National Water-Quality Assessment (NAWQA) Project
U.S. Geological Survey Circular 1291
Appendix 7D. Statistical summaries of organochlorine pesticide compounds in whole fish, 1992-2001
Summary statistics for organochlorine pesticide compounds and selected pesticide groups in whole fish are presented below as a function of land-use class (agricultural, urban, undeveloped, and mixed land-use sites). The summary statistics include detection frequencies computed two ways: detections greater than or equal to a threshold concentration of 5 µg/kg wet weight, and detections at any concentration. Statistics also include selected concentration percentiles (50th, 75th, 90th, 95th, and maximum) for each land-use class.
Statistical summaries by land-use class:
Table 7D-1. Agricultural land use.
Table 7D-2. Urban land use.
Table 7D-3. Undeveloped land use.
Table 7D-4. Mixed land use.
How the summary statistics were calculated:
All statistics are based on one sample per stream site. Low-level detections of pesticides were not censored at the reporting level by the laboratory. A detailed description of the approach for analyzing organochlorine pesticides in whole fish and computing statistical summaries is provided in Appendix 8C.
For each of the four land use classes, and for each detection threshold, the frequency of detection for each pesticide compound in whole fish was calculated as follows:
(n / N) * 100
Where N is the number of samples analyzed for the pesticide compound and n is the number of samples in which the specific pesticide compound was detected at or above the threshold. Samples with missing data were dropped from the analysis. Samples with raised reporting levels (that is, reported as not detected above a specified concentration that is higher than the reporting level, such as <10 µg/kg for a compound with a reporting level of 5 µg/kg; see Appendix 8C) were treated as missing data, and were dropped from the analysis.
Percentiles of concentration were computed using the Statistical Analysis System (SAS) Proc Univariate procedure. Samples with missing data were dropped from the analysis. Samples were ranked from low to high concentrations within each land-use class, and all nondetections were ranked lower than any detection within the class. Any calculated percentile less than the lowest detected concentration in the land-use class was considered to be a nondetection at the reporting level and is expected to be less than the lowest detection in the land-use class.
Summary statistics are presented for individual pesticide compounds analyzed in whole fish, as well as for two groups of related pesticide compounds, total DDT and total chlordane. (The component compounds making up each of these two pesticide groups are provided in footnotes to Appendix 1B.) In each sample, concentrations of total DDT and total chlordane were determined by summing concentrations of the individual components of each group, with missing data and nondetections treated as zero concentrations. Therefore, summed concentrations are lower-bound estimates of the concentrations for total DDT and total chlordane.
Notes for using tables:
Reporting level: This is the concentration consistently reported by the NWQL for a given pesticide compound in whole fish, which reflects the sensitivity of the method for that compound. It is determined by the NWQL on the basis of quality-control data. For samples in which a given compound was not detected, the value is reported as the reporting level, along with a "less than" remark code. For example, a routine non-detection of dieldrin in fish would be reported as <5 µg/kg (wet weight).
Number of sites: This is the number of sites (which also equals the number of samples) used in the calculation of detection frequencies and concentration percentiles for this compound. For fish, the number of sites is the same whether detection frequencies are computed using either (a) all detections or (b) the 5 µg/kg detection threshold.
Detection frequencies: The detection frequency indicates the percentage of sites (which also equals the number of samples) at which a given compound was detected, which is a function of the sensitivity of the analytical method used for that compound. In whole fish, detection frequencies were calculated for two detection thresholds: (1) “All” detections (detections at any concentration—some as low as 0.8 µg/kg wet weight); (2) detections at or greater than 5 µg/kg wet weight. Most pesticide compounds (all except toxaphene) analyzed by NAWQA in fish have the same reporting level (5 µg/kg). However, because detections below the reporting level are sometimes (but not consistently) reported, comparison of detection frequencies among different compounds should be done at a common detection threshold. (See discussion of "censoring" data in Appendix 8C). For fish, the common reporting threshold is 5 µg/kg for all pesticide compounds except toxaphene, which is a complex mixture and has a reporting level of 200 µg/kg—this means that the detection frequencies for toxaphene in tables 7D-1 through 7D-4 cannot be compared directly with those of the other compounds analyzed in fish.
Frequency of detection, in percent, was rounded to the hundredths place.
Percentiles of concentration: Concentrations measured for each pesticide are summarized using percentiles and the maximum concentration. The 50th, 75th, 90th, and 95th concentration percentiles for each land-use class are provided. The tables also include a column for "Maximum," which is the highest measured concentration at any site within this land-use class.
Percentiles provide information about the concentrations at selected points in the cumulative frequency distribution of the ranked concentrations. For example, concentrations of p,p’-DDT in whole fish were less than or equal to 64 µg/kg at 95 percent of agricultural sites (or samples), less than or equal to 42 µg/kg at 90 percent of agricultural sites, less than or equal to 8.6 µg/kg at 75 percent of agricultural sites, and less than the reporting level (not detected) at 50 percent of agricultural sites. If percentiles are detected concentrations (as opposed to nondetections), then the percentiles also may be interpreted as the percentage of samples in which concentrations were greater than or equal to a given concentration. For example, concentrations of p,p’-DDT in whole fish were greater than or equal to 64 µg/kg at 5 percent of agricultural sites, greater than or equal to 42 µg/kg at 10 percent of agricultural sites, and greater than or equal to 8.6 µg/kg at 25 percent of agricultural sites (table 7D-1).
Nondetections are denoted as “ND.”
Pesticide names are presented in Appendix 1B.
Information on sampling sites and their characteristics is presented in Appendix 5C.
Downloadable concentration data are presented in Appendix 6D.
Additional information on approach and methods is presented in Appendix 8C.
For more information, contact:
Lisa H. Nowell
U.S. Geological Survey
NAWQA Pesticide Synthesis Project
6000 J Street
Sacramento , CA 95819-6129
voice: (916) 278-3096
fax: (916) 278-3070