Correspondence Comment on “Pharmaceuticals, Hormones, and Other Organic Wastewater Contaminants in U.S. Streams, 1999-2000: A National Reconnaissance” The paper by Kolpin et al. (1) reviewed the results of a nationwide reconnaissance of organic wastewater contaminants by the United States Geological Survey (USGS). The paper represents a significant contribution toward the understanding of the occurrence of compounds, such as personal care products and pharmaceuticals, following their intended use. We would like to comment on two points that would mitigate some of the unintended bias of this publication: (i) By reporting a median concentration for only those samples where analytes were detected, the paper suggests the presence of emerging contaminants in U.S. water at higher median concentrations than is actually the case. (ii) The inclusion of data with reported concentrations of target contaminants in field blanks is contrary to accepted practice and overestimates the extent of contamination associated with these emerging contaminants. The median is generally used as a summary statistic to represent the center of the population distribution from which the sample data are pulled. In the sample data set, 50% of the values are above and 50% of the values are below the median. In their Table 1 (Summary of Analytical Results of Streams Sampled for 95 Organic Wastewater Contaminants), each compound is listed with summary statistics N (number of samples), RL (reporting level), freq (frequency of detection), max (maximum concentration), and med (median detectable concentration). A footnote to the table indicates that the median listed is “the median detectable concentration”. This method of reporting the median of a data set ignores all of the data points that are below the analytical detection limit. These values are termed censored data. By reporting the median detectable concentration and ignoring all of the data below the reporting limits, the table gives misleading information. A similar presentation of the data is found in Figure 2, which presents boxplots with measured concentrations for the 30 most frequently detected organic wastewater contaminants (OWCs). The boxplots show the concentration distribution truncated at the reporting level. Again, the presentation of primarily those values above the reporting levels and ignoring the “less than” data in determining the percentiles and the medians is a misrepresentation of the population from which the samples are drawn. Other investigators have developed methods to handle censored data. Helsel recommended using either robust probability plot or distributional maximum likelihood estimation (MLE) procedures for estimating the median (2). Liu et al. reported on a “model selection procedure, termed forward censored regression, for selecting an appropriate model for environmental data with censored values” (3). Other relevant publications are listed in refs 4-8. In a response to comments by Ericson et al. (9), the USGS noted that its approach to reporting the median value was thought to be a better indication of the measured value than the maximum concentration found. However, this method results 1052
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in a reported median value that suggests the concentration of the compound in the environment is significantly higher than was actually detected. The reported results of this investigation would be more meaningful had the authors used an applicable statistical method to include the censored data in their assessment of the results of this important project. If the number of detects for a particular compound was insufficient to use these methods (e.g., greater than 50% nondetects), then the median should have been reported as below the level of detection. The second data reporting issue we note is field blank contamination. The Kolpin et al. paper reports concentrations of cholesterol using two analytical methods. Method 5 resulted in detection of this compound in 84.3% of the locations tested and a median reported concentration of 0.83 µg/L. It is reported in the paper that all of the field blanks for Method 5 had concentrations of cholesterol with a median concentration of 0.09 µg/L. Cholesterol concentrations from 0.005 to 0.18 µg/L (i.e., less than two times the median concentration in field blanks) were assumed to be below the RL. Data for cholesterol in the reagent blanks were “not currently available” in the Water Quality Data Report found at the USGS Web page (10). These data may have been instructive in evaluating the source of contamination in the field blanks. It is standard good laboratory practice to reject data when the target analyte is detected in the field blank. The U. S. Environmental Protection Agency (U.S. EPA) guidance is to reject data when the reported concentration of the target analyte is less than 10 times greater than that in the field blank (11). On the basis of the reported median concentrations, cholesterol data developed using Method 5 should have been disqualified from the study because of field blank contamination. Since cholesterol is one of the most frequently detected compounds in the USGS data set, the authors have overstated the extent of contamination from emerging contaminants by not properly rejecting these data. The USGS data reported in this Journal will become the national database against which all future analyses will be compared. The USGS should revise the median value and sample concentrations where blanks had contamination to ensure that the reported data provide an accurate snapshot of surface water concentrations of these compounds.
Literature Cited (1) Kolpin, D. W.; Furlong, E. T.; Meyer, M. T.; Thurman, E. M.; Zaugg, S. D.; Barber, L. B.; Buxton, H. T. Environ. Sci. Technol. 2002, 36 (6), 1202-1211. (2) Helsel, D. R. Environ. Sci. Technol. 1990, 24 (12), 1767-1774. (3) Liu, S.; Lu, J.-C.; Kolpin, D. W.; Meeker, W. Q. Environ. Sci. Technol. 1997, 31 (12), 3358-3362. (4) Gilliom, R. J.; Helsel, D. R. Water Resour. Res. 1986, 22 (2), 135-146. (5) Gilliom, R. J.; Hirsch, R. M.; Gilroy, E. J. Environ. Sci. Technol. 1984, 18 (7), 530-535. (6) Helsel, D. R.; Cohn, T. A. Water Resour. Res. 1988, 24 (12), 1997-2004. (7) Helsel, D. R.; Gilliom, R. J. Water Resour. Res. 1986, 22 (2), 147-155. (8) Shumway, R. H.; Azari, R. S.; Kayhanian, M. Environ. Sci. Technol. 2002, 36 (15), 3345-3353. (9) Kolpin, D. W.; Furlong, E. T.; Meyer, M. T.; Thurman, E. M.; Zaugg, S. D.; Barber, L. B.; Buxton, H. T. Environ. Sci. Technol. 2003, 37. 1054. 10.1021/es020943r CCC: $25.00
2003 American Chemical Society Published on Web 01/23/2003
(10) U.S. Geological Survey. Water-quality data for pharmaceuticals, hormones, and other organic contaminants in U.S. streams, 1999-2000; http://toxics.usgs.gov/pubs/OFR-02-94/index.html (reviewed July 25, 2002). (11) U.S. Environmental Protection Agency. Guidance on evaluation, resolution, and documentation of analytical problems associated with compliance monitoring; EPA 821-B-93-001; U.S. Goverment Printing Office: Washington, DC, June 1993.
Alice E. Till* Pharmaceutical Research and Manufacturers of America 1100 15th Street NW Washington, D.C. 20005 ES020943R
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