Harnessing the Complex Behavior of Dense and Viscous Reatment Fluids as a Strategy for Aquifer Remediation

There is a critical need for developing practical and inexpensive techniques for remediating DNAPL sources and dilute plumes of chlorinated solvents in aquifers and in lower permeability zones occurring at depth. An alternative to conventional techniques involves the use of a semi-passive schemes based on in situ oxidation. In this study, we have explored two different remedial concepts with both sad-tank experiments and complimentary modeling. The first involves the use wells to deliver a dense and viscous sodium silicate solution mixed with an oxidant (e.g., potassium permanganate) and sequester it within some treatment zone. Flow tank experiments show that dense unstably stratified silicate solutions sink slowly and form instabilities. The concentrations of the viscous solutions declined significantly over relatively short distances due to convective mixing of dense and less dense fluids. W modeled these experiments with MITSU3D, a coupled flow and mass transport code that can address variable density/viscosity problems in three dimensions.  The second approach aims to better sequester treatment chemicals by taking advantage of the time-delayed gelling properties of silicate solutions. In effect, it should be possible to create a slow-release material for oxidants in situ but also a coating over contaminant-bearing lenses to reduce rates of diffusion out of these lenses. A small flow-tank experiment provided an initial proof-of-concept. An injected silicate solution gelled in place after 6 hours after placement. Tracers used in the silicate solution slowly diffused out of the gel, providing the slow release. After 20 days, approximately 60% of the initial tracer concentration left in the gelled zone. Work is now underway to model this system and to evaluate key properties of the gel.