Environmental Engineering Reference
In-Depth Information
5.1.5
Factors Affecting Decay of Toxaphene Residues
The U.S. EPA banned the use of Toxaphene in 1990. The observed decline in
toxaphene concentrations in fi sh tissue and the low observed toxaphene concentra-
tions in watershed soils and sediments are partly attributable to the natural removal of
toxaphene from the watershed. The half-life of toxaphene in soil is reported as ranging
from 1 to 14 years (US EPA 1999 ). The wide range is attributable to apparently
differing degradation rates for toxaphene under aerobic and anaerobic conditions
(US DHHS 1996 ). Under anaerobic conditions the half-life of toxaphene in soil and
sediment has been reported to be on the order of weeks to months (Callahan et al.
1979 ). However, under aerobic soil conditions, Nash and Woolson ( 1967 ) reported a
half-life of 11 years. Assuming that aerobic conditions are most common in the
Newport Bay watershed suggests a half-life on the order of 11 years. At this rate and
given that the use of toxaphene was banned in 1990 and excluding other loss mecha-
nisms, the mass of toxaphene in the agricultural soils of Newport Bay and Watershed
would have declined by at least 63% over the past 16 years due solely to natural
removal. Assuming anaerobic conditions—conditions typical of sediments submerged
in water, such as bay sediments—the half-life of toxaphene is on the order of weeks
or months. This suggests sediments, most of which remain consistently submerged in
the watershed, should currently contain very little toxaphene. The half-life for toxa-
phene in the watershed that was estimated using red shiner fi sh tissue data (3.4 years)
is consistent with these estimates for the half-life of toxaphene in watershed soils.
5.2
NAS Fish Guidance to Protect Wildlife
The SARWQCB ( 2006 ) have decided to use the NAS ( 1972 ) guidance of 100 ppb
toxaphene in fi sh to protect wildlife consuming fresh water fi sh in the Watershed.
As is apparent from viewing the regression in Fig. 30 , the current level in red shiners
is well below this guidance. Even so, the SARWQCB has chosen to base impair-
ment on the older fi sh data that exceed the guidance and to require a sediment target
to achieve the fi sh target of 100 ppb.
5.3
New York State Sediment Guidance to Protect Wildlife
US EPA Region IX ( 2002 ) and SARWQCB ( 2006 ) staff have decided to use the
New York State Department of Conservation ( 1998 ) screening level of 0.1 ppb as a
TMDL target for toxaphene in sediments. The following is a detailed look at the
scientifi c basis for this guidance.
New York State chose the equilibrium partitioning method for the derivation of a
sediment screening level for toxaphene (New York State Department of
Environmental Conservation 1998 ). The calculation began with a New York State
water column criterion of 0.005 ppb (
ʼ
g/L) toxaphene. Multiplication by a K oc for
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