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The Quality of Our Nation's Waters
Pesticides in the Nation's Streams and Ground Water, 1992-2001

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U.S. Geological Survey Circular 1291

Appendix 8B. Analytical Approach and Methods for Pesticides in Ground Water

INTRODUCTION

This appendix provides information on the approach and methods used for analysis of ground-water data summarized in USGS Circular 1291, Pesticides in the Nation's Streams and Ground Water, 1992-2001: The Quality of Our Nation's Waters (Gilliom and others, 2006).

APPROACH AND METHODS

Study design and site selection: Ground-water studies in each Study Unit were designed in accordance with the NAWQA national design strategy for the occurrence and distribution of water-quality conditions (Gilliom and others, 1995). The national study design for ground water focuses on water-quality conditions in major aquifers, with emphasis on recently recharged (shallow) ground water. Ground-water data summarized in USGS Circular 1291 (Gilliom and others, 2006) were collected from two types of ground-water studies: (1) land-use studies, and (2) major-aquifer studies. Major-aquifer studies are referred to as study-unit surveys in Gilliom and others (1995).

Ground-water sites sampled for the NAWQA program were not selected to be statistically representative of the Nation’s land use or any other factor. Rather, the NAWQA national design is intended to provide a framework for comparing water-quality among sites to improve the understanding of the relation between water quality and land use, pesticide use, soils, climate, and other natural or human influences.

Major-aquifer studies: Major-aquifer studies (MASs) are designed to assess the chemical quality of regionally extensive aquifer systems that represent important current or future ground-water resources in each Study Unit. Major aquifers selected for study are expected to be relatively homogenous in water-quality characteristics compared to the Study Unit as a whole. Within each MAS area, all existing wells that are suitable for sampling are inventoried. Protocols and procedures for well selection are provided in Lapham and others (1995). Wells are deemed suitable for sampling if (among other considerations) they are not under the influence of known ground-water contamination and are thought to provide samples representative of the chemical quality of the targeted ground-water resource. From the inventoried wells, 20 to 30 are selected for sampling using a grid-based random site-selection approach (Scott, 1990). Most wells sampled for MASs are either public- or domestic-supply wells. Because public- and domestic-supply wells are typically finished well below the ground-water table and have relatively large contributing areas (as compared to shallow observation wells), it is assumed that water samples collected from these wells represent the influence of multiple or ‘mixed’ land uses.

Land-use studies: Land-use studies (LUSs) are designed to assess the concentrations and distribution of water-quality constituents in recently recharged (shallow) ground water associated with regionally extensive combinations of land-use settings and hydrogeologic conditions in each Study Unit. The primary land-use settings sampled are agriculture and urban because pesticide use is greatest in these settings; however, to a lesser degree, undeveloped areas are also sampled. Assessment of these settings is accomplished by sampling a combination of low-capacity, pre-existing wells and (or) installing and sampling observation wells that are finished near (typically within 20 feet of) the water table. Protocols and procedures for well selection and installation of observation wells are provided in Lapham and others (1995). Typically, 20 to 30 wells are sampled per LUS. Wells and well locations selected for a LUS are randomly distributed throughout the occurrences of the targeted setting (combination of land use and hydrogeologic conditions) using the grid-based random selection method described in Scott (1990). Prior to installing or sampling a well, the land-use setting within a 500-meter buffer around the site is confirmed to represent the targeted setting.

Sample collection and processing: Ground-water and quality-control samples were collected and processed following procedures and protocols described in Koterba and others (1995). Before a water-quality sample is collected, the well is purged of standing water to ensure that the sample represents the chemistry of the aquifer rather than water standing in the well casing. Well purging continues until at least 3 casing volumes of water have been removed and 5 successive measurements of temperature, dissolved oxygen, specific conductance, and pH are each within specified tolerances. All sample collection and processing equipment that comes in contact with sample water is constructed of Teflon, glass, aluminum, or stainless steel. Sampling equipment is cleaned with a dilute solution of phosphate-free detergent and rinsed with deionized water and pesticide-grade methanol prior to sample collection. Water samples are collected in 1-liter pre-baked amber glass bottles after being filtered using pre-combusted glass-fiber filters with a nominal 0.7-µm pore diameter to remove suspended particulate matter. Filtered samples are placed on ice and shipped to the U.S. Geological Survey’s (USGS) National Water Quality Laboratory (NWQL) in Denver, Colorado for analysis. To ensure that sample collection and handling procedures do not introduce contamination to water-quality samples, specified quality-control samples are collected for each ground-water study. The field quality-control program included the collection of field blank water samples to asses potential contamination, replicate water samples to asses variability, and field matrix spikes to assess potential pesticide degradation or matrix effects. Contamination in field blank water samples is summarized in Martin and others (1999). Variability in replicate water samples is summarized in Martin (2002). Pesticide recovery in laboratory reagent spikes and field matrix spikes is summarized in Martin (1999).

Pesticides, analytical methods, and reporting levels: Most NAWQA water samples were analyzed for 75 pesticides and 8 pesticide degradates (Gilliom and others, 2006, Appendix 1A). These pesticides account for approximately 78 percent of the total amount (by weight) of pesticides used for agriculture in the United States in 1997 (Gilliom and others, 2006, figure 3-4) and a substantial portion of urban and suburban use. Water samples were analyzed for pesticides at the USGS NWQL. Two analytical methods were used extensively during the first cycle (1992-2001) of the NAWQA program—GCMS and HPLC. Both methods use solid-phase extraction to remove pesticides from filtered water samples.

The GCMS method was the primary analytical method used by all Study Units. The GCMS method provides low-level analyses of 47 pesticides or degradates by gas chromatography/mass spectrometry and selected-ion monitoring (Zaugg and others, 1995). The pesticide acetochlor was added to the GCMS method in June 1994 (Lindley and others, 1996); consequently some wells used in the national assessment do not have analyses of acetochlor.

The HPLC method was used extensively by Study Units that began investigations in 1991 and 1994. The HPLC method provides low-level analyses of 36 additional pesticides or degradates by high-performance liquid chromatography and photodiode-array detection (Werner and others, 1996). This method was used less frequently by Study Unit investigations that were initiated in 1997; thus, about one third of the sites used in the national assessment did not have analyses by HPLC.

Low-level detections of pesticides by these analytical methods are not censored at the reporting limit. All detections (pesticides conclusively identified by retention time and spectral characteristics) are quantified and concentrations less than the reporting limit are reported with an “E” remark to indicate that the concentration--but not the presence--is estimated. In addition, concentrations less than the lowest calibration standard, concentrations extrapolated above the highest calibration standard, or samples diluted to bring concentrations within the range of the calibration standards also are remarked “E” (Oblinger Childress and others, 1999, p. 8-10). Concentrations in excess of 20.0 µg/L determined by GCMS or in excess of 1.50 µg/L determined by HPLC generally are above the calibration curve of the method and are remarked “E.” Data users should infer that the uncertainty in the concentration for a concentration remarked “E” is expected to be relatively larger than that for a concentration without an “E” remark.

Any detections of five pesticides analyzed by GCMS (azinphos-methyl, carbaryl, carbofuran, deethylatrazine, and terbacil) and six pesticides analyzed by HPLC (aldicarb, aldicarb sulfone, aldicarb sulfoxide, chlorothalonil, dichlobenil, and DNOC) are reported with an “E” remark, regardless of concentration. These pesticides have lower or more variable recovery relative to the other pesticides analyzed by the method (Zaugg and others, 1995, p. 35; Werner and others, 1996, pp. 27, 34; U.S. Geological Survey, 1998). The presence or absence of an “E” remark for any pesticide compound was not considered in the statistical analysis of the concentration data.

The types and numerical values of reporting levels have changed through time (table A). A reporting level is the “less than” concentration reported when a pesticide is determined to not be present (for example: < 0.005 µg/L). Procedures used by NWQL to set reporting levels, the types of reporting levels, and considerations for data analysis are discussed in Oblinger Childress and others (1999). All reporting levels for pesticide non detections reported by the NWQL (except those for samples with analytical difficulties resulting in “raised reporting levels,” discussed below) were changed to the maximum value of the long-term method detection limit (LT-MDL) for water years 1992-2002 (MAXLTMDL in table A). The maximum value of the LT-MDL was determined by review of NWQL records of annual values of LT-MDL.

Table A. Reporting levels for pesticides analyzed in NAWQA water samples, 19922002 water years.

[Pesticide and degradates are sorted by analytical method and NWIS parameter code. GCMS, gas chromatography/mass spectrometry; HPLC, high-performance liquid chromatography; NWIS parameter code, the number used to identify a pesticide in the U.S. Geological Survey National Water Information System; CRL, common reporting level for the indicated water year, MAXCRL, maximum common reporting level for water years 1992–2002; MAXLTMDL, maximum long-term method detection level for water years 1992–2002; µg/L, micrograms per liter; na, pesticide not analyzed]

Method NWIS parameter code Pesticide CRL 1992 (µg/L) CRL 1993 (µg/L) CRL 1994 (µg/L) CRL 1995 (µg/L) CRL 1996 (µg/L) CRL 1997 (µg/L) CRL 1998 (µg/L) CRL 1999 (µg/L) CRL 2000 (µg/L) CRL 2001 (µg/L) CRL 2002 (µg/L) MAXCRL (µg/L) MAXLTMDL (µg/L)
GCMS 04024 Propachlor 0.007 0.007 0.007 0.007 0.007 0.007 0.007 0.007 0.007 0.01 0.01 0.01 0.005
GCMS 04028 Butylate 0.002 0.002 0.002 0.002 0.002 0.002 0.002 0.002 0.002 0.002 0.002 0.002 0.001
GCMS 04035 Simazine 0.005 0.005 0.005 0.005 0.005 0.005 0.005 0.005 0.005 0.011 0.005 0.011 0.006
GCMS 04037 Prometon 0.018 0.018 0.018 0.018 0.018 0.018 0.018 0.018 0.018 0.015 0.015 0.018 0.007
GCMS 04040 Deethylatrazine 0.002 0.002 0.002 0.002 0.002 0.002 0.002 0.002 0.002 0.006 0.006 0.006 0.003
GCMS 04041 Cyanazine 0.004 0.004 0.004 0.004 0.004 0.004 0.004 0.004 0.004 0.018 0.018 0.018 0.009
GCMS 04095 Fonofos 0.003 0.003 0.003 0.003 0.003 0.003 0.003 0.003 0.003 0.0027 0.0027 0.003 0.001
GCMS 34253 alpha-HCH 0.002 0.002 0.002 0.002 0.002 0.002 0.002 0.002 0.002 0.0046 0.0046 0.0046 0.002
GCMS 34653 p,p'-DDE 0.006 0.006 0.006 0.006 0.006 0.006 0.006 0.006 0.006 0.0025 0.0025 0.006 0.001
GCMS 38933 Chlorpyrifos 0.004 0.004 0.004 0.004 0.004 0.004 0.004 0.004 0.004 0.005 0.005 0.005 0.003
GCMS 39341 gamma-HCH 0.004 0.004 0.004 0.004 0.004 0.004 0.004 0.004 0.004 0.004 0.004 0.004 0.002
GCMS 39381 Dieldrin 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.0048 0.0048 0.0048 0.002
GCMS 39415 Metolachlor 0.002 0.002 0.002 0.002 0.002 0.002 0.002 0.002 0.002 0.013 0.013 0.013 0.006
GCMS 39532 Malathion 0.005 0.005 0.005 0.005 0.005 0.005 0.005 0.005 0.005 0.027 0.027 0.027 0.014
GCMS 39542 Parathion 0.004 0.004 0.004 0.004 0.004 0.004 0.004 0.004 0.004 0.007 0.01 0.01 0.005
GCMS 39572 Diazinon 0.002 0.002 0.002 0.002 0.002 0.002 0.002 0.002 0.002 0.005 0.005 0.005 0.003
GCMS 39632 Atrazine 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.007 0.007 0.007 0.004
GCMS 46342 Alachlor 0.002 0.002 0.002 0.002 0.002 0.002 0.002 0.002 0.002 0.0024 0.0045 0.0045 0.002
GCMS 49260 Acetochlor na na 0.002 0.002 0.002 0.002 0.002 0.002 0.002 0.0041 0.006 0.006 0.003
GCMS 82630 Metribuzin 0.004 0.004 0.004 0.004 0.004 0.004 0.004 0.004 0.004 0.006 0.006 0.006 0.003
GCMS 82660 2,6-Diethylaniline 0.003 0.003 0.003 0.003 0.003 0.003 0.003 0.003 0.003 0.0017 0.006 0.006 0.003
GCMS 82661 Trifluralin 0.002 0.002 0.002 0.002 0.002 0.002 0.002 0.002 0.002 0.009 0.009 0.009 0.005
GCMS 82663 Ethalfluralin 0.004 0.004 0.004 0.004 0.004 0.004 0.004 0.004 0.004 0.009 0.009 0.009 0.005
GCMS 82664 Phorate 0.002 0.002 0.002 0.002 0.002 0.002 0.002 0.002 0.002 0.011 0.011 0.011 0.006
GCMS 82665 Terbacil 0.007 0.007 0.007 0.007 0.007 0.007 0.007 0.007 0.007 0.034 0.034 0.034 0.017
GCMS 82666 Linuron 0.002 0.002 0.002 0.002 0.002 0.002 0.002 0.002 0.002 0.035 0.035 0.035 0.018
GCMS 82667 Parathion-methyl 0.006 0.006 0.006 0.006 0.006 0.006 0.006 0.006 0.006 0.006 0.006 0.006 0.003
GCMS 82668 EPTC 0.002 0.002 0.002 0.002 0.002 0.002 0.002 0.002 0.002 0.002 0.002 0.002 0.001
GCMS 82669 Pebulate 0.004 0.004 0.004 0.004 0.004 0.004 0.004 0.004 0.004 0.0016 0.0041 0.0041 0.002
GCMS 82670 Tebuthiuron 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.016 0.016 0.016 0.008
GCMS 82671 Molinate 0.004 0.004 0.004 0.004 0.004 0.004 0.004 0.004 0.004 0.0016 0.0016 0.004 0.001
GCMS 82672 Ethoprop 0.003 0.003 0.003 0.003 0.003 0.003 0.003 0.003 0.003 0.005 0.005 0.005 0.002
GCMS 82673 Benfluralin 0.002 0.002 0.002 0.002 0.002 0.002 0.002 0.002 0.002 0.01 0.01 0.01 0.005
GCMS 82674 Carbofuran 0.003 0.003 0.003 0.003 0.003 0.003 0.003 0.003 0.003 0.02 0.02 0.02 0.01
GCMS 82675 Terbufos 0.013 0.013 0.013 0.013 0.013 0.013 0.013 0.013 0.013 0.017 0.017 0.017 0.009
GCMS 82676 Pronamide 0.003 0.003 0.003 0.003 0.003 0.003 0.003 0.003 0.003 0.0041 0.0041 0.0041 0.002
GCMS 82677 Disulfoton 0.017 0.017 0.017 0.017 0.017 0.017 0.017 0.017 0.017 0.021 0.021 0.021 0.011
GCMS 82678 Triallate 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.0023 0.0023 0.0023 0.001
GCMS 82679 Propanil 0.004 0.004 0.004 0.004 0.004 0.004 0.004 0.004 0.004 0.011 0.011 0.011 0.005
GCMS 82680 Carbaryl 0.003 0.003 0.003 0.003 0.003 0.003 0.003 0.003 0.003 0.041 0.041 0.041 0.021
GCMS 82681 Thiobencarb 0.002 0.002 0.002 0.002 0.002 0.002 0.002 0.002 0.002 0.0048 0.0048 0.0048 0.002
GCMS 82682 Dacthal 0.002 0.002 0.002 0.002 0.002 0.002 0.002 0.002 0.002 0.003 0.003 0.003 0.002
GCMS 82683 Pendimethalin 0.004 0.004 0.004 0.004 0.004 0.004 0.004 0.004 0.004 0.01 0.022 0.022 0.011
GCMS 82684 Napropamide 0.003 0.003 0.003 0.003 0.003 0.003 0.003 0.003 0.003 0.007 0.007 0.007 0.003
GCMS 82685 Propargite 0.013 0.013 0.013 0.013 0.013 0.013 0.013 0.013 0.013 0.023 0.023 0.023 0.011
GCMS 82686 Azinphos-methyl 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.05 0.05 0.05 0.02
GCMS 82687 cis-Permethrin 0.005 0.005 0.005 0.005 0.005 0.005 0.005 0.005 0.005 0.006 0.006 0.006 0.003
HPLC 04029 Bromacil na 0.035 0.035 0.035 0.035 0.035 0.035 0.035 0.06 0.09 0.09 0.09 0.04
HPLC 38442 Dicamba na 0.035 0.035 0.035 0.035 0.035 0.035 0.035 0.043 0.043 0.11 0.11 0.05
HPLC 38482 MCPA na 0.05 0.05 0.05 0.05 0.05 0.17 0.17 0.17 0.08 0.2 0.2 0.1
HPLC 38487 MCPB na 0.035 0.035 0.035 0.035 0.035 0.14 0.14 0.13 0.13 0.26 0.26 0.13
HPLC 38501 Methiocarb na 0.026 0.026 0.026 0.026 0.026 0.026 0.026 0.026 0.07 0.07 0.07 0.03
HPLC 38538 Propoxur na 0.035 0.035 0.035 0.035 0.035 0.035 0.035 0.08 0.12 0.12 0.12 0.06
HPLC 38711 Bentazon na 0.014 0.014 0.014 0.014 0.014 0.014 0.014 0.035 0.035 0.05 0.05 0.03
HPLC 38746 2,4-DB na 0.035 0.035 0.035 0.035 0.035 0.24 0.24 0.1 0.1 0.25 0.25 0.13
HPLC 38811 Fluometuron na 0.035 0.035 0.035 0.035 0.035 0.035 0.035 0.06 0.06 0.06 0.06 0.03
HPLC 38866 Oxamyl na 0.018 0.018 0.018 0.018 0.018 0.018 0.018 0.018 0.018 0.16 0.16 0.08
HPLC 39732 2,4-D na 0.035 0.035 0.035 0.035 0.035 0.15 0.15 0.11 0.11 0.16 0.16 0.08
HPLC 39742 2,4,5-T na 0.035 0.035 0.035 0.035 0.035 0.035 0.035 0.04 0.04 0.07 0.07 0.04
HPLC 39762 2,4,5-TP na 0.021 0.021 0.021 0.021 0.021 0.021 0.021 0.06 0.025 0.025 0.06 0.03
HPLC 49235 Triclopyr na 0.05 0.05 0.05 0.05 0.05 0.25 0.25 0.25 0.07 0.07 0.25 0.04
HPLC 49236 Propham na 0.035 0.035 0.035 0.035 0.035 0.035 0.035 0.035 0.09 0.22 0.22 0.11
HPLC 49291 Picloram na 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.09 0.09 0.09 0.04
HPLC 49292 Oryzalin na 0.019 0.019 0.019 0.019 0.019 0.31 0.31 0.31 0.28 0.28 0.31 0.14
HPLC 49293 Norflurazon na 0.024 0.024 0.024 0.024 0.024 0.024 0.024 0.042 0.042 0.042 0.042 0.021
HPLC 49294 Neburon na 0.015 0.015 0.015 0.015 0.015 0.015 0.015 0.07 0.017 0.07 0.07 0.03
HPLC 49296 Methomyl na 0.017 0.017 0.017 0.017 0.017 0.017 0.017 0.017 0.017 0.47 0.47 0.24
HPLC 49297 Fenuron na 0.013 0.013 0.013 0.013 0.013 0.013 0.013 0.07 0.07 0.07 0.07 0.03
HPLC 49299 DNOC na 0.035 0.035 0.035 0.035 0.035 0.42 0.42 0.42 0.25 0.25 0.42 0.13
HPLC 49300 Diuron na 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.06 0.056 0.12 0.12 0.06
HPLC 49301 Dinoseb na 0.035 0.035 0.035 0.035 0.035 0.035 0.035 0.06 0.09 0.09 0.09 0.04
HPLC 49302 Dichlorprop na 0.032 0.032 0.032 0.032 0.032 0.032 0.032 0.032 0.05 0.12 0.12 0.06
HPLC 49303 Dichlobenil na 0.02 0.02 0.02 0.02 0.02 1.2 1.2 0.07 0.049 0.09 1.2 0.05
HPLC 49304 Dacthal monoacid na 0.017 0.017 0.017 0.017 0.017 0.017 0.017 0.039 0.07 0.07 0.07 0.04
HPLC 49305 Clopyralid na 0.05 0.05 0.05 0.05 0.05 0.23 0.23 0.23 0.42 0.42 0.42 0.21
HPLC 49306 Chlorothalonil na 0.035 0.035 0.035 0.035 0.035 0.48 0.48 0.48 0.13 0.25 0.48 0.07
HPLC 49308 3-Hydroxycarbofuran na 0.014 0.014 0.014 0.014 0.014 0.014 0.014 0.11 0.11 0.11 0.11 0.05
HPLC 49311 Bromoxynil na 0.035 0.035 0.035 0.035 0.035 0.035 0.035 0.04 0.07 0.07 0.07 0.03
HPLC 49312 Aldicarb na 0.016 0.016 0.016 0.016 0.016 0.55 0.55 0.21 0.21 0.21 0.55 0.1
HPLC 49313 Aldicarb sulfone na 0.016 0.016 0.016 0.016 0.016 0.1 0.1 0.1 0.2 0.2 0.2 0.1
HPLC 49314 Aldicarb sulfoxide na 0.021 0.021 0.021 0.021 0.021 0.021 0.021 0.021 0.021 0.27 0.27 0.14
HPLC 49315 Acifluorfen na 0.035 0.035 0.035 0.035 0.035 0.035 0.035 0.09 0.05 0.05 0.09 0.04
HPLC 61188 Chloramben methyl ester na 0.011 0.011 0.011 0.011 0.011 0.42 0.42 0.14 0.14 0.21 0.42 0.11

Water-quality data: The ground-water data summarized in USGS Circular 1291 were compiled March 29, 2002 and were available through January 15, 2002. Because not all sampling for ground-water networks was completed by January 2002, the dataset was subsequently augmented in 2004 to include samples collected post January 2002. The compiled data were checked to ensure the exclusion of quality-control samples and unusually high pesticide concentrations were verified by Study-Unit personnel.

Some analytical values were not used in the national assessment. The maximum “common” reporting level for water years 1992-2002 (MAXCRL in table A) was used to identify nondetections of pesticides that could be attributed to sample analytical difficulties such as matrix interference. Any analytical value reported as a nondetection at a concentration greater than the maximum common reporting level (a “raised” reporting level) was deleted from the data set. The common reporting level is the most frequently occurring reporting level for each pesticide and water year (for example, CRL 1985 in table A) in the water-quality data set. Analytical data remarked “V” also were deleted from the data set. Analytical data remarked “V” by Study-Unit personnel indicates that a value was affected by a significant amount of contamination (U.S. Geological Survey, 1997).

Selection of samples for the national assessment: Multiple samples may have been collected and analyzed for pesticides from a single well to accommodate various purposes such as trends analysis and quality control. To preclude bias in summary statistics from wells that were sampled more than once, the data set was processed such that each well had a single pesticide analysis. Wells that were a part of multiple networks (for example, a LUS and MAS) were assigned to one network based on well type: observation wells were assigned to the LUS and other well types were assigned to the MAS. One sample from well 401658074505001 was used in two MASs (delrsus1 and linjsus3). In total, 5,047 ground-water samples that were collected from 187 ground-water studies were summarized in USGS circular 1291.

Summary statistics: Summary statistics are based on one ground-water sample per site. Missing data for individual pesticides were not used in the calculation of summary statistics. Approximately one third of the ground-water sites did not have pesticides analyzed by HPLC. As a consequence, detection frequencies and concentration percentiles for pesticides analyzed by HPLC are based on a smaller number of sites than pesticides analyzed by GCMS. For this and other reasons (e.g., matrix interference, broken sample bottles, etc.), data for all 83 targeted pesticides (Gilliom and others, 2006, Appendix 1A) are not available at every ground-water site.

Detection frequencies: Detection frequencies were calculated at 4 thresholds: (1) “all” detections (detections at any concentration--some as low as 0.001 micrograms per liter (µg/L)), (2) detections at or greater than 0.01 µg/L, (3) detections at or greater than 0.1 µg/L, and (4) detections at or greater than 1.0 µg/L. 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.

Percentiles of concentration: Concentrations measured for each pesticide are summarized using percentiles and the maximum measured concentration. Concentration data for each pesticide were ranked from low to high with nondetections ranked below the lowest reported concentration. The 50th, 75th, 90th, and 95th percentiles are calculated and provide information about the magnitude of concentrations at selected points in the cumulative frequency distribution of the 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.

Additional methods and approach: Additional information on methods and approach for ground-water analyses for individual figures in Circular 1291 are provided in the online supplemental information section of each figure.

REFERENCES

Gilliom, R.J., Alley, W.M., and Gurtz, M.E., 1995, Design of the National Water-Quality Assessment Program: Occurrence and distribution of water-quality conditions: U.S. Geological Survey Circular 1112, 33 p. Available at: http://water.usgs.gov/pubs/circ/circ1112/

Gilliom, R.J., Barbash, J.E., Crawford, C.G., Hamilton, P.A., Martin, J.D., Nakagaki, Naomi, Nowell, L.H., Scott, J.C., Stackelberg, P.E., Thelin, G.P., and Wolock, D.M., 2006, Pesticides in the Nation's streams and ground water, 1992-2001--The quality of our Nation's waters: U.S. Geological Survey Circular 1291, 171p. Available at: URL http://water.usgs.gov/nawqa/pnsp/pubs/circ1291/

Koterba, M.T., Wilde, F.D., and Lapham, W.W., 1995, Ground-water data-collection protocols and procedures for the National Water-Quality Assessment Program: Collection and documentation of water-quality samples and related data. U.S. Geological Survey Open-File Report 95-399, 113 p.

Lapham, W.W., Wilde, F.D., and Koterba, M.T., 1995, Ground-water data-collection protocols and procedures for the National Water-Quality Assessment Program: Selection, installation, and documentation of wells, and collection of related data. U.S. Geological Survey Open-File Report 95-398, 69 p.

Lindley, C.E., Stewart, J.T., and Sandstrom, M.W., 1996, Determination of low concentrations of acetochlor in water by automated solid-phase extraction and gas chromatography with mass-selective detection: Journal of the AOAC International, v. 79, no. 4, p. 962-966.

Martin, J.D., 1999, Quality of pesticide data for environmental water samples collected for the National Water-Quality Assessment Program, 1992-96, and examples of the use of quality-control information in water-quality assessments: accessed March 21, 2006 at http://water.usgs.gov/nawqa/pnsp/pubs/qcsummary.html

Martin, J.D., 2002, Variability of pesticide detections and concentrations in field replicate water samples collected for the National Water-Quality Assessment Program, 1992-97: U.S. Geological Survey Water-Resources Investigations Report 01-4178, 84 p. Available at: http://in.water.usgs.gov/newreports/martin-pest.pdf

Martin, J.D., Gilliom, R.J., and Schertz, T.J., 1999, Summary and evaluation of pesticides in field blanks collected for the National Water-Quality Assessment Program: 1992-95: U.S. Geological Survey Open-File Report 98-412, 102 p. Available at: http://water.usgs.gov/nawqa/pnsp/pubs/files/ofr98412.pdf

Oblinger Childress, C.J., Foreman, W.T., Connor, B.F., and Maloney, T.J., 1999, New reporting procedures based on long-term method detection levels and some considerations for interpretations of water-quality data provided by the U.S. Geological Survey National Water Quality Laboratory: U.S. Geological Survey Open-File Report 99-193, 19 p. Available at: http://water.usgs.gov/owq/OFR_99-193/index.html

Scott, J.C., 1990, Computerized stratified random site-selection approaches for design of a ground-water-quality sampling network. U.S. Geological Survey Water-Resources Investigations Report 90-4101, 109 p.

U.S. Geological Survey, 1997, NWIS--New remark code (V) for water-quality data: Office of Water Quality Technical Memorandum 97.08. Available at: http://water.usgs.gov/admin/memo/QW/qw97.08.html

_____, 1998, Changes in reporting levels and data qualifiers for selected pesticides and degradation products in schedules 2050 and 2051: National Water Quality Laboratory Technical Memorandum 98.03A. Available at: http://nwql.usgs.gov/Public/tech_memos/nwql.98-03A.html

Werner, S.L., Burkhardt, M.R., and DeRusseau, S.N., 1996, Methods of analysis by the U.S. Geological Survey National Water Quality Laboratory--Determination of pesticides in water by Carbopak-B solid-phase extraction and high-performance liquid chromatography: U.S. Geological Survey Open-File Report 96-216, 42 p. Available at: http://wwwnwql.cr.usgs.gov/Public/pubs/OFR96-216/OFR96-216.html

Zaugg, S.D., Sandstrom, M.W., Smith, S.G., and Fehlberg, K.M., 1995, Methods of analysis by the U.S. Geological Survey National Water Quality Laboratory--Determination of pesticides in water by C-18 solid-phase extraction and capillary-column gas chromatography/mass spectrometry with selected-ion monitoring: U.S. Geological Survey Open-File Report 95-181, 49 p. Available at: http://wwwnwql.cr.usgs.gov/Public/pubs/OFR95-181/OFR95-181.html

Pesticide names 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.

Summary statistics are presented in Appendix 7B.

For more information, contact:

Ground water:
Paul Stackelberg
U.S. Geological Survey
NAWQA Pesticide Synthesis Project
425 Jordan Road
Troy , NY 12180
voice: (518) 285-5652
fax: (518) 2285-5601
email: pestack@usgs.gov

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