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Apatite Barrier

Photo of apatite barrier emplacement at a field site
Apatite barrier placement

Apatite barrier placement by infiltration (instead of injection well placement) allows sequestration of contaminants in the vadose zone as well as the saturated zone, eliminates drilling costs, and allows treatment of contaminant zones close to river banks.

Apatite barriers are used to sequester heavy metals and are typically emplaced in the path of a contaminant plume by injection through an array of drilled wells. In contaminated zones subject to the influence of riverine hydrodynamics, emplacement of an apatite barrier is complicated by river stage influence on apatite solution flow. It is usually desirable to perform the injection at high river stage, when formations are saturated, followed by a moderate river stage to induce flow towards the river. There may be only a few short periods throughout the year when conditions are optimal for apatite injection. Infiltration of apatite solution from the ground surface extends the amount of time available for apatite emplacement and immobilizes contaminants in the vadose zone that would not be treated by well injection. Additionally, areas closer to the riverbank can be treated using infiltration because access for heavy trucks and drilling equipment is not a requirement. It is expected to be less costly to implement apatite emplacement with infiltration, because drilling costs are eliminated. PNNL researchers are using laboratory-scale and intermediate-scale studies along with computer flow modeling to develop an infiltration strategy that defines both the precipitation rate of apatite-forming solutions and the kinetics of contaminant sequestration processes under the variably saturated conditions found in riverine formations. The combined effects of biogeochemical and coupled precipitate/flow processes will be quantified so that an infiltration strategy that produces a relatively uniform spatial distribution of the apatite precipitate can be achieved at field scale. This emplacement method should find broad application in remediation of contaminant plumes extending through the vadose and saturated zones and may become the emplacement method of choice in many types of formations.

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