COMPATIBILITY OF SAMPLING AND LABORATORY PROCEDURES EVALUATED FOR THE 1985 3-SHIP INTERCOMPARISON STUDY ON LAKE ERIE

To update the information concerning data intercomparability from Great Lakes monitoring programs, a three-ship laboratory comparison was conducted on the central basin (mid-lake) of Lake Erie in August 1985, in which eight water quality parameters were measured Participating were ships operated by the U.S. Environmental Protection Agency, the Ohio State University, and Environment Canada. Two separate comparison studies were conducted, the first to identify that portion of variability due to analytical variability (AV), and the second to assess the overall variability (OV) in whole-ship operations, including water sampling procedures at two depths, epilimnion and hypolimnion. In general, differences in mean levels were due primarily to laboratory procedures, with significant differences occurring between at least one pair of laboratories for every parameter. For all parameters, except chloride and nitrate plus nitrite, these differences ranged from 30 to nearly 100 percent of the mean in the water layer with the lower concentration and from 12 to 95 percent in the layer with higher concentration. The results for some parameters were overly precise for one or more laboratories, which led to differences as small as 3 to 6 percent of the mean being significant (nitrate plus nitrite and chloride). However, large laboratory differences were observed for soluble reactive silica even though laboratory precision was high. These results are discussed in view of the quality assurance program of the EPA laboratory, the August 1985 monitoring of central basin stations by the three agencies, and the magnitude of changes in total phosphorus in Lake Erie over time. It is concluded that, although at least one laboratory exhibited precision better than that of the 1985 monitoring program, the differences between laboratories estimated from the intercomparison study are large enough to be of practical significance and thus careful consideration is necessary before combining results from monitoring programs of the different agencies.