Environmental Engineering Reference
In-Depth Information
-DDE, the main metabolite of
concern, has a K
ow
of 6.956 (de Bruijn et al.
1989
). After application to soils, DDT
may be lost through both volatilization and biodegradation. Volatilization tends to
be the more important removal mechanism initially, while biodegradation is more
important later in the removal process (US DHHS
2002
). As a result of both these
processes, DDT removal from soils tends to be non-linear, and thus the fi rst 50% of
DDT tends to be removed from soil more quickly than subsequent halves, such that
the half-life of DDT in soil may increase over time (US DHHS
2002
).
A variety of studies have been conducted to characterize the half-life of DDT and
its metabolites. In temperate climates, the half-life of DDT in soil has been reported
to range from 2.3 to 16.7 years (Lichtenstein and Schultz
1959
; Racke et al.
1997
;
Stewart and Chisholm
1971
).
Although it has been suggested that other non-organochlorine pesticides, such as
dicofol, continue to be used in the watershed and may include small amounts of DDT,
the SARWQCB (
2006
) concluded that dicofol contains minimal levels of DDT and is
therefore an “inconsequential continuing source in the watershed.” Mischke et al.
(
1985
) concluded that DDT levels in dicofol were too low to account for the DDT soil
residues found in their 1985 study of agricultural residues in California soils.
As discussed below, recent data from Newport Bay and Watershed indicate that
DDT concentrations are declining in all media where historical and recent data are
available, and these data have been used to estimate the half-life of DDT in the local
watershed.
DDT is strongly hydrophobic; for example,
p
,
p
′
3.1.1
Agricultural Soils
Table
1
presents historical DDT concentrations for agricultural soils in the Newport
Bay watershed. In general, these soils seem to exhibit a downward trend in DDT
concentrations over time, which is expected given a DDT half-life of less than
Table 1
DDT concentrations in agricultural soils in the Newport Bay Watershed
0-12 inch Sample Depth
12-24 inch Sample Depth
>24 inch Sample Depth
Range of
Detected
Total DDT
(ppm)
Total
Non-
detect
Samples
Detection Limits
(ppm)
Range of
Detected Total
DDT (ppm)
Total Non-
detect
Samples
Range of
Detected Total
DDT (ppm)
Total Non-
detect
Samples
Year
Total
Samples
Total
Samples
Total
Samples
1985
0.001 - 1.750
12
0
--
1987
0.034 - 1.500
10
0
0.025 - 2.140
10
4
--
10
10
0.016
1988
0.027 - 1.090
10
1
0.095 - 0.150
10
8
0.550 - 0.550
10
9
<0.027 - 0.064
1989
0.024 - 0.791
15
6
0.052 - 0.707
10
5
0.016 - 0.333
19
13
0.016
1990
0.102 - 0.900
4
1
0.110 - 0.910
2
0
0.020 - 0.197
7
3
0.016
1991
0.019 - 0.488
34
4
0.010 - 0.490
32
14
--
1995
0.085 - 0.806
19
1
--
2000
0.007 - 0.132
28
0
0.005
2002
0.005 - 1.620
174
34
0.020 - 0.073
27
17
--
2004
0.002 - 2.000
230
167
0.002 - 0.300
45
36
0.002 - 0.2
2006
0.013 - 0.157
6
1
0.005 - 0.005
6
5
0.005
Sources
: Unpublished technical report provided by the SARWQCB (1985); unpublished technical
reports provided by The Irvine Company (1985-2006); no data were available for shaded areas and
for entries noted with '--'
Search WWH ::
Custom Search