| Abstract: | Field blanks are quality-control samples
used to assess contamination in environmental
water samples. Contamination is the unintentional
introduction of a chemical (pesticides
in this instance) into an environmental water
sample from sources such as inadequately
cleaned equipment, dirty hands, dust, rain, or
fumes. Contamination causes a positive bias in
analytical measurements that may need to be
considered in the analysis and interpretation of
the environmental data. Estimates of pesticide
contamination in environmental water samples
collected for the National Water-Quality
Assessment (NAWQA) Program are used to
qualify, where needed, interpretations of the
occurrence and distribution of pesticides in the
surface and ground waters of the United States.
Field blanks collected from 1992 to
1995 as part of the NAWAQA Program
were analyzed for 88 pesticides and pesticide
metabolites. Of 47 pesticides determined
by gas chromatography/mass spectrometry,
23 were detected at least once in 175 surfacewater
field blanks and 15 were detected at
least once in 145 ground-water field blanks.
The most frequently detected pesticides in
surface-water field blanks were atrazine
(in 10.9 percent of blanks), simazine (9.1 percent),
and metolachlor (4.6 percent). The most
frequently detected pesticides in ground-water
field blanks were p,p?-DDE (4.1 percent)
and atrazine (2.8 percent). The maximum
pesticide concentration detected by gas chromatography/
mass spectrometry in a surfacewater
field blank was 0.120 microgram per
liter (mg/L) for pronamide; the maximum
concentration detected in a ground-water field
blank was 0.013 mg/L for chlorpyrifos and
prometon. Of 41 pesticides determined by
high-performance liquid chromatography,
diuron and 2,4-D were detected once in
109 surface-water field blanks and bromacil,
diuron, and fenuron were detected once in
104 ground-water field blanks. Except for a
detection of 2,4-D at 0.230 mg/L, the detectable
concentrations of these pesticides were
less than or equal to 0.020 mg/L.
Field blanks showed no evidence of
contamination by most pesticides. Of the
88 pesticides for which the field blanks were
analyzed, 63 were not detected in field blanks
from surface-water sites and 70 were not
detected in field blanks from ground-water
sites. Therefore, environmental data for the
pesticides not detected in field blanks can be
interpreted without qualification for contamination.
Field blanks did show evidence of
contamination by some pesticides. Most
of the pesticides detected in field blanks,
however, were detected more frequently and
at higher concentrations in environmental
water samples. Two criteria were used to
evaluate the need to consider contamination
in water-quality assessments: (1) a ratio of the
frequency of pesticide detection in environmental
water samples to the frequency of
detection in field blanks of 5.0 or less and (2) a ratio of the median concentration detected
in environmental water samples to
the maximum concentration detected in field
blanks of 2.0 or less. These criteria indicate
that contamination, for the majority of the
pesticide data collected for the NAWQA
Program, probably does not need to be
considered in the analysis and interpretation
of (1) the frequency of pesticide detection
or (2) the median concentration of pesticides
detected. Contamination must be considered,
however, in detection frequency for cispermethrin,
pronamide, p,p?-DDE, pebulate,
propargite, ethalfluralin, and triallate in surface
water and fenuron, benfluralin, pronamide,
cis-permethrin, triallate, chlorpyrifos, trifluralin,
propanil, p,p?-DDE, bromacil, dacthal,
diazinon, and diuron in ground water. Contamination
also must be considered in median
concentrations detected for pronamide,
p,p?-DDE, propargite, napropamide, and
triallate in surface water and benfluralin,
cis-permethrin, triallate, chlorpyrifos, trifluralin,
p,p?-DDE |
