A laboratory based investigation into the effects of in situ oxidative treatments of DNAPLs on porosity and permeability

Abstract

DNAPLs, or Dense Non-Aqueous Phase Liquids, are organic compounds that have densities in excess of 1 kg/L, and are immiscible in water. They represent a particular problem when present in groundwater because of their physical and chemical properties, most particularly their density and a tendency toward high interfacial tension. Many methods for removing these materials have been tested in recent decades; chemical oxidation has been one that has received a great deal of attention. This investigation is meant to gain more understanding of the effects of chemical oxidation using potassium permanganate on the porosity and permeability of porous media in an aquifer. Three columns were constructed with manometers and electrodes emplaced to take discrete measurements through the length of the columns. Two different size ranges of sandy media were used; a fine grained portion was placed in the bottom of the columns to effectively create an aquitard, allowing a DNAPL, in this case trichloroethylene (or TCE) to be emplaced directly in the column. A coarser grained material was used in the upper portion of the columns to allow for the easy passage of fluid through the pore spaces, and room for precipitation of by-products. A potassium permanganate (KMnCU) solution was then pumped through the columns, and measurements of hydraulic head changes taken with time. The electrodes were used to collect conductivity breakthrough curves that could then be used to calculate porosity. Changes in hydraulic conductivity and porosity were compared with potassium permanganate concentration. The results proved difficult to interpret, largely due to an error in the TCE injection. Rather than forming an even layer, the TCE found preferential flow paths, and spread throughout the columns. Resulting changes in hydraulic conductivity and porosity did reveal lowering porosity with increased KMn04 exposure.