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

Abstract

A mean residence time of 3.8years was calculated for groundwater feeding a first-order stream by plotting the timed-decay for the R-enantiomer of metolachlor ethane sulfonic acid.

Method to Evaluate the Age of Groundwater Inputs to Surface Waters by Determining the Chirality Change of Metolachlor Ethanesulfonic Acid (MESA) Captured on a Polar Organic Chemical Integrative Sampler (POCIS)

“…Chiral MESA analyses were conducted per previously described methods , with some minor modifications. MESA was analyzed by LC–MS/MS on an Acquity H-Class Plus UPLC with Xevo TQ-Smicro MS/MS (Waters Corporation).…”

“…These techniques, however, are limited to groundwater only because once the groundwater enters the stream waters, the volatile tracers re-equilibrate with the atmosphere. We have recently discovered a new method to estimate groundwater mean age or mean residence time using the changes in the enantiomeric ratio of the metabolite of metolachlor, MESA (metolachlor ethanesulfonic acid, (2-[(2-ethyl-6-methylphenyl)­(2-methoxy-1-methylethyl)­amino]-2-oxoethanesulfonic acid)) …”

“…Finally, the S -stereoisomers of metolachlor have long been known to have much greater herbicidal activity. ,, S -metolachlor (88% S -isomer) was first introduced to the markets in 1997 and was extensively the only formulation available by 1999. − Thus, the old MESA pool (oldMESA, formed before 1999) will be racemic and the new MESA pool (newMESA) will be mostly the S -isomer. By calculating the change of the enantiomeric ratio, and because MESA is not volatile like the atmospheric contaminants previously mentioned, MESA can be an excellent chronological marker of the groundwater and surface waters …”

“…The purpose of the current study is to evaluate POCISs deployed at five sites across the US where metolachlor has been used before and after 1999 to obtain time-weighted averages of the MESA enantiomeric ratio. We compared the effectiveness of the POCIS to capture MESA over time and versus grab samples, examined the precision of MESA extraction from stored POCIS discs, and validated the extraction method and a modified LC–MS/MS method used previously . We also examined the potential influence of chirality on the degradation of metolachlor to MESA.…”

“…By calculating the change of the enantiomeric ratio, and because MESA is not volatile like the atmospheric contaminants previously mentioned, MESA can be an excellent chronological marker of the groundwater and surface waters. 7 1.2. Study Objectives.…”