Public Health Concentrations for Metolachlor and Metolachlor Degradates Ethanesulfonic Acid and Oxanilic Acid in Groundwater

This report describes the Office of Environmental Health Hazard Assessment (OEHHA) derivation of the public health concentrations (PHC) in water for metolachlor and its environmental degradates metolachlor ethanesulfonic acid (MESA) and metolachlor oxanilic acid (MOXA).  The PHCs derived are 1300 parts per billion (ppb) for MESA, 3200 ppb for MOXA, and 7 ppb for metolachlor.  This report includes the risk characterization of human consumption of drinking water at the detected levels for a lifetime.  OEHHA conducts this evaluation under the California Pesticide Contamination Prevention Act (PCPA) (DPR, 2017a).  The California Department of Pesticide Regulation (DPR) initiated the evaluation process to determine if the detected levels of MESA and MOXA at concentrations from 0.059 to 20.2 ppb would “pollute” the groundwater.  The term pollute is defined in Food and Agriculture Code Section 13142(j)1 as “to introduce a pesticide product into the groundwaters of the state resulting in an active ingredient, other specified ingredient, or a degradation product of a pesticide above a level that does not cause adverse health effects, accounting for an adequate margin of safety.”