Environmental Engineering Reference
In-Depth Information
source of DDT in eggs during the fi rst year of reproduction. DDT in eggs thereafter
will increase as a result of dietary DDT accumulated during each subsequent year.
Egg laying and starvation can partially offset these life-long increases.
For Newport Bay, a hen laying eggs having DDT levels at the bottom of the
range is probably in its fi rst year of reproduction and has, by feeding location and
prey selection, not been exposed to signifi cant levels of DDT. These hens are most
likely feeding several miles out in the Pacifi c Ocean in an area far from DDT
contamination.
A hen laying eggs having DDT levels at the top of the range is probably an older bird
(some live up to 20 years) with a history of frequenting locations and consuming
prey that are high in DDT. In Southern California, the highest DDT levels in fi sh are
found off of the Palos Verdes Peninsula. For several miles around the Los Angeles
County Sanitation Districts outfall, the ocean bottom and aquatic biota have been
contaminated by DDT manufacture wastes. The next highest levels are found in rivers
draining from Mexico into California. The Alamo and New Rivers that drain into
the Salton Sea and the Tijuana River that drains into the Pacifi c Ocean are examples.
These rivers drain agricultural lands in Mexico where DDT was used long after the
U.S. ban in 1972. In addition, the continuing use of DDT to control malaria along the
southwestern coast of Mexico is also a signifi cant exposure location. The next high-
est levels occur in rivers, estuaries and bays that receive agricultural drainage in
California. Newport Bay is such an example, although for the Newport Bay Watershed
the downward trend in DDT releases has been accelerated in recent decades by the
conversion of agricultural lands to urban uses, a change that reduces the erosion of
soils where DDT was historically used. The least contaminated aquatic biota can be
found in areas of the Pacifi c Ocean not associated with contaminated effl uents and
in estuaries that do not receive drainage from lands containing DDT residues from
past agricultural and other applications. Most of the fi sh-eating avian species resi-
dent to Newport Bay and Watershed are migratory, so all of the above exposure
scenarios are possible.
When interpreting studies on the effects of DDT on avian reproduction, one must
be aware of the wide range of egg residue levels. Average egg residue levels, aver-
age shell thinning and average hatching failure do not reveal the full range of effects
(i.e., from no effect to severe effect) that can occur in individual birds at the same
location and in the same data set. Thresholds should be estimated from the full
range of effects in individual birds and not from the average effect for a particular
study. For example, the average egg residue of 4 ppm DDT may result in a 30%
hatching failure. Examination of the individual data may reveal that the hatching
failure only occurred at egg levels above 10 ppm, so the threshold for hatching failure
is closer to 10 ppm than 4 ppm.
Since the U.S. ban of DDT in 1972, residues in the aquatic environment and in
bird eggs have declined to levels that, in most locations are below thresholds for
eggshell thinning and in almost all locations, below thresholds for hatching failure.
The CH2M Hill study found that DDT levels in eggs of six avian species were
below thresholds for hatching failure, with perhaps minimal egg shell thinning in
one species. The lack of a correlation of DDT levels with shell thickness in Forster's
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