Environmental Engineering Reference
In-Depth Information
applications, such as the commercial nurseries formerly located in the upper part
of the Watershed. Recent studies indicate that the acute toxicity of sediments from
Upper Newport Bay is diminishing (Orange County Watersheds 2008-2011).
Understanding sediment loads is important to understanding the fate of organo-
chlorines, because these chemicals bind tightly to soil and sediment particles.
Annual sediment loads discharged from the Watershed were estimated at approxi-
mately 250,000-275,000 t during the rapid urbanization period of the 1980s and
1990s (US EPA Region IX 1998 ). Much of the sediment load resulted from
in-stream erosion (Trimble 1997 ). Because of the volume of sediments deposited
within the Bay, the Upper Bay was dredged in 1983, 1985, 1988, 1999 and 2010
(Newport Bay Conservancy 2013 ). Implementation of the sediment TMDL
(SARWQCB 1998 ) has led to reduced sediment loads.
Stream stabilization and other measures have reduced sediment loads to the Bay. 1
Flow rate and suspended sediment discharge samples collected at San Diego Creek
showed that although average annual fl ow volume for the years 2000-2005 was
roughly equivalent to the average annual fl ow volume for 1983-1999, average annual
sediment discharge for the latter period was only 42% of the average annual sedi-
ment discharge for 1983-1999 (see footnote 1). Orange County's consultant attrib-
uted this reduction in sediment load to land development, effectively capping soils,
and to erosion control measures in the watershed. Moreover, this consultant found
that “[a]s the San Diego Creek watershed becomes further developed, less and less
watershed supply of sediment is released during storm events (see footnote 1).”
In the next section, we address the fate of DDT in the Watershed and the science
underlying the DDT TMDL targets.
3
DDT
3.1
Levels in the Environment
DDT was fi rst used as an insecticide in California around 1944 and was in wide use
by 1947. In 1963, the California Department of Food and Agriculture declared it a
restricted material, and 1972 was the last year that DDT was applied to crops in the
state (Mischke et al. 1985 ).
According to the United States Department of Health and Human Services
(US DHHS 2002 ), commercial DDT is a mixture of several congeners, and typi-
cally has a composition of 65-80% p , p
-DDT. In the envi-
ronment, DDE and DDD are the major degradation products of DDT. DDT and its
congeners are persistent in the environment and have been found in various animal
species, in water, in soil, and in sediment (US DHHS 2002 ). As indicated below and
except as otherwise indicated, DDT is used to represent the sum of all measured
DDT congeners.
-DDT and 15-21% o , p
1 County of Orange Resources and Development Department 2006.
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