Analysis of metolachlor ethane sulfonic acid chirality in groundwater: A tool for dating groundwater movement in agricultural settings

Authors: Rice, Clifford; McCarty, Gregory; Bialek Kalinski, Krystyna; ZABETAKIS, KARA - University Of Maryland; TORRENTS, ALBA - University Of Maryland; Hapeman, Cathleen

Submitted to: Science of the Total Environment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/2/2016
Publication Date: 5/12/2016
Citation: Rice, C., Mccarty, G.W., Bialek Kalinski, K.M., Zabetakis, K., Torrents, A., Hapeman, C.J. 2016. Analysis of metolachlor ethane sulfonic acid chirality in groundwater: A tool for dating groundwater movement in agricultural settings. Science of the Total Environment. 560-561:36-43.

Interpretive Summary: Conservation measures are frequently implemented to minimize the pollutant losses in agricultural production, but assessing the effectiveness of these practices is difficult. Determining the age of groundwater can tell scientists and land managers how long nutrients have been in the water and if a newly-implemented mitigation strategy is working. Thus, accurate tools are needed to estimate the age of groundwater. Recently, we discovered a method to date groundwater using the molecular structure of a commonly-used herbicide and its metabolite. Many biologically-active compounds can exist as two distinct compounds which are mirror images of each other, often referred to as left-handed (S for sinister) and right-handed (R for rectus). Frequently, one compound is more active than the other. Metolachlor is a widely-used herbicide and exists in R and S forms, and the S form is more biologically active. For many years, metolachlor was applied as a 50:50 mixture of R and S, but today it is applied as mostly (88%) S-metolachlor. Once metolachlor enters the soil profile, it can be metabolized by soil microbes into MESA (metolachlor ethane sulfonic acid). MESA too exists as R and S forms. Groundwater samples were analyzed over a period of years in a field that was under continuous maize culture. The age of the groundwater was estimated by examining the change in the ratio of R and S forms of MESA over time. This tool has great potential in assisting scientists in the placement of conservation measures to ensure maximum effectiveness.

Technical Abstract: Chemical chirality of pesticides can be a useful tool for studying environmental processes. The chiral forms of metolachlor ethane sulfonic acid (MESA), an abundant metabolite of metolachlor, and metolachlor were examined over a 6 year period in groundwater and a groundwater-fed stream in a riparian buffer zone. This buffer zone bordered cropland under continuous maize culture where annual treatments with metolachlor were made. Three years prior to this study, racemic (rac) metolachor was applied but after two years this product was changed to S-metolachlor. This new metolachlor product is enriched by 88% in the S-enantiomer of metolachlor while rac-metolachlor contains an equal mix of the R-and S- enantiomers. Chiral analyses of the samples in this study showed increasing abundance of the S-enantiomeric forms with time for both MESA and metolachlor. Results for metolachlor showed the change to S-enriched enantiomer was erratic but quite rapid. However, for MESA, a slower exponential increase in enrichment to the S-form was observed in both the first order stream (R2 = 0.8) and in groundwater (R2 = 0.95). These data were converted to a decay function to describe depletion of rac-MESA entering the first order stream. The resulting mean residence time for this process was 3.8 years. Data such as this have potential for dating groundwater processes which could be uniquely linked to agriculture which is needed to assess the effectiveness of certain conservation and mitigation measures.