Miljøprojekt Nr. 1202 2007 Teknologiudviklingsprogrammet for jord- og grundvandsforurening – Modelsimuleringer af PCEoprensning ved kemisk oxidation i moræneler

Modelsimuleringer af PCEoprensning ved kemisk oxidation i moræneler

Summary

The effect of and time required for remediation of clayey till with embedded sand lenses contaminated with PCE by in-situ chemical oxidation with permanganate was evaluated by solute transport modeling. A numerical solute transport model was set up based on field data from a trial in-situ chemical oxidation remediation of PCE contaminated clayey till at Dalumvej 34B to evaluate the governing hydro-geological and reaction processes related to in-situ chemical oxidation as a remediation strategy for clean-up of sites contaminated with chlorinated solvents. Various scenarios have been conducted with changing conditions regarding: geology, hydraulics, reaction zones, distribution and concentration of pollutant, and injection of permanganate.

The simulations show that as long as the remediation process is running (i.e. permanganate solution is being administered), in-situ chemical oxidation can effectively control the discharge of chlorinated solvents in groundwater from the site. During the remediation process, the discharge can be controlled by regulating the amount of added permanganate or by injecting permanganate in numerous injection wells. Pumping can increase the water flow in the sand lenses and thereby enhance the horizontal spreading of the injected permanganate.

The simulations demonstrate the difficulty of cleaning up solvents in the clayey till matrix due to diffusion limitations. Mass balance results reveal that up to 80 per cent of the pollutants remain in the clay matrix after 10 years of remediation, even though complete mass removal is accomplished in the hydraulic active sand lens.

In particular the distance between the sand lenses and the reaction of permanganate with the natural organic matter and other reduced species (described by the natural oxidant demand (NOD) of the clayey material) is of crucial importance for remediation efficiency. Higher remediation efficiency is achieved, if the distance between the hydraulically active sand lenses (or fractures) is lower or if the oxidizing agent can diffuse further into the clay matrix before it is reduced by the NOD (i.e. the NOD is significantly lower). For the given conditions in the reduced part of the clayey till at the Dalumvej site, the simulations show that 60-90% of the added permanganate is reduced by the NOD. It is found that a complete clean-up of the clayey till matrix could be achieved, if the oxidizing agent was not reduced by the presence of organic compounds (NOD=0), i.e. the sole reaction was between the oxidizing agent and the solvents.

If the remediation strategy has a limited timeframe, e.g.. 10 years, a considerable rebound of solvents to the sand lenses is found as a result of continued diffusion from the contaminated clayey till matrix. With time the rebound concentration is found to reach a level at near the same concentration level as the breakthrough for a scenario without remediation. Complete mass removal from the matrix will require a very long remediation timeframe.

In the simulations, the remediation efficiency with regard to mass removal can partly be improved by enhancing the permanganate dosage until a maximum diffusion gradient is established between the clay matrix and the sand lenses. Greater remediation efficiency is achieved for geological materials where the reaction between permanganate and solvents is less affected by reaction with organic matter and other reduced species in the clay matrix (i.e. materials of lower NOD).

The conducted simulations show that chemical oxidation as a remediation strategy can be used to regulate discharge of chlorinated solvents from a site like Dalumvej 34B. However, the remediation efficiency is effectively limited by the time perspective for solvents to diffuse out of the clay matrix. Therefore, the remediation strategy must be conducted over a very long period of time before complete clean-up is established, unless the distance between the sand lenses is lower (or reduced) or diffusion of the oxidizing agent into the clay matrix is less retarded by reaction with natural organic matter and other reduced species (lower NOD).