13    UNSATURATED ZONE GAS-PHASE VOC
BIODEGRADATION:
THE IMPORTANCE OF WATER POTENTIAL
Patricia A. Holden, Graduate Researcher
Department of Environmental Science, Policy & Management
James R. Hunt, Professor
Environmental Engineering
Department of Civil Engineering
Mary K. Firestone, Chair and Professor
Department of Environmental Science, Policy & Management
University of California, Berkeley
Berkeley, California 94720
INTRODUCTION
Bioremediation of the unsaturated zone using indigenous microorganisms holds promise as a
cost-effective and environmentally-compatible cleanup option. Both field and laboratory results
indicate that microbial decomposition of hydrocarbon vapors in some subsurface soils may
occur1-2 and that forced air advection, called bioventing, through such systems increases the
overall cleanup rate by enhancing volatilization and gas phase mass transfer to microbial communities.3 More conclusive evidence of biological removal stems from carbon dioxide isotopic
analysis during field studies or from measuring the percent of recovered label in 14C laboratory
studies.4 Despite the evidence supporting in situ bioremediation of volatile organic compounds
(VOCs). the majority of laboratory and field studies to date can best be described as phenome-
nalistic. If in situ bioremediation is to be cost-effectively engineered, a firm understanding
of related process factors and an ability to control those factors must be gained. Sims et al. proposed five environmental factors that influence microbial activity during bioremediation:
(1) available soil water, (2) oxygen, (3) redox potential, (4) pH, (5) nutrients, and (6) temperature. We propose that soil water potential is a limiting constraint, and in fact is the unifying determinant of many individual factors deemed important to VOC biodegradation in unsaturated
soils.
DEFINITION OF SOIL WATER POTENTIAL
The amount of water in soil is typically reported as water content, either gravimetric or volumetric.6 Water content is easily measured and is a useful and familiar description of the composite soil matrix. As a measured parameter, however, water content provides no intrinsic description of the physical properties of soil water that govern either its movement or its influence on
other soil constituents.
Soil water potential, \\t, is defined as the potential energy per unit volume of soil water.6-7 Energy per volume is equivalent to pressure and i|/ is most commonly reported in units of MPa or
bars. Total soil water potential. \\i, has two primary components: \|/s, solute potential, and \)/m.
matric potential. A useful expression relating \|/s to solute concentration is the modified van't
Hoff relation 7 :
ys = <tryCRT (1)
50th Purdue Industrial Waste Conference Proceedings, 1995, Ann Arbor Press, Inc.. Chelsea. Michigan 48118.
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