Miguel F.Acevedo
ABSTRACTS
ESTIMATING PESTICIDE EXPOSURE IN TIDAL
STREAMS OF LEADENWAH CREEK, SOUTH
CAROLINA
Journal of
Toxicology
and Environmental Health 52:295-316. 1997
Miguel F. Acevedo, Magdiel Ablan, Kenneth L. Dickson and William T.
Waller,
Institute of Applied Sciences, University of North Texas, Denton TX
76203,
Foster L. Mayer and Michael Morton, Gulf Breeze Environmental Research
Laboratory, US Environmental Protection Agency, Gulf Breeze FL 32561.
Acevedo and Ablan, also at the Center for Modeling and Simulation
(CESIMO),
Universidad de Los Andes, Mérida, Venezuela.
ABSTRACT
This paper estimates the potential exposure of estuarine organisms to
two
pesticides (azinphosmethyl and fenvalerate) in a tidal stream of
Leadenwah
Creek near the Edisto River, South Carolina, during four runoff
episodes.
Exposure is calculated from simulation runs of the one-dimensional
transport
equation solved by an implicit finite difference method. Calibration
was
done for each episode by adjusting three conditions (runoff starting
time,
duration and flow) and a correction to the dispersion coefficient in
order
to match the continuously measured salinity transients. First-order
rate
constants used by the fate component were calculated from half-life
values
reported in the literature. Baseline scenarios for each episode and
each
pesticide were derived by using the same conditions obtained in the
salinity
runs and adjusting the pesticide loading in order to mimic the few data
points of measured pesticide concentrations. In all baseline scenarios,
pesticide concentration rises following the initial burst of runoff
(also
noticeable as an abrupt drop in salinity) and then oscillates forced by
the tidal cycle . These oscillations are dominated by transport while
fate
imposes a secular decaying trend. Ten additional scenarios for each
episode
were obtained from the baseline scenario by randomly varying three
pesticide
load parameters (starting time and duration of runoff, and pesticide
discharge)
using a Latin Hypercubes design. Two exposure metrics were calculated
from
the simulated and the measured pesticide concentration: maximum and
time
average, which was obtained by integrating the curve and dividing by
the
time period. The metrics calculated from the baseline runs are
relatively
close to the data-derived metrics, because the baseline runs attempted
to mimic the data. For each one of the two metrics and all
pesticide-episode
combinations, several statistics of the set of eleven scenarios were
also
calculated: minimum and maximum, mid-range, mean, standard deviation
and
median. The mean ± standard deviation interval of the
simulation-derived
value consistently bracket the data- derived value for the maximum
metric,
but not for the time- average metric. This may indicate that even if
the
maximum value is correctly captured in the field sample, the
time-average
exposure could be in error when calculated directly from the field data
due to under sampling of the pesticide time series. The methodology
developed
here attempts to reconstruct the possible exposure from the sparse
sampling
of the pesticide concentration during the runoff episodes; only when
the
number of field samples is high and regularly spaced is it possible to
have confidence in the reconstruction of the curve. The shape of the
curve
cannot be inferred from the field measurements alone; as expected,
tidal
movement makes the pesticide concentration swing up and down. This
result
has important implications because the biological community would be
subject
to repetitive pulses of exposure to the chemicals. The baseline
simulations
can be used to derive a pulse-exposure metric by calculating the sum of
ratios of the time average of the threshold-exceeding concentrations to
the time-average of the toxic threshold during intervals of
above-threshold
concentration. This metric is species-specific and extrapolates
laboratory
toxicity data in order to compare pulse-exposure to mortality rates
measured
in the field.
KEYWORDS: exposure, metrics, tidal streams, modeling, transport,
fate.
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