Mitigation of the Herbicide Metolachlor in a Fractured Bedrock Aquifer: Insights from Multi-Element Compound-Specific Stable Isotope Analysis

Abstract:

Pesticides are increasingly detected in groundwater worldwide and pose a major threat to ecosystems and public health. Despite their ubiquity, little is known about the long-term fate of such compounds at the field scale due to the difficulty in demonstrating their in situ degradation. Compound-Specific Isotope Analysis (CSIA) is a promising tool to address this issue. During the transformation of organic contaminants, molecules with light isotopes (e.g., ¹²C, ³⁵Cl) are usually degraded at a different rate than those with heavy isotopes (e.g., ¹³C, ³⁷Cl), creating a fractionation in the undegraded molecules and providing direct evidence of the compound transformation. This approach has been limited for a long time to legacy contaminants, but recent analytical progresses have made the application of CSIA to pesticides realistic at typical environmental concentrations (i.e., sub-μg/L range).

This study focuses on a metolachlor (halogenated organic herbicide) plume discovered in 1992 in a fractured dolostone aquifer in Ontario, Canada. Sequestration of mass in the matrix led to the present-day conditions in which back-diffusion from the rock matrix into groundwater moving in the fracture network results in a persistent low concentration plume. However, data indicate that metolachlor concentrations have been recently decreasing.

The goal of this study was to assess whether this observed decrease of concentrations could be associated to metolachlor degradation in the aquifer. Several multi-level systems have been sampled and groundwater was analyzed for hydrochemistry and multi-elements CSIA (carbon, chlorine) was also conducted for metolachlor. CSIA demonstrated metolachlor degradation within the plume, indicating that biotransformation contributes to the observed mitigation. Data also indicate that different transformation pathways may be responsible for metolachlor degradation in this aquifer, although this aspect should be further explored in process-specific studies. A large fractionation was observed for chlorine compared to carbon isotopes. Chlorine therefore proved to be a particularly sensitive indicator of metolachlor degradation and a highly relevant parameter to include in future studies.