Analysis of ground water by different laboratories: a comparison of chloride and nitrate data, Nassau and Suffolk counties, New York

Water samples from wells in Nassau and Suffolk Counties were analyzed for chloride and nitrate. Two samples were collected at each well; one was analyzed by the U.S. Geological Survey, the other by a laboratory in the county from which the sample was taken. Results were compared statistically by paired-sample t-test to indicate the degree of uniformity among laboratory results. Chloride analyses from one of the three county laboratories differed significantly (0.95 confidence level) from that of a Geological Survey laboratory. For nitrate analyses, a significant difference (0.95 confidence level) was noted between results from two of the three county laboratories and the Geological Survey laboratory. The lack of uniformity among results reported by the participating laboratories indicates a need for continuing participation in a quality-assurance program and exercise of strong quality control from time of sample collection through analysis so that differences can be evaluated. (Kosco-USGS)