environmental setting, water quality, and ecological indicators of

criteria for the protection of drinking water or aquatic life
(fig. 12). Concentrations of fipronil exceeded USEPA
(2002b) numeric targets for TMDL’s (table 3). Although
atrazine concentrations in three samples collected in the
spring exceeded 3 µg/L, the MCL (U.S. Environmental
Protection Agency, 2000a) was not exceeded because it
is based on an annual average of quarterly samples. Concentrations
larger than 3 µg/L were not detected in samples
collected during other times of the year, and the
average annual concentrations did not exceed 3 µg/L for
sites sampled throughout the year.
Concentrations were determined for 109 pesticides
and degradation products in samples from 29 wells.
Concentrations were detected in less than 1 percent (25)
of the analyses for ground-water samples; 19 pesticides
and degradations products were detected (fig. 11). Pesticides
and degradation products were detected in samples
from 11 wells; samples from wells 6 and 35 each
accounted for 24 percent of the detections. Pesticides
and degradation products detected most frequently were
the herbicides bentazon (3 detections) and atrazine
(5 detections).
Concentrations were determined for 109 pesticides
and degradation products in surface-water samples; concentrations
were detected in 20 percent of these analyses.
Concentrations of 47 pesticides and degradation products
were detected in surface water; at least 3 pesticides
were detected in all samples. Annual maximum concentrations
of different pesticides and pesticide degradation
products usually occurred during the spring, about the
time ricefields were drained, as shown in figure 13 for
the Mermentau site. More than 190 samples collected
from 24 surface-water sites were analyzed for
51 hydrophilic pesticides and degradation products
(app. 2); 28 pesticides were detected. A total of 36 samples
collected from Des Cannes, Lacassine, and Mermentau
sites were analyzed for 58 hydrophobic
pesticides and degradation products. Nineteen pesticides
were detected, and at least 3 were detected in more than
70 percent of the samples.
The herbicides atrazine, molinate, and tebuthiuron,
and the insecticide fipronil were detected in samples collected
from all surface-water sites. Herbicides were
detected more frequently and in larger concentrations
than insecticides. Atrazine occurred in over 92 percent
of the samples; concentrations in 3 samples exceeded the
criterion (3 µg/L) for drinking water, and concentrations
in 10 samples exceeded the criterion for the protection of
aquatic life (1.8 µg/L). Molinate occurred in more than
86 percent of the samples and at the largest concentration
(154 µg/L) for any of the pesticides detected in the study
area. Tebuthiuron and malathion each were detected
once in concentrations exceeding criteria for the protec-
26
tion of aquatic life. Fipronil concentrations at 17 sites
exceeded the freshwater chronic numeric target concentration
for TMDL of 2.3 µg/L (table 3), and at 3 sites
exceeded the acute numeric target concentration for
TMDL of 4.6 µg/L (U.S. Environmental Protection
Agency, 2002b).
Atrazine is the most commonly used herbicide in
the United States (Hayes and others, 2002). Two atrazine
degradation products, 2-hydroxyatrazine and deethylatrazine,
were detected in 83 and 90 percent of the samples.
Although concentrations at Riceville (5.23 µg/L)
and Lacassine (5.83 and 8.91 µg/L) exceeded 3 µg/L, the
USEPA (2000a) MCL (3 µg/L) was not exceeded
because it is based on an annual average of quarterly
samples. Concentrations at Riceville, Mermentau, and
Lacassine exceeded the criterion for the protection of
aquatic life (1.8 µg/L). There is some evidence that atrazine
disrupts the endocrine system in some amphibians at
concentrations less than 1 µg/L (Hayes and others,
2002). Concentrations greater than 1 µg/L were detected
at Des Cannes, Mermentau, Riceville, Lake Arthur,
Lacassine, Tortue, Church Point, and Mallet sites. Concentrations
greater than 0.1 µg/L were detected at all
sites except Boggy, Castor, Blue, Upper Des Cannes, and
Caney.
Tebuthiuron, a urea-based broad-spectrum herbicide,
is used to control weeds in rights-of-way, industrial
sites, and rangeland (Oregon State University, 1996).
Tebuthiuron was detected at all the surface-water sites
and in 95 percent of all surface-water samples. The largest
concentration (6.33 µg/L) was detected at the Des
Cannes site and was the only detection exceeding the criterion
(1.6 µg/L) for the protection of aquatic life
(table 3).
Malathion, an organophosphate insecticide, is used
for public health mosquito control (U.S. Environmental
Protection Agency, 2000b), recommended for control of
household insects by homeowners (Louisiana State University
Agricultural Center, 2002), and recommended for
control of stinkbugs associated with rice agriculture
(Louisiana State University Agricultural Center, 2001).
Malathion was detected at 16 surface-water sites; the
largest concentration (0.113 µg/L) occurred at the
Lacassine site and was the only detection that exceeded
the criterion (0.1 µg/L) for the protection of aquatic life
(table 3).
In 1999, carbofuran was replaced with fipronil, a
relatively new phenypyrazole insecticide, to control the
rice water weevil. Carbofuran was sprayed in areas
where the rice water weevil was observed, whereas
fipronil is used prophylactically as a seed coating prior to
planting (fig. 13). Fipronil was detected in more than