Page 351 - gas transport in porous media

CHAPTER 22

ENVIRONMENTAL REMEDIATION OF VOLATILE
ORGANIC COMPOUNDS

RON FALTA
Clemson University, Clemson, SC, USA

22.1 INTRODUCTION

Over the past 25 years, groundwater and subsurface contamination by volatile
organic compounds (VOCs) has emerged as a widespread problem in industrialized
countries. This class of compounds is broad, and it includes a variety of com-
mon organic chemicals such as aromatic hydrocarbons (e.g., benzene, toluene,
and xylenes) aliphatic hydrocarbons, (e.g., pentane, hexane, and cyclohexane)
and halogenated hydrocarbons (e.g., carbon tetrachloride, tetrachloroethylene, and
trichloroethylene). These chemicals are widely used as chemical feedstocks, as
cleaning solvents and degreasers, and, in the case of hydrocarbons, as fuels.
These chemicals, besides having a high vapor pressure, tend to have low aqueous
solubilities, on the order of 50 to 2000 mg/l. The low solubility of most VOCs tends
to result in high Henry’s constants, and strong partitioning into the gas phase from
the aqueous phase.
Unfortunately, many of these common chemicals are also toxic, and have very
low maximum contaminant levels (MCLs) in drinking water. For example, benzene,
which is one of the most common industrial chemicals in the world, has an aqueous
solubility of about 1760 mg/l, yet the United States MCL is only 5 ug/l.
Due to their low aqueous solubility, VOCs typically exist as nonaqueous phase
liquids (NAPLs), and they are often released to the environment in this form. The
hydrocarbon fuel mixtures such as gasoline, kerosene, diesel, jet fuel, and fuel oil
form LNAPLs (lighter than water NAPLs), while the chlorinated cleaning solvents
form DNAPLs (denser than water NAPLs). Due to capillary trapping, complete
removal of NAPLs from the subsurface by mobilization is nearly impossible, and
conventional fluid pumping operations typically leave behind NAPL saturations of
5 to 25%.

353
C. Ho and S. Webb (eds.), Gas Transport in Porous Media, 353–370.
© 2006 Springer.