Metolachlor and its primary soil degradation product, metolachlor ethane sulfonic acid (MESA); exist as four enantiomeric compounds. During degradation, the chiral centers are unaffected, resulting in virtually no changes in the enantiomeric ratio. In the US, highly S-enriched metolachlor was introduced as the commercial replacement for racemic metolachlor in 1999. Because the enantiomeric ratio is maintained, the enantiomeric ratio of MESA in the groundwater can be used to estimate groundwater flushing times. This concept was examined in a small first order catchment draining 22 ha of cropland that had received metolachlor applications since 1998. This riparian stream was fed by upwelling groundwater from the fields. The chiral abundances of MESA in the receiving stream were measured over a 7 year time span, beginning in 2000. Using this data a flushing time of 3.8 years for groundwater entering a stream was calculated. The delay in the shift from the racemic form of MESA to the S-enriched form was used to model the movement of the stored groundwater.

Chirality in metolachlor can be used to date grounwater in agricultural settings

253rd National Meeting (2017)

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