DDT, Chlordane, Toxaphene and PCB Residues in Newport Bay and Watershed: Assessment of Hazard to Wildlife and Human Health - Reviews of Environmental Contamination and Toxicology - page 54

20 years (Lichtenstein and Schultz 1959 ; Racke et al. 1997 ; Stewart and Chisholm
1971 ) and the fact that DDT use was discontinued in the early 1970s. However, the
data reported in Table 1 were not sampled from soils at the same locations.
Given that no data were available showing the amounts of DDT historically applied
to different areas of the watershed, the DDT data on agricultural soils cannot be
used to assess trends over time or local DDT half-life values.
Several agricultural soil DDT data points have also been reported in Mischke
et al. ( 1985 ) for Orange County. Total DDT concentrations in that report ranged
from 0.32 to 2.96 ppm for three different sample locations. However, the precise
locations of these samples could not be identifi ed from the report, and thus the data
were not useful for establishing trends in agricultural soil DDT concentrations in
the Newport Bay watershed.
Peak DDT concentrations at the 12-24 in. depth were generally comparable to
concentrations in the top 12 in., while peak DDT concentrations in samples collected
from a depth of 24 or more inches were roughly two to sixfold lower than concentra-
tions at the surface. Sampling locations for several sampling years are presented in
Fig. 2 ; note that exact locations for samples collected in 1988, 1991, and 1995 are not
known. Data from those years are shown using the average concentration in the
approximate location of sample collection.
If one conservatively assumes a half-life of 20 years for DDT in soil, that DDT was
banned in 1972, and if other losses or removal mechanisms are excluded, the mass of
DDT in the agricultural soils of the Newport Bay watershed would have declined by
approximately 71% over the past 36 years solely from soil degradation.
Because DDT adsorbs strongly to soil particles, the predominant pathway for
movement in the watershed is via soil erosion. Two related changes within the
watershed have served to minimize the transport of DDT to the waters within the
Newport Bay watershed. First, urban development initially has led to a conversion
of land use away from agricultural use and toward residential, commercial, and
industrial development, which increases the impervious land and minimizes direct
erosion from land surfaces. Much of this land use conversion has occurred on land
that was in agricultural production prior to 1972, when DDT was in use, and much
of the land currently in agricultural production was fi rst farmed after 1972, indicat-
ing that DDT would not have been applied to these areas. Second, several measures,
including channelization and construction of and improvements to the fl ood control
system, have resulted in decreased sediment loads being delivered to the Bay, and
therefore, decreased sediment yield of the watershed over time. 2 Development in the
watershed has and will continue to reduce the amount of DDT available to biota in
the watershed.
2 WRC Consulting Services Inc, Historical Sediment Load Examination, San Diego Creek
Watershed. Report prepared for County of Orange, Resources and Development Management
Department. June 28, 2006.