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An Informal Assessment of Interlaboratory Variability of Mining Effluents

How variable are effluent toxicity testing data? One common response to this question is that the variability of toxicity tests is comparable to the variability associated with analytical chemistry data. For toxicity testing, this is based in large part on data derived from reference toxicant tests employing analytical-grade solutions. What about the variability associated with toxicity of realworld mining effluents, which are to be tested in the upcoming metal mining environmental effects monitoring (EEM) program in Canada? Few interlaboratory data sets using real mine effluents currently exist. In this presentation, we review available data comparing analytical and toxicological data variability among laboratories, using the relative percent difference (RPD) as a yardstick. Recognizing that the method detection limit (MDL) used in analytical variability lacks an obvious counterpart in toxicity testing, we found that toxicity test interlaboratory variability is considerably greater than analytical variability, in particular for analyses conducted at concentrations well above the MDL. Interlaboratory variability is important where toxicity test results are used to demonstrate compliance or where they may be used, for example, in an EEM program to trigger follow-up action. Our assessment demonstrates the importance of studying this issue further. An understanding of the issue of interlaboratory variability using mining effluents can only be achieved through a comprehensive round-robin testing study using real-world mining effluents. Introduction Asking a room full of analytical chemists the question “How variable are routine chemical analyses of mine effluents and receiving waters?” is sure to trigger a lively discussion with no guarantee of a consensus emerging at the end. However, they will likely agree on one issue— variability itself is variable, and in a fairly predictable way: as analyte concentration decreases and approaches the Method Detection Limit (MDL), variability increases greatly. In fact, the MDL is a function of the variability of replicate analyses of a blank sample; and so the continuing drive toward more sensitive analytical techniques is really no more than a continuing effort to reduce analytical variability. Aquatic toxicologists are also concerned with the variability associated with ecotoxicity tests, and have sought to minimize it by developing detailed and standardized test methods, (i.e., Environment Canada biological test methods) and the routine implementation of reference toxicant tests as tools in improving laboratory quality assurance and quality control. How variable are these tests? As with analytical chemistry, opinions vary, but a recent evaluation of a large data set involving two reference toxicants (ESG International and B. Zajdlik & Associates, 2002) indicates that aquatic toxicity tests using reference toxicants can exhibit variability in a range similar to that of routine chemical analyses (e.g., metals). Although this is encouraging, this may not reflect reality when it comes to the variability of effluents. Analytical variability is quite variable and the MDL is important to understanding it; but toxicity testing has no obvious counterpart to the MDL (other than, possibly, the “allowable” mortality in control exposures in acute toxicity tests). So, to avoid the problem of comparing “apples and oranges”, any comparison of toxicity testing variability and analytical variability needs to consider the impact of the MDL. Another limitation in our understanding of toxicity test variability is that most of the available data involve reference toxicants, not whole effluents. As valuable as this information is, there is no guarantee that it elucidates the variability of toxicity test data associated with mine effluents and/or receiving waters. We informally conducted our own interlaboratory variability assessment of chemical analyses and toxicity tests, based on data available to us from several different mine operations. Our objective, using “real-world” mine effluent data, was to answer the question: is toxicity test data variability similar to analytical variability in routine analyses conducted at concentrations well above the MDL? The answer, we have found, is no; toxicity test interlaboratory variability is much higher, and is actually comparable to analytical variability at or near the MDL. This preliminary conclusion, which needs to be confirmed through a statistically rigorous study, has significant implications for the use and interpretation of toxicity test data, especially in a regulatory context.