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In Situ Uranium Stabilization Through Polyphosphate Remediation (Paperback)
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In Situ Uranium Stabilization Through Polyphosphate Remediation (Paperback)
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The Hanford Site, in south-eastern Washington State, is a former
nuclear defence production facility. A groundwater plume containing
uranium, originating from a combination of purposeful discharges of
wastewater to cribs, trenches, and ponds, along with some
accidental leaks and spills with nuclear fuel fabrication
activities, has persisted beneath the Hanford Site 300 Area for
many years. The uranium plume is just upstream of the city of
Richland municipal water supply intake on the Columbia River.
Despite the cessation of uranium releases and the removal of
shallow vadose zone source materials, the remedial action objective
to lower the concentration of groundwater uranium to the U.S.
Environmental Protection Agency (EPA) maximum contaminant level
(MCL) concentration of 30 g/L has not been achieved within the
anticipated 10-year time period. Despite several decades of
studies, effective uranium cleanup strategies remain elusive for
contamination in deep subsurface settings that prevail at a number
of U.S. Department of Energy (DOE) sites in the western United
States. Numerous strategies have been proposed including iron
barriers, soluble reductive agents, microbial stabilisation via
reduction and precipitation, and emplacement of solid phosphate
barriers. While these all have merit, each encounters limitations
for deep subsurface remediation in an oxidative environment
(Peterson et al. 2005; Wellman et al. 2005b; Wellman et al. 2006c;
McGrail and Mattigod 1999; DOE 2005a; DOE. 2005b). Polyphosphate
technology has been demonstrated to delay the precipitation of
phosphate phases for controlled in situ formation of stabile
phosphate phases which control the long-term fate of uranium
(Wellman et al. 2005b; Wellman et al. 2006c). Precipitation of
phosphate minerals occurs when phosphate compounds degrade in water
to yield the orthophosphate molecule (PO43 ). Accordingly, a
detailed understanding of polyphosphate degradation and reaction
kinetics, in the context of site-specific information, allows the
technology to be tailored as a time-released source of phosphate
for direct subsurface treatment without a drastic change in
hydraulic conductivity of the target aquifer. A site-specific
treatability test was conducted to optimise polyphosphate
remediation technology for implementation through a pilot-scale
field technology demonstration to accelerate monitored natural
attenuation of the uranium plume within the Hanford 300 Area
aquifer. A focused application of polyphosphate was conducted in a
source or "hot spot" area to reduce the inventory of available
uranium that contributes to the groundwater plume through direct
precipitation of uranyl-phosphate solids and secondary containment
via precipitation of apatite acting as a long-term sorbent for
uranium. The general treatability testing approach consisted of
initial site characterisation and set-up, a polyphosphate injection
test, and post-treatment performance assessment. Fundamental
science studies were conducted with site- specific sediment and
groundwater to develop an effective remediation scheme for
deployment of polyphosphate technology. In addition to remediating
a portion of the plume, the data from this test provide valuable
information for designing a full-scale remediation of uranium in
the aquifer at the 300 Area of the Hanford Site. Results provide a
detailed understanding of the fundamental underpinnings necessary
to evaluate the efficacy and potential use of polyphosphate
technology at other sites with varying geochemical and hydrodynamic
conditions.
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