environmental setting, water quality, and ecological indicators of

were sampled monthly November 1998-September 2001
and semimonthly February-June 1999 and 2000. Fourteen
short-term sites were selected to characterize the
occurrence and distribution of land-use effects on water
quality in the basin. These sites were sampled monthly
February-June 2000. Nineteen sites were selected to
compare water-quality characteristics with drainage area
and agricultural land-use intensity March-May 2001.
These included the 3 long-term sites in the Mermentau
River Basin, 1 long-term reference site in the Calcasieu
River Basin, 8 of the short-term sites sampled in 2000,
and 7 additional short-term sites. Bed-sediment samples
were collected once at 17 surface-water sites during lowflow
conditions August-September 2000.
Two methods were used to group sites. Nineteen
ecological data-collection sites were grouped before data
collection according to the study design based on drainage
area and agricultural intensity and grouped statistically
after data collection using ordination (CCA) and
classification (cluster analysis) techniques on surfacewater
quality, habitat, and aquatic invertebrate data
(table 1, fig. 7). Drainage areas greater than or equal to
70 mi 2 were considered large basins and those smaller
than 70 mi 2 were considered small basins. Sites were
selected to represent water-quality conditions influenced
by differences in agricultural intensity as indicated by the
percentage of the basin used for rice cultivation. In this
report, the term “high-intensity” describes an area where
at least 45 percent of the drainage area is used for rice
cultivation; “low-intensity” is used to describe an area
where less than 45 percent of the drainage area is used
for rice cultivation. A Geographic Information Systems
(GIS) coverage was used to determine basin areas
(Joseph Holmes, Louisiana Department of Environmental
Quality, written commun., 2001). Rice areas also
were determined from this GIS coverage and verified by
county agricultural agents within parishes in the basin.
Data Analysis
Concentrations of nutrients, major inorganic ions,
trace elements, and pesticides were evaluated in relation
to land use and to national drinking-water regulations or
criteria. Concentrations in ground water were compared
with those in surface water. Nutrients, major inorganic
ions, and trace elements were detected frequently in both
ground water and surface water and are discussed
concurrently. Maximum pesticide concentrations are
compared to available aquatic-life criteria, and concentrations
for one insecticide, fipronil, are compared to
numeric targets for total maximum daily loads (TMDL’s)
established by USEPA (2002b). Pesticides were detected
14
infrequently in ground water and, other than in general
terms, are discussed separately from those in surface
water.
Primary and Secondary Drinking Water Regulations
(U.S. Environmental Protection Agency, 2000a)
were used to evaluate concentrations of nitrate and
selected major inorganic ions, as shown below.
Constituent Concentration
Drinking Water
Regulation
Nitrate 10 mg/L MCL 1
Chloride 250 mg/L SMCL 2
Sulfate 250 mg/L SMCL 2
Iron 300 µg/L SMCL 2
Manganese 50 µg/L SMCL 2
1 Maximum Contaminant Level.
2 Secondary Maximum Contaminant Level.
Primary Drinking Water Regulations and health
advisories were used to evaluate concentrations of pesticides
(table 3). Primary and Secondary Drinking Water
Regulations and health advisory levels were established
by USEPA (2000a) and include the following:
1. Enforceable Maximum Contaminant Levels
(MCL’s), established to protect public health
by limiting the levels of contaminants in
drinking water;
2. Nonenforceable Secondary Maximum
Contaminant Levels (SMCL’s), available for
some nutrients and major inorganic ions,
established to limit cosmetic (such as skin or
tooth discoloration) or aesthetic (such as
taste, odor, or color) effects in drinking water;
and
3. Nonenforceable health advisory (HA) levels,
established to avoid adverse noncarcinogenic
effects for a lifetime of exposure.
Aquatic-life criteria for pesticides in freshwater
include USEPA water-quality criteria (2002a), USEPA
TMDL’s (2002b), and Canadian water-quality guidelines
for the protection of aquatic life (Canadian Council of
Ministers of the Environment, 2001) (table 3). These
criteria are based on single-chemical toxicity tests and do
not consider synergistic or antagonistic effects of pesticide
mixtures. Criteria from USEPA (2002a) are
estimates of the highest concentration to which an
aquatic community can be exposed indefinitely without