Acephate Refined Drinking Water Assessment for Registration Review
https://www.epa.gov/pesticides/epa-publishes-updated-risk-assessments-chemical-acephate
Acephate will initially enter the environment via direct application (e.g., liquid spray) to use sites. It may
move off-site via spray drift and runoff (dissolution in runoff water). Additionally given its high mobility,
acephate may move into groundwater systems, however this movement is inhibited by its low
persistence in soil. Based on the vapor pressure of acephate and its calculated Henry’s Law Constant, it
is not expected that volatilization will be a significant route of dissipation route. Rapid degradation of
acephate in soil yields the intermediate degradate methamidophos [max of 90% of the applied
radioactivity (AR) formed] which in turn is non-persistent in soil. Acephate and methamidophos both
hydrolyze slowly with half-lives of 169 days and 27 days, respectively, at pH 7 conditions. In aerobic
aquatic water/sediment systems, acephate degraded to methamidophos with a half-life of nearly a
month and maximum formation of 26% of the AR in the total system. In an anaerobic aquatic system
acephate degraded to methamidophos with a half-life of 6.4 days and formed a maximum of 27% of the
AR in the total system. DMPT (O,S-dimethyl phosphorothioate) (a major degradate in soil and alkaline
hydrolysis) and O-des-methyl methamidophos (a major hydrolysis degradate at neutral pH) were also
identified as degradation products of acephate in laboratory soil and water studies.
In summary, while the majority of acephate residues in soil will rapidly degrade to methamidophos,
once in the aquatic environment (via either runoff or direct deposition from spray drift) acephate will
degrade at a slower rate and in lower quantities to methamidophos. Therefore, the assumption of 100%
conversion to methamidophos made in the 2017 DWA was refined in this updated DWA in favor of a FD
modeling approach to quantify the potential mixture of acephate and methamidophos residues in
drinking water.