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)

We previously discovered a method to estimate the groundwater mean residence time using the changes in the enantiomeric ratio of MESA (metolachlor ethane sulfonic acid, (2-[(2-ethyl-6-methylphenyl)(2-methoxy-1-methylethyl)amino]-2-oxoethanesulfonic acid), a metabolite of the herbicide metolachlor. However, many grab samples would be needed for each watershed over an extended period, and this is not practical. Thus, we examined the use of a polar organic chemical integrative sampler (POCIS) deployed for 28 days combined with a modified LC-MS/MS method to provide a time-weighted average of the MESA enantiomeric ratio. POCISs equipped with HLB discs were deployed at five sites across the United States where metolachlor was used before and after 1999 and compared the effectiveness of the POCIS to capture MESA versus grab samples. In addition, an in-situ POCIS sampling rate (Rs) for MESA was calculated (0.15 L/d), the precision of MESA extraction from stored POCIS discs was determined, and the effectiveness of...