Effects of electroosmosis on natural soil: Field test

A unique configuration of horizontal sheet-like electrodes was used in the field at a site in Ohio that was underlain by silty clay glacial drift to i nduce electroosmotic flow and to characterize the effects of electroosmosis on soil properties (e.g., electrical conductivity and pH). The lower elect rode was created at a depth of 2.2 m by filling a flat-lying hydraulic frac ture with granular graphite, and the upper one was a metallic mesh placed a t a depth of 0.4 m and covered with sand. The electrodes were attached to a DC power supply, creating an electrical gradient of 20-31 V/m and a curren t of 42-57 A within approximately 20 m(3) of soil. Total energy applied was 5,500 kW . h during approximate 4 months of operation. Electroosmotic flow rates of 0.6-0.8 L/h were observed during tests lasting several weeks, alt hough total flow rate (electroosmotic plus hydraulic) was strongly influenc ed by fluctuations of the ground-water table. The ratio of applied current to voltage decreased from 0.9 to 0.6 A/V and was mainly due to a decrease i n electrical conductivity of the soil. A low pH front developed at the anod e and migrated toward the cathode. The velocity of the pH front per unit vo ltage gradient was 0.014 (cm/day)/(V/m). This was 40 times slower than what has been reported from laboratory experiments using kaolinite as a medium. These results confirm the feasibility of using horizontal electrodes at sh allow depths, but they also underscore some important differences between t he geochemical effects observed during field tests in natural soils and tho se seen in laboratory tests using ideal materials.