In Situ Decontamination of Soil

Abstract

Abstract Methods for decontamination of soil in situ are receiving increased attention, as the requirement to clean these sites has become a national concern. The continual and costly liability is an important corporate incentive to clean contaminated soils. The development of an effective technology for in situ decontamination is a priority issue. Some in situ decontamination processes are similar to enhanced oil recovery schemes and use an integrated heating approach. One such method consists of heating by conduction and convection in conjunction with electrical heating. Advantages of electrical heating are that the energy can be focussed and the soil heated uniformly. The integrated heating approach has been extensively tested in the United States in a joint venture between Lawrence Livermore National Laboratories (LLNL) and the University of California at Berkeley. The most significant test was done at an abandoned Naval Air Base, now named the LLNL Gasoline Spill Site. It has been demonstrated that such an integrated approach can remove hydrocarbon contaminates from in situ at an accelerated rate. The objective of this paper is to present a mathematical model that solves the heat transfer problem for the integrated heating process. The model is also used to investigate the practicality of the process in terms of energy requirements and the time it takes to clean the soil. The mathematical model combines all of the principal heat transfer mechanisms of the process. These are electrical heating, convection, conduction and accumulation of energy. In addition, conductive heat losses from the sides and bottom of the heated volume and the substantial variation of electrical resistivity with temperature are included. Calculations made using the mathematical model developed in this paper are in good agreement with results obtained with the general-purpose reservoir simulator, TETRAD. Introduction Increasing the temperature of a contaminated soil helps in the removal of the hydrocarbons. The introduction of heat increases the solubility of the hydrocarbon with the water phase, increases equilibrium concentrations in the vapour phase and causes easier desorption from the clay mineral. These factors can accelerate the clean up process several times(1). Some proposed methods consider an integrated heating approach, consisting of heating by conduction and convection at the same time as heating electrically(2, 3). This approach has been successfully tested in the United States by Lawrence Livermore National Laboratories (LLNL) at the LLNL Gasoline Spill Site(2, 4). At this site, hydrocarbons have accumulated in situ and are in communication with the ground water system. Vertical injection/production wells completed with electrodes are strategically located to heat the soil. The heating of the soil is primarily by steam injection. The electrical heating is supplemental and is used to heat regions where the steam will not flow. The estimated cost to clean the contaminated soil(2), which has an area of about 2,000 m2 and is 15 m thick is of the order of $(CDN) 50/m3. The electrical heating component of the above process is limited because of the use of vertical electrodes spaced far apart. The current density at the electrode surface is large and decreases rapidly with distance.