The history of Pond B
Pond B is one of several reservoirs
that were constructed on the Department of Energys Savannah
River Site (SRS) to serve as secondary cooling systems for nuclear
production reactors. Pond B was filled in 1961 and received thermal
effluents from R Reactor until the reactor was shut down in 1964.
Since 1964, water levels in the pond have been maintained by precipitation
and groundwater seepage. Extensive macrophyte (aquatic vegetation)
communities have developed in the shallower waters (littoral zone)
of the pond, and a diverse animal community, including at least seven
fish species, has become established.
contamination in Pond B
During R Reactor operations from 1961-1964, radionuclides
(tritium, cesium-137, strontium-90, americium-241, cerium-244, and
plutonium-239, 240) were released to Pond B in effluents. By 1984,
cesium-137 (137Cs) accounted for 99% of the total measured
radioactivity in the pond, and of the 137Cs inventory,
99% resided in the ponds sediments. Because 137Cs
is chemically similar to potassium, it can be assimilated by aquatic
organisms and thus can enter the food chain.
with evaluating change in radiocesium levels in pond sediments
|The Pond B cooling
reservoir on the Savannah River Site received heated effluent from
R Reactor from 1961 until 1964. Since cessation of ractor
operations, macrophyte communities have developed and a diverse
animal community has become established.
Given that most of a lake or reservoirs 137Cs
inventory resides in the sediments, export of 137Cs from
the whole ecosystem can be calculated as the difference between sediment
137Cs inventory estimates from two points in time. However,
because of high spatial variability in the 137Cs contents
of individual sediment samples, there is considerable uncertainty
associated with estimates of whole-ecosystem inventories. Therefore,
using this method to estimate 137Cs export will also involve
about the export of radiocesium from Pond B
|Minimum detectable difference
in sediment 137Cs inventory as a function of sample size.
The dashed line indicates the estimated export rate of 137Cs in
surface water between 1984 and 1994. The sample size required
to detect a change in the sediment 137Cs inventory equal to the
surface water export is indicated by the open circle.
Based on the mean 137Cs
inventories in sediment cores collected from Pond B in 1984 and 1994,
it was previously estimated that the 137Cs inventory of
the sediments had declined by 49% during 1984-1994. This decline represented
an effective (ecological) half-life of 10 years and an annual loss
of 6.5% of the inventory. The annual loss solely from radioactive
decay is only 2.3%. Thus, it seemed that the amount of 137Cs
in the pond was declining at a rate of 4.2% per year beyond the known
loss due to radioactive decay. One possibility was that 137Cs
was being carried out of the pond in surface water flowing through
the ponds outlet canal. If this was the case, then a substantial
amount of the 137Cs from Pond B was being exported to downstream
ecosystems such as Par Pond. However, because of the large uncertainties
associated with the sediment inventory estimates, it was also possible
that there was no statistically significant difference between the
two inventory estimates after accounting for radioactive decay.
of change in sediment inventory
|Annual discharge of water and
estimated export of 137Cs from Pond B during 1986-1991.
We tested the hypothesis that
the sediment 137Cs inventory at Pond B declined faster
than expected from radioactive decay. To minimize the error from variability
among locations, we used only cores collected from the same 30 locations
in both years (additional locations had been sampled in 1984). The
mean rate of decline in the 137Cs inventory beyond natural
radioactive decay was 2.6% per year, but the 95% confidence interval
about this estimate was very broad, ranging from a loss of 7.7% per
year to an increase of 0.8% per year. Therefore, we could not conclude
that a significant decline beyond natural radioactive decay had occurred.
Given the actual sample size of 30 pairs of sediment cores, only a
decline of 5.8% per year or more could have been detected.
of radiocesium export in surface water
Based on hydrologic data from
1986-1991, we estimate that the amount of 137Cs lost from
the pond in surface water outflow was no more than 0.6% per year of
the sediment inventory. We believe surface water outflow to be the
only plausible mechanism for the loss of 137Cs from Pond
B (the loss of 137Cs in animals leaving the pond is miniscule
by comparison). To detect a loss of 0.6% per year by sampling sediments
in 1984 and 1994 would have required nearly 1,700 samples in each
|Aerial view of Pond B on the
SRS, showing approximate locations of 30 sites sampled in 1984 and
1994 to determine the inventory of 137Cs in pond sediments.
data from Pond B yielded no conclusive evidence of a decline of
the 137Cs inventory beyond radioactive decay and surface
Export of 137Cs
in surface water from Pond B probably averages <0.6% per year
of the sediment 137Cs inventory.
To have detected
a change in sediment inventory equal to the surface water export
of 137Cs would have required an impracticably large number
of samples in each year.
Pond B seems
to retain 137Cs effectively, preventing it from moving
into downstream ecosystems in appreciable quantities.
in Pond B
to Research Snapshots)