Intermediate-scale experiments to investigate subsurface flow and transport of an apataite solution in support of Sr-90 sequestration at the Hanford 100N Area
EMSL Project ID
20899
Abstract
The attached proposal has recently been funded after an extensive peer review. The unique capabilities of the Subsurface Flow and Transport Experimental Laboratory in the EMSL are needed to complete Task 5 (Table 1, Page 4).Strontium-90 (Sr-90) discharge from past-practice liquid waste disposal sites in the Hanford 100N area have resulted in Sr-90 release to groundwater, Columbia River, and biota on the Columbia River bank. Although liquid discharges were terminated in 1993, Sr-90 adsorbed on aquifer solids remains as a continuing source to the Columbia River. Due to Sr-90 radioactive decay and adsorption, only the Sr-90 within 100 feet of the river bank is at risk of discharge to the river, so the inland pump and treat system (at a cost of $1M/yr) that removes Sr-90 does not reduce potential discharge to the river and therefore does not meet the remedial action objectives of the record of decision (ROD). A treatability test plan is in place to install a 90-meter (300-foot) apatite permeable reactive barrier (PRB) test wall near the shore in FY2006. The proposed treatability test for the treatment technology is consistent with the requirements and remedial action objectives of the ROD for Interim Action issued in 1999 (Thompson 2005). Sr-90 sequestration by this technology occurs by injection of Ca-citrate-PO4 solution, in situ biodegradation of citrate resulting in apatite [Ca6(PO4)10(OH)2] precipitation, adsorption of Sr-90 to the apatite, then apatite recrystallization with Sr-90 substitution for Ca (permanent) with the natural radioactive decay of Sr-90. Laboratory-scale studies have quantified the sequential processes of this technology in 100N area sediment. The current implementation plan is to inject a low concentration (10 mM citrate) apatite-forming solution, which should result in sufficient removal capacity of Sr-90 for ~10 years followed by higher concentration injections to develop further capacity. Because most (50 - 70%) of the Sr-90 contamination is in the shallow, variably saturated Hanford Fm., the most efficient means of treating this zone may be surface infiltration of the apatite-forming solution, but there has been no development of this technology for vadose zone application.
We propose to develop an infiltration strategy that defines the precipitation rate of an apatite-forming solution and Sr sequestration processes under variably saturated (low water content) conditions. We will develop this understanding through small-scale column studies, intermediate-scale 2-D experiments, and numerical modeling, to quantify individual and coupled processes associated with apatite formation and Sr-90 transport during and after infiltration of the Ca-citrate-PO4 solution. Development of capabilities to simulate these coupled biogeochemical processes during both injection and infiltration will be used to determine the most cost effective means to emplace an in situ apatite barrier with a longevity of 300 years to permanently sequester Sr-90 until it decays. Biogeochemical processes that will be investigated are citrate biodegradation and apatite precipitation rates at varying water contents as a function of water content. Coupled processes that will be investigated include the influence of apatite precipitation (which occupies pore space) on the hydraulic and transport properties of the porous media during infiltration.
Project Details
Project type
Exploratory Research
Start Date
2006-10-01
End Date
2007-10-01
Status
Closed
Released Data Link
Team
Principal Investigator
Team Members