Section Seven
OTHER BIOLOGICAL PROCESSES
52 THE EFFECTS OF SECONDARY CARBON
SOURCES AND ELECTRON ACCEPTORS ON THE
BIOTRANSFORMATION OF ALACHLOR
AND PROPACHLOR
Gregory G. Wilber, Assistant Professor
Guigen Wang, Graduate Student
School of Civil and Environmental Engineering
Oklahoma State University
Stillwater, Oklahoma 74075
INTRODUCTION
The detection of agricultural chemicals in both raw ground waters and finished drinking waters has resulted in increased concern about the fate, transport, and health effects of these compounds upon their release. It has also resulted in the addition of many herbicides to the list of
drinking water contaminants regulated by the United States Environmental Protection Agency.
Among these are two commonly used acetanilide herbicides, alachlor (2-chloro-2',6'-diethyl-N-
methoxymethyl acetanilide) and propachlor (2-chloro-N-isopropyl acetanilide), which are applied as pre- and post-emergence herbicides on numerous crops including corn and wheat.1 Both
of these pesticides have been detected in ground water samples in Oklahoma and in other agricultural areas in the U.S.2-3 Current standards of the Safe Drinking Water Act include a maximum contaminant level (MCL) of 2 pg/L for alachlor.4 Propachlor currently has only monitoring
requirements, and may have a specific MCL established in the future.
In order to better assess the risks associated with the introduction of these herbicides into the
environment, particularly with respect to their transport within subsurface drinking water
sources, information is needed regarding their fate once they have reached the subsurface.
Among the major fate processes which must be considered is microbial biotransformation. It is
known that among the important factors affecting the biotransformation of such xenobiotic
chemicals are the dominant electron acceptor condition and the presence of other, more readily
degraded exogenous carbon sources (i.e., a primary substrate).5-9
Alachlor and propachlor, the structures of which are shown in Figure 1, have been demonstrated to be biotransformable under a variety of conditions. Novick and coworkers report observing cometabolic biotransformation of alachlor and propachlor in sewage and lake water samples.'011 Propachlor degradation appeared to require the presence of more readily degraded
primary substrates.10 However, propachlor has also been shown to serve as the sole source of
carbon and energy to at least two distinct microbial species.12 Additional research which has directly compared the acetanilide herbicides has found in general that those compounds with fewer
and smaller substituents (such as propachlor) are more readily degraded than those more heavily
substituted (such as alachlor).13- 14 Wilber and Parkin report the biotransformation of alachlor, in
continuous-flow, acetate-fed biofilm reactors and batch reactors, under four different electron acceptor conditions.915 In all cases, the presence of acetate was required for the transformations to
continue, though the effect was most pronounced in the sulfate-reducing system. Later, Wilber
and Garrett also reported the cometabolic biotransformation of propachlor by an acetate-fed nitrate-reducing mixed culture, although the necessity of nitrate was not tested.8 Several field stud-
50th Purdue Industrial Waste Conference Proceedings, 1995, Ann Arbor Press, Inc., Chelsea, Michigan 48118.
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