Other Technologies

Indirect methods of oxygenation such as air sparging are less efficient because the injected air follows a limited number of preferential flow paths and only a small percentage of ground water is effectively aerated.  Pulsing air injections can induce some mixing, but even with optimal operation, the amount of oxygen that becomes dissolved in water rarely reaches the equilibrium saturation achieved when water is in contact with a standard atmosphere (11.277 mg/L at 10° C).

Dispersion of dissolved compounds in ground water results primarily from shifts in gradients and flow paths.  Actual mixing through diffusion may be limited to a few inches to a few feet per year.  Diffusion limitations are the primary reason for the failure of peroxygens to oxygenate large volumes of the saturated zone.  Even where peroxygen socks are placed in large diameter wells or gravel-filled boreholes of higher permeability than the surrounding aquifer, ground water modeling indicates that the plume width of effectively oxygenated ground water downstream of the well or borehole may be as narrow as twice the diameter of the borehole.

In the early years of aerobic in-situ bioremediation, hydrogen peroxide was used to deliver a dissolved oxygen (DO) equivalent that was much higher than could be achieved by injecting oxygen alone.  Hydrogen peroxide is soluble in water at very high concentrations and degrades to water and DO (approximately 47.1% effective oxygen delivered by weight).  The degradation of peroxide is catalyzed by metals such as iron, and by biological enzymes found in all microbes. Peroxide solutions can be stabilized with phosphates and other additives to prevent iron from rapidly degrading peroxide, but the microbes must degrade peroxide to water and oxygen immediately or be killed.

At the first demonstration of in-situ bioremediation (US EPA, 1990), 700 mg/L of hydrogen peroxide (equivalent DO concentration of 329.7 mg/L) was injected into a line of five injection wells.  Hydrogen peroxide was never detected as close as 30 feet down-gradient in spite of a ground-water flow velocity as great as 10 feet per day in the injection area.  Dissolved oxygen concentrations 30 feet down gradient from the injection area ranged from 40 to 60 mg/L and oxygen concentrations in the unsaturated zone were as high as 60% by volume, indicating that as much as 98% of the equivalent DO was lost from solution.