INTERACTION OF SPECIFIC TOXIC ORGANIC CHEMICALS
PERCOLATING THROUGH A SOIL
Robert J. Osipoff, Design Engineer
Williams & Works, Inc.
Grand Rapids, Michigan 49501
Neil J. Hutzler, Assistant Professor
John C. Crittenden, Assistant Professor
Department of Civil Engineering
Michigan Technological University
Houghton, Michigan 49931
INTRODUCTION
The presence of toxic organic chemicals in the nation's groundwater poses a serious
threat to the environment and to public health. Unlike surface waters, groundwaters move
very slowly which means chemicals have little opportunity to be dispersed. Furthermore,
once the groundwater has been contaminated, cleanup is very difficult. Groundwater protection requires proper design of landfills and sites for the land application of wastewater.
Describing the transport of toxic organic chemicals in the sod environment is essential
for proper selection of sites for land disposal of hazardous wastes. Research on the behavior of pesticides in sod has identified several possible fates of organic chemicals [1,2,3].
The direction and rate of groundwater flow govern the direction and velocity of chemical
movement. Retention of toxic organics by sod wdl depend on the amount of chemical
reversibly and irreversibly adsorbed onto sod particles whde biological degradation, hydrolysis and chemical oxidation wdl determine the persistence of organic chemicals. Some organics may be lost to the atmosphere by volatdization. If the waste is deposited on the surface,
runoff or wind erosion may move the chemical away from the disposal site. In addition to
these mechanisms, the movement of organics may be influenced by the formation of humic
acid complexes or by changes in sod water such as pH variation. Although aU these mechanisms can be operative, contamination of aquifers wdl depend primardy on whether toxic
organic chemicals are adsorbed by the sod matrix or are biodegraded by sod microorganisms.
The adsorption of organic chemicals onto sod is a function of the sod particle properties,
the chemical properties, and the sod water characteristics [2,3]. Clay mineral and organic
content are among the most important sod properties. Expanding clays such as montmord-
lonite show a greater affinity for organics than nonexpanding, probably because they have
a much higher surface area. The organic fraction provides various functional groups such as
carboxyl, amino and phenolic hydroxyl which can react with chemicals in the aqueous
phase. The surface pH of the sod also is important in adsorption. The preferential adsorption
of one chemical over another for a given sod is related to chemical characteristics such as
structure, functional groups, molecular size, water solubdity, polarity and the value of the
dissociation constant relative to sod surface pH. Molecular structure and size influence a
chemical's movement in and around sod particles whde the functional groups influence
chemical reactions. Soluble and polar compounds are less likely to be adsorbed and acidic
compounds are more likely to be adsorbed when the value of their dissociation constant
is less than the surface pH of the sod [4]. FinaUy, constituents in the water that compete
for adsorption sites will also reduce the amount of a given chemical that is adsorbed.
Biodegradation represents a major route of degradation for many organic chemicals
applied to sod, yet, the factors that influence biodegradation are not wed understood [3],
In general, conditions that favor microbial activity, favor biodegradation, but the mechanisms by which sod microorganisms breakdown specific compounds depend on the microbial
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