environmental setting, water quality, and ecological indicators of

Water-quality data were compared to drinkingwater
and aquatic-life criteria and bed-sediment data
were compared to aquatic-life criteria. Water-quality
data were analyzed statistically and related to agricultural
land-use intensity. Samples from 29 wells and 24
stream sites analyzed for selected nutrients, major inorganic
ions, and pesticides indicate the following:
Nutrients
Concentrations of nutrients and major inorganic
ions in ground and surface water generally were
largest in the southeastern part of the study area, an
area of intense agriculture.
The only drinking water regulation exceeded in a
ground-water sample was that for nitrate in a well
located in an area previously used for raising poultry
and swine.
Concentrations of nutrients from ground water used
for agricultural irrigation were not high enough to
contribute to eutrophication in surface water.
The greatest potential for eutrophic conditions in
surface water, based on nutrient thresholds,
occurred March-May, at about the same time or
shortly after ricefields were drained.
Major Inorganic Ions and Trace Elements
Secondary Maximum Contaminant Levels established
by the U.S. Environmental Protection
Agency were exceeded in samples from 20 wells—
sulfate in 1 well, chloride in 4 wells, iron in 7 wells,
and manganese in 17 wells.
Secondary Maximum Contaminant Levels were
exceeded in samples from all surface-water sites—
iron at 8 sites, and manganese at all sites.
The maximum concentrations of sodium and chloride
in surface water occurred at Bayou Lacassine
near Lake Arthur and probably resulted from saltwater
intrusion induced by reverse flow during the
drought.
Pesticides
Fewer pesticides and degradation products were
detected in ground water than in surface water.
Concentrations were lower in ground water than in
surface water, and did not exceed drinking water or
aquatic life criteria. Concentrations were detected
in less than 1 percent of all analyses; 19 pesticides
and degradation products were detected in samples
from 11 wells.
Pesticides and degradation products most frequently
detected in ground water were the herbicides
bentazon and atrazine.
Concentrations of 47 pesticides and degradation
products were detected in surface water. At least 3
43
pesticides were detected in all surface-water samples.
In 72 percent of the samples at least
5 hydrophylic pesticides were detected, and in more
than 70 percent of the samples at least 3 hydrophobic
pesticides were detected.
Although atrazine concentrations in three surfacewater
samples exceeded 3 µg/L (micrograms per
liter), the Maximum Contaminant Level established
by the U.S. Environmental Protection Agency was
not exceeded because it is based on an annual average
of quarterly samples. Concentrations larger
than 3.0 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.
Pesticides concentrations in surface water exceeded
aquatic-life criteria for atrazine (1.8 µg/L), tebuthiuron
(1.6 µg/L), and malathion (0.1 µg/L).
Fipronil was detected in concentrations exceeding
the numeric targets for acute total maximum daily
loads (2.30 µg/L) at 3 surface-water sites and the
numeric targets for chronic total maximum daily
loads (4.6 µg/L) at 14 sites.
Maximum pesticide concentrations in surface water
usually occurred in the spring at about the same
time or shortly after ricefields were drained.
Concentrations of DDE in bed sediment at Des
Cannes and Church Point sites exceeded interim
freshwater sediment-quality guidelines for the protection
of aquatic life.
Fipronil sulfide was detected at all bed-sediment
sites, but there are no current (2002) guidelines with
which to evaluate the environmental effects of
fipronil and degradation products.
The study design, based on drainage basin area and
agricultural land-use intensity, was used to determine
differences in natural and human-related influences on
surface-water quality at 19 ecological data-collection
sites. Aquatic invertebrate communities were used as
indicators of surface-water quality and habitat conditions
at these sites. Canonical correspondence analysis (CCA)
identified four significant environmental variables
(instream cover score, percentage of open canopy, and
concentrations of dissolved oxygen and maximum dissolved
fipronil) that described the distribution of aquatic
invertebrate communities among ecological data-collection
sites. Cluster analysis revealed four site groups
which separated ecological data-collection sites geographically
within the study. Environmental data and
aquatic invertebrate metrics within the study design and
CCA-assigned site groups were compared and indicate
the following: