In situ controlled-release schemes for controlling the spread of organic contaminants in roundwater

摘要

In situ chemical oxidation with KMnO4 is being used for the remediation of groundwater contaminated with chlorinated solvents. Most applications involve schemes with active flushing of source zones. This study illustrates new possibilities for using 'controlled release' approaches with oxidants for plume control and more generally in water treatment. KMnO4 is extremely soluble in water. Controlled-release permanganate solids rely on diffusional release modes, which effectively add treatment chemicals slowly in reactive barriers. We have investigated a variety of practical approaches to employ slow release forms in reactive barrier systems. An integral part of this work has involved the development of a code capable of modeling controlled-release systems has been developed to fully explore issues involving the controlled, reactive transport, and mixing of active agents in complex aquifer settings. The minimal dispersion in the system and the geometry of the solids provided unique challenges in the modeling. This study examines several possibilities of mixing the MnO4 -with a TCE plume in a reactive barrier. The first creates a well-mixed, stable MnO4-zone with predetermined size (D×L=8m×2m) and concentration ranges (1.5～20 mg/L) by cyclical patterns of pumping (e.g., four one-day injection/withdrawal pumping periods over 24 d). This type of mixing zone can be maintained for long times with periodic well mixing and replacements of exhausted controlled-release forms. Another promising approach to mixing couples funnel-and-gates with controlled-release system. Two-dimensional solid forms are designed with the model to achieve almost complete mixing and an optimum MnO4- concentration.The development of this general code together with the novel mixing schemes opens possibilities for other types of controlled-release barriers created by reductants, nutrients, bioactive chemicals, and agents for in situ redox manipulations. Coupled use of the generalized code with hydrologic data will help to optimize the design and operation of controlled-release systems in practice.