National Water-Quality Assessment (NAWQA) Project
U.S. Geological Survey Circular 1291
Appendix 7B. Statistical summaries of pesticide compounds in ground water, 1992-2001
Summary statistics for pesticide compounds in ground water are presented below as a function of land use and analytical method. The summary statistics include detection frequencies at selected thresholds ( detections greater than or equal to 1 µg/L, detections greater than or equal to 0.1 µg/L, detections greater than or equal to 0.01 µg/L, and detections at any concentration) and selected percentiles of concentration (50th, 75th, 90th, 95th, and maximum) for each land-use class: agricultural, urban, undeveloped, and mixed land-use sites.
Table 7B-1. Agricultural land use.
Table 7B-2. Urban land use.
Table 7B-3. Undeveloped land use.
Table 7B-4. Mixed land use.
How the summary statistics were calculated:
All statistics are based on one sample per well. For each of the four land use classes, and for each detection threshold, the frequency of detection for each pesticide compound 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. The detection frequency values were multiplied by 100% to express frequency of detection as a percentage.
Percentiles of concentration were computed by ranking concentrations from low to high. Low-level detections of pesticides were not censored at the reporting level by the laboratory. All detections (pesticides conclusively identified by retention time and spectral characteristics) were quantified and reported by the laboratory. Therefore, all nondetections were ranked lower than any detection for the calculation of percentiles for land-use classes. Any calculated percentile less than the lowest detected concentration in the land-use class was considered to be a nondetection at the maximum value of the long-term method detection level for 1992-2001 and is expected to be less than the lowest detection in the land-use group. A detailed description of analytical approach and methods for statistical summaries is provided in appendix 8B.
Notes for using tables:
Frequency of detection: Detection frequencies indicate how often a compound was detected in samples. Four detection frequencies are provided in the tables: "All" detections, detections at concentrations greater than or equal to 0.01 µg/L, detections at concentrations greater than or equal to 0.1 µg/L, and detections at concentrations greater than or equal to 1 µg/L. The values for "All" detections provide the percentage of detections for a given compound, but should not be directly compared among compounds because reporting levels (detection capabilities) varied among compounds. Because maximum long-term detection levels varied, detection frequencies were calculated using three common detection thresholds (0.01, 0.1, and 1 µg/L). The use of these detection thresholds facilitates comparisons among compounds by censoring detections to a common reference concentration. Adjustments of this type are essential in order to answer questions like "is compound x detected more often than compound y?"
For most pesticides analyzed by GCMS, the lowest appropriate detection threshold for comparisons among compounds is 0.01 µg/L. Several compounds (azinphos-methyl for example) have maximum long-term detection levels that exceed the 0.01 µg/L threshold. In these cases, the detection frequency is preceded by a ">" symbol to indicate that the true percentage of samples with concentrations greater than the threshold probably is greater than or equal to that reported in the table. For most pesticides analyzed by HPLC, the lowest appropriate detection threshold for comparisons among compounds is 0.1 µg/L. Nonetheless, several compounds have maximum long-term detection levels that exceeded the threshold. In these cases, the detection frequencies may underestimate the true percentage of samples with concentrations greater than the threshold and detection frequencies are preceded with a ">" symbol. All of the pesticides can be compared at the 1 µg/L threshold. Use of this threshold censors the majority of the pesticide detections, but allows comparisons among pesticides measured at high concentrations. Frequency of detection, in percent, was rounded to the hundredths place.
Percentiles of concentration: Concentrations measured for each pesticide are summarized using percentiles. The 50th, 75th, 90th, and 95th percentiles of concentration by land-use class are provided. Percentiles provide information about the magnitude of concentrations at selected points in the cumulative frequency distribution of ranked concentrations. For example, concentrations of atrazine at the 1,406 wells in agricultural land-use studies were less than or equal to 0.0175 µg/L in 75% of the wells, and less than or equal to 0.358 µg/L in 95% of the wells (table 7B-1). The tables include a column for "Maximum." The concentrations in this column are the maximum measured concentrations for each pesticide in each land-use class. Concentrations in the tables are rounded to the thousandths place. Nondetections are denoted as “ND.”
Pesticide names and analytical methods are presented in Appendix 1A.
Information on sampling sites and their characteristics is presented in Appendix 5B.
Downloadable concentration data are presented in Appendix 6B.
Additional information on analytical approach and methods is presented in Appendix 8B.
For more information, contact:
Paul E. Stackelberg
U.S. Geological Survey
NAWQA Pesticide Synthesis Project
425 Jordan Road
Troy , NY 12180
voice: (518) 285-5652
fax: (518) 285-5601