The term “prefield test” denotes an in situ test of contaminated soil in preparation for in situ treatment of the soil by a method called “electrokinetically enhanced bioremediation” (EEB). A prefield test yields data that are helpful in designing and operating an efficient and cost-effective EEB system.

Test Cathode and Anode Wells can be positioned in this pattern or any of a variety of other patterns, depending on the size and nature of the soil region of interest. The minimum number of electrode wells needed for a prefield test is three.
EEB was described in “Engineered Bioremediation of Contaminated Soil” (KSC-12045), NASA Tech Briefs, Vol. 25, No. 7 (July 2001), page 58. To recapitulate: EEB involves the utilization of controlled flows of liquids and gases into and out of the ground via wells, in conjunction with electrokinetic transport of matter through pores in the soil, to provide reagents and nutrients that enhance the natural degradation of contaminants by indigenous and/or introduced micro-organisms. An EEB system includes injection and electrode wells, pumps, reservoirs of chemicals, and other components needed to control the movements of charged anionic and cationic as well as noncharged chemical species and micro-organisms through the ground.

It has been standard practice, in preparing to design systems for in situ treatment of contaminated soil, to perform bench-scale laboratory tests on samples of soil from contaminated sites to determine hydrogeological, physical, and chemical parameters of soils and contaminants. A prefield test yields additional information that cannot be obtained from a bench-scale test and thus makes it possible to design a superior treatment system for a specific contaminated site. The additional information pertains to electrical conductivity and other parameters that vary spatially because of spatial variations in such properties of the soil as porosity, density of packing of particles, and chemical properties of pore fluid/soil interfaces. The data from a prefield test make it possible to optimize such design and operating parameters as applied voltages and currents and the positions of electrode wells, in order to treat the contaminated soil efficiently and more nearly uniformly.

In preparation for a prefield test, one inserts multiple test electrodes at different locations dispersed over the soil region of interest. At least one test electrode must be an anode and at least one must be a cathode (see figure). During the test, known dc voltages and currents are applied to the soil via the test electrodes. Voltage probes are inserted in the soil at various depths and at numerous horizontal positions between the test electrodes. The voltage readings as functions of position are used to generate a three-dimensional map of the test electric field.

The inhomogeneities of the test electric field are related to the inhomogeneities of the soil and the positions of the test electrodes, and can be used to guide the subsequent placement of working electrode wells for remediation of the soil. A rule of thumb calls for the placing of the working electrode wells so that at locations far from the electrode wells but still within the region of soil to be treated, the electric field should be at least 10 to 20 percent as strong as the electric fields near the electrode wells.

Other parameters can also be measured during a prefield test:

  • It can be useful to measure the temperature of the soil at various positions between the test electrodes and the temperatures of the test electrode wells as functions of applied currents.
  • The volumes of fluids in the electrode wells can be measured over time to determine rates of electro-osmotic flow through the soil. It may also be useful to track rates of electro-osmotic flow functions of applied voltages.
  • Voltage drops across electrode-well walls can be measured for use in determining the optimum well-wall materials for particular soil conditions.
  • The pH of the soil near a test electrode well can be measured while releasing a pH-adjusting solution from the well at a known rate. The result of this measurement provides guidance for adjusting the pH of the soil during treatment.

This work was done by Dalibor Hodko of Lynntech, Inc., for Kennedy Space Center. For further information, access the Technical Support Package (TSP) free online at www.nasatech.com/tsp  under the Materials category.

In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to

Dalibor Hodko Lynntech, Inc.
7610 Eastmark Drive Suite 202
College Station, TX 77840
(979) 693-0017

Refer to KSC-12160, volume and number of this NASA Tech Briefs issue, and the page number.