toxicological profile for malathion - Agency for Toxic Substances and ...

MALATHION 189
6. POTENTIAL FOR HUMAN EXPOSURE
obtained from the Suwannee River which contained “a large amount of colored materials.” It is assumed
that the referenced “colored materials” are derived from humic acids which, as known photosensitizers,
can contribute to indirect photolysis in water systems. Aside from the early studies referenced here, very
little other information on the photolysis of malathion in water was found in the literature. In a non-U.S.
study, the rapid degradation of malathion in natural (estuarine) water exposed to ambient sunlight and
temperature was reported; the half-life of malathion in estuarine water from the Ebre Delta area of Spain
was 4.4–4.9 days (LaCorte et al. 1995). The contribution of photolysis to the overall degradation rate for
the compound was not determined. In a study of malathion in river water, sea water, and groundwater in
Hawaii, Miles and Takashima (1991) found that although degradation was rapid (mean half-life of
4.7 days), photodegradation and biodegradation of the compound were not important; degradation
proceeded mainly by an elimination reaction. Based on these data, the direct photolysis of malathion in
water is not an important fate process.
Microbial degradation of malathion in water has been studied in different types of water. A study was
conducted to determine the degradation rate of malathion at 20 EC in sterile (autoclaved; pH 8.20) and
unsterile (pH 8.05) filtered seawater and in a seawater/sediment (pH 7.3–7.7) microcosm (Cotham and
Bidleman 1989). Reported half-lives were 3.3 days in sterile seawater, 2.4 days in unsterile seawater, and
2 days in the seawater/sediment microcosm. When the values determined for the seawater systems were
normalized to pH 8.0, the half-life of malathion was approximately twice as long in sterile seawater, at
5.3 days, as in unsterile seawater (half-life of 2.6 days; Cotham and Bidleman 1989). These researchers
noted that the more rapid degradation in the seawater/sediment system relative to the unsterile seawater
system, which had a higher pH (leading to more rapid hydrolysis), indicates that microbial activity or
interaction of malathion with the sediment was a contributing factor (in addition to hydroxide-catalyzed
hydrolysis) to the degradation of the compound. Half-lives of 92–96 hours for malathion (1 mg/L) in
seawater were reported by Bourquin (1977); significant malathion degradation was not observed in sterile
seawater. Degradation products observed in the study were malathion monocarboxylic acid and
malathion dicarboxylic acid; malaoxon was not detected as a degradation product.
In a study of the degradation of malathion by isolated salt-marsh microorganisms, 11 of the 15 bacterial
cultures were able to degrade malathion as a sole carbon source and the remaining 4 were able to degrade
malathion by cometabolism when 0.2% peptone was added as an additional source of carbon (Bourquin
1977). An isolated salt-marsh fungus was unable to degrade malathion without the addition of 0.2%
peptone. The study author attributed the degradation of malathion by bacterial cultures to a
carboxylesterase system that leads to the formation of the mono- and dicarboxylic acids, and a delayed