Selected chloroacetanilide herbicides (acetochlor, alachlor, and metolachlor) in ground water

DESCRIPTION OF CONTENTS OF FILE GW1PEST2.DAT


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| DATA FILE FORMAT |
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columns  1-15  Station ID number
columns 21-26  Date (YYMMDD format)
columns 31-34  Time
columns 41-49  Water level (Storet code 72019, feet below land surface)
column  51     Remark for dissolved metolachlor
columns 52-59  Dissolved metolachlor (Storet code 39415, in micrograms per liter)
column  61     Remark for dissolved acetochlor
columns 62-69  Dissolved acetochlor (Storet code 49260, in micrograms per liter)
column  71     Remark for dissolved alachlor
columns 72-79  Dissolved alachlor (Storet code 46342, in micrograms per liter)


NOTES: A < symbol in the remarks field indicates the pesticide was not
       detected in the sample.


       An E in the remarks field indicates the concentration given is
       estimated.  (An E remark associated with a low concentration typically
       indicates the pesticide was positively identified by spectral
       characteristics and retention time but that the concentration is
       less than the method detection limit.  An E remark associated with a
       high concentration typically indicates the reported concentration
       exceeded the range of the calibration standards and was estimated by
       extrapolation.)


       Water levels or concentrations less than zero (for example -999.999)
       indicate no data are available for that observation.




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| STATION INFORMATION |
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Data are available for 94 wells.  Data were collected between June 1994 and
August 1995.  Well location, depth, and screened interval information may be
obtained by retrieving the file 'gw1sta.txt'.




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| METHODS          |
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All samples were analyzed by the U.S. Geological Survey National Water Quality
Laboratory.  Samples were collected using the methods of Koterba and others
(1995).  Pesticides were determined by Gas Chromatography/Mass Spectrometry.
Prior to September 1992 the method of Sandstrom and others (1992) was used for
alachlor and metolachlor.  From September 1992 on, the method of Zaugg and
others (1995) was used for alachlor and metolachlor.  The method of Lindley and
others (1996) was used for acetochlor.




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| REFERENCES       |
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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.


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.


Sandstrom, M.W., Wydoski, D.S., Schroeder, M.P., Zamboni, J.L., and Foreman,
W.T., 1992, Methods of Analysis by the U.S. Geological Survey National Water
Quality Laboratory--Determination of organonitrogen herbicides in water by
solid-phase extraction and capillary-column gas chromatography/mass spectrometry
with selected-ion monitoring: U.S. Geological Survey Open-File Report 91-519,
34 p.


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.