy criteria established by the Network Operations Subcommittee increased from an average of 61 percent for intersite-comparison studies 1 through 10 to an average of 78 percent for studies 11 through 24. The operators of 12 percent of the sites that participated in studies 1 through 24 accounted for 29 percent of the pH determinations that failed to meet the measurement-accuracy criteria. Excluding study 22 because of the different pH measurement-accuracy criteria that were applied, a nonparametric correlation analysis indicated a statistically significant association between the median pH of the reference solution and the percentage of site operators reporting pH values that met the Network Operations Subcommittee measurement-accuracy criteria in studies 11 through 24 (a=0.05, r=-0.71). No statistically significant correlation between the median pH of the reference solution and the percentage of site operators reporting pH values that met the measurement-accuracy criteria was determined for studies 1 through 10. The percentage of site operators reporting specific-conductance values within the measurement-accuracy criteria varield from 88 to 97 percent when the median specific conductance of the reference solution was less than 25 microsiemens per centimeter at 25 degrees Celsius. When the median specificconductance value of the reference solution was between 30 and 60 microsiemens per centimeter at 25 degrees Celsius, 38 to 82 reported values that met the measurement-accuracy criteria. A nonparametric correlation analys icant association between the median specific percent of the site operators is indicated a statistically signifconductance of the reference solution and the percentage of site operators reporting specific-conductance values that met the measurement-accurac^ criteria in studies 1 through 24 (a=0.05, r=-0.82).

Summary of the National Atmospheric Deposition Program/National Trends Network intersite-comparison program, November 1978-November 1989