Correspondence Comment on “Misinterpretation of Drinking Water Quality Monitoring Data with Implications for Risk Management” Received for review October 18, 2006. Accepted March 1, 2007. Dear ES&T Editor, I was drawn to this recent ES&T article by its title (1). The people in question were water professionals and environmental engineering and science professors, and most of them had more than 10 years experience in the water business. How could they misinterpret drinking water quality monitoring data with implications for risk management? As I quickly went through the article, I found relief as the monitoring scenario presented by the authors is hypothetical and the accusation is primarily based on the surveyed answers to a statistical question that lacks practical applicability as well as a legal basis for real-time decision-making in response to a positive monitoring result of a contaminant like atrazine in drinking water. “Misinterpretation of drinking water quality monitoring data....” (1) is a broad charge with significant consequences (at the minimum, it could shake the public trust in the water industry). It would have been better if the authors supported their conclusions with real case studies. The hypothetical monitoring scenario presented by the authors (1) is not realistic, as the primary concern for a contaminant like atrazine in drinking water in the United States is its concentration above the maximum contamination level (MCL) not its presence or absence with respect to the methods detection limit. The MCLs are enforceable drinking water standards and are established on the basis of health effects, treatment capability, and monitoring availability and costs. According to the Code of Federal Regulations (2), only the actual monitoring data (analytical results) should be used and the procedure to determine compliance with the MCLs is straightforward. I do not believe that the water professionals would miss this regulatory requirement, which is the most important interpretation of drinking water quality monitoring data, even though they might not answer the authors’ statistical question correctly for whatever reasons. The current MCL for atrazine in the United States is 3 µg/L (2), which is substantially higher than the detection limits of analytical methods for atrazine (0.1 to 0.01 µg/L (3-5)). Therefore, a person in charge of the water treatment plant should not and will not take any special actions simply because of a positive monitoring result above the method detection limit. Furthermore, compliance with the MCLs should be determined based on the analytical results obtained at each sampling point (2). An organic contaminant like atrazine in drinking water at a concentration around the methods detection limit or even at the MCL does not cause immediate hazards or short-term acute health problems. Also, the contaminant typically occurs infrequently and some monitoring results could be false positive, as noticed by the authors as well. The federal law (2) therefore typically allows a water utility to determine compliance with the MCL by a running
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annual average of all samples taken at each sampling point. For a water utility that conducts monitoring annually or less frequently, the state may require a confirmation sample when the level of a contaminant is greater than the MCL at a sampling point (2). These practices under the law (2) could be additional reasons why many respondents chose to take no immediate or serious action (such as closing down the water treatment plant), to check and confirm the positive monitoring result, and to request re-sampling tests for the hypothetical monitoring scenario. The people in question were never offered the opportunity to examine the experimental evidence (original monitoring data/analytical results) and to know how the frequency of true positive results was established by the authors for water samples from consumers’ taps (note: the positive results should be determined based on the MCL not the method detection limit). The basic questions are (1) how many water samples would have to be taken for the possible determination of the frequency of one positive result truly present in every 1000 water samples, and (2) was this frequency determined based on water samples from all the consumers’ taps (as a whole) or from each individual tap (sampling point)? Theoretically, the frequency of true positive results tells precisely the risk probability with the concerned pollutant in the water samples. If it is known a priori and the circumstance can be assumed to be the same, then the frequency of true positive results (regardless the probability of a positive test result being true) can be used to predict the future risk probability and decide the (financial and technical) needs to address it in an overall risk management plan. For determining compliance with the MCLs, however, it is neither feasible nor necessary to know the frequency of true positive results or the probability of a positive test result being true, and the predictions based on the past experimental evidence or “best intelligence” play no direct role as the law (2) specifies that the actual monitoring data obtained at each sampling point should be used.
Literature Cited (1) Rizak, S. N.; Hrudey, S. E. Misinterpretation of drinking water quality monitoring data with implications for risk management. Environ. Sci. Technol. 2006, 40 (17), 5244-5250. (2) Code of Federal Regulations, Part 141, Title 40, July 1, 2003; pp 341-565. (3) WHO. Guidelines for Drinking-Water Quality, 2nd ed.; World Health Organization: Geneva, 1996; Vol. 2. (4) Ma, W. T.; Cai, Z. W.; Jiang, G. B. Determination of atrazine, deethylatrazine and simazine in water at parts-per-trillion levels using solid-phase extraction and gas chromatography/ ion trap mass spectrometry. Rapid Commun. Mass Spectrom. 2003, 17 (24), 2707-2712. (5) Seitz, W.; Schulz, W.; Weber, W. H. Novel applications of highly sensitive liquid chromatography/mass spectrometry for the direct detection of ultra-trace levels of contaminants in water. Rapid Commun. Mass Spectrom. 2006, 20 (15), 2281-2285.
Shaoying Qi Department of Civil & Environmental Engineering University of Illinois at Urbana-Champaign ES062505G
10.1021/es062505g CCC: $37.00
2007 American Chemical Society Published on Web 03/30/2007
