Inventory
and spatial pattern of 137Csin
a pond:
a
comparison of two survey methods
J. P. Abrahama, F. W. Whickera, T. G. Hintonb,*,
D. J. Rowana
aDepartment
of Radiological Health Sciences, Colorado State University, Fort Collins, CO
80523, USA bUniversity of Georgia, Savannah River Ecology Laboratory, Drawer E,
Aiken, SC 29802, USA
Received 21 September 1998; received in revised form 13
January 2000; accepted 28 January 2000
Abstract
The sediments of Pond A, a former Savannah River Site
cooling pond for R-reactor, were
contaminated with 137Cs between 1954 and
1964.Pond A is unique because it
is very shallow
and contains an extremely high density of aquatic
vegetation and old, undecomposed tree
stumps which modify normal sedimentation processes and
cause special radiological characterization challenges.To determine the most efficient technique for estimating inventory and
spatial patterns of 137Cs, we measured
exposure rates at 124 sediment surface locations with
two types of thermoluminescent dosimeters (TLDS) and
compared them to estimates calculated
from 137Cs measured in 58 extracted sediment
cores.The mean net exposure rate (±
1SEM)
measured at the sediment water interface with a UD-802
multi-element TLD (differentially
shielded lithium borate and calcium sulfate) was 40 ± 4 µR h-1, while the corresponding value
measured with a CaF2 TLD was 64 ±10µR h-1. Both sets of TLD measurements were
found to correlate well with each other (R2 =
0.88, p < 0.001), and moderately well with theoretical
calculations derived from 137Cs activity concentrations measured in
sediment cores (R2 > 0.50).The corresponding mean exposure rate calculated from
the sediment data, 69 ± 10 µR h-1, was
likely an over-estimate resulting from the core
sampling bias created by the large number of tree
stumps.Overall,
peak 137Cs activity occurred at ~ 2-4 cm depth in the sediment cores,
with
> 99% in the top 20 cm of sediment.The
total 137Cs inventory of Pond A was estimated as
4.1 ± 0.5 x 1010 Bq, with most activity
located in the deeper portions.Approximately
1 % of the137Cs activity thought to have been released
by R-Reactor can be accounted for in Pond A, with
an additional 53% estimated from other work to be in
the much larger Pond B, and Par Pond,
located further down the drainage.However, the mean deposition in Pond A
(7.9 x 109 Bq ha -1) was higher than either Pond B (4.0 x
109 Bq ha -1), or Par Pond
(1.4 x 109 Bq ha -1).It was
concluded that, although the TLD method was more efficient and
could employ more sampling locations to estimate
spatial pattern, a reasonable amount of
coring was essential to determine depth distribution,
radionuclide composition, and to interpret the TLD data. Optimal
estimation and characterization efficiency can benefit from simultaneous
application of both techniques.
Abraham, J.P., F.W. Whicker, T.G. Hinton, and D.J.
Rowan. 2000. Inventory and spatial pattern of
137
Cs in a pond: a comparison of two survey methods. Journal of Environmental
Radioactivity 51:157-171.
