Geoscience Reference
In-Depth Information
17.3.1 Radionuclides
Radionuclides released to the environment through anthropogenic processes induce
groundwater contamination. The long half-life of some species in this group affects
the nature of some of the induced changes in the composition of groundwater. For
example, significant quantities of elemental plutonium (Pu) have been introduced
into the environment as a result of nuclear weapons testing and production, as well as
from nuclear power plant accidents. Radionuclides may reach groundwater mainly by
migration as dissolved solutes and by colloidal-facilitated transport when insoluble or
poorly soluble contaminants are sorbed on colloidal surfaces. A specific example is
plutonium (Pu) contamination originating from the Benham, USA underground
nuclear test site, which was detected in groundwater collected in wells 1.3 km
downgradient (Kersting et al.
1999
). More than 90 % of Th, Pu, and other radio-
nuclides found in the aquifer were associated with the transport of naturally occurring
colloidal particles (e.g., clays or zeolites, \1 lm in size) in the groundwater.
Additional data on plutonium concentration, solute/colloid distribution, and
isotopic composition in groundwater at the Hanford Site (Columbia River, U.S.
DOE 100 K-area) are found in Dai et al. (
2005
). Concentration patterns of
239
Pu
measured in unfiltered samples of water in a well transect through the site are
shown in Fig.
17.11
. It was found that
239
Pu is associated with the colloidal
fraction of the groundwater in 7-29 % of the wells sampled. The authors consider
that a high colloidal abundance at the Hanford Site
239
Pu may be associated with
the oxidation state defined by the geochemical environment.
The oxidation state of plutonium largely controls its migration through sedi-
ments and aquifers; plutonium can exist simultaneously in groundwater in four
oxidation states: Pu
III
,Pu
IV
,Pu
V
, and Pu
VI
. In a long-term field lysimeter exper-
iment, Kaplan et al. (
2006
) observed that Pu moved more rapidly (overall rate of
12 cm/yr) when it was applied initially as Pu
VI
than as Pu
III
or Pu
IV
. The tendency
to form complexes, hydrolyze, polymerize, and/or sorb to solids follows the
effective charge of the ion: Pu
4+
[ Pu
VI
O
2
[ Pu
3+
[ Pu
V
O
2
+
(Choppin
1983
).
Moreover, the plutonium isotopic state may provide more complete knowledge on
the contamination source. Fuel-grade Pu has a
240
Pu/
239
Pu ratio between 0.07 and
0.18, while reactor-grade Pu has a
240
Pu/
239
Pu ratio of 0.18:0.30 (Taylor et al.
2001
; Pellaud
2002
). Measuring Pu isotope ratios at the Hanford Site 100 K-Area
groundwater sample, Dai et al. (
2005
) evaluated the reactor waste source by
comparing to
241
Pu (Fig.
17.12
). While the overall concentrations of Pu were low,
giving general information on Pu contamination of groundwater, the Pu isotopic
compositions confirmed the local sources of Pu contamination.
A rapid change in groundwater chemistry was observed under the Hanford
reservation nuclear facility (USA) when, in 1984, liquid waste from a uranium
extraction operation was routed to a newly constructed crib. Within a few months,
uranium concentrations in nearby groundwater monitoring wells increased by 170
times their previous level and reached more than 5000 times the current EPA
drinking water standard. Despite intensive reclamation of the polluted site, the
