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)

Authors: Plummer, Rebecca; Hapeman, Cathleen; Rice, Clifford; McCarty, Gregory; Schmidt, Walter; Downey, Peter; Moorman, Thomas; Douglass, Elizabeth; Strickland, Timothy - Tim; Pisani, Oliva; Bosch, David - Dave; Elkin, Kyle; Buda, Anthony

Submitted to: Journal of Agricultural and Food Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/29/2020
Publication Date: 1/29/2020
Citation: Plummer, R.E., Hapeman, C.J., Rice, C., McCarty, G.W., Schmidt, W.F., Downey, P.M., Moorman, T.B., Douglass, E.A., Strickland, T.C., Pisani, O., Bosch, D.D., Elkin, K.R., Buda, A.R. 2020. 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). Journal of Agricultural and Food Chemistry. 68(8):2297-2305. https://doi.org/10.1021/acs.jafc.9b06187.
DOI: https://doi.org/10.1021/acs.jafc.9b06187

Interpretive Summary: Models are used to calculate the delivery of nutrients from groundwater to surface waters. These models require various landscape measurements including the age of the groundwater. A new method was discovered to measure groundwater age using a degradation product of a commonly-used herbicide which is frequently found in groundwater and surface waters. However, this method would require collecting numerous samples which would not be practical. This study examines the effectiveness of a new passive sampler and a modified analytical procedure to obtain the data needed to calculate the age of groundwater in a watershed. Experiments were carried out to validate and calibrate this technique. Results of this work will be used in models to evaluate the delivery of nutrients from groundwater to surface waters under various soil, agronomic, and land use conditions and to examine the effectiveness of conservation practices to protect water quality.

Technical Abstract: Modeling nutrient fate requires the use of landscape parameters, such as, the mean age or mean residence time of groundwater entering surface waters. We recently discovered a new 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) combined with a modified LC/MS-MS method to provide a time-weighted average of the enantiomeric ratio of MESA. POCIS 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 and versus grab samples. In addition, an in-situ POCIS sampling rate for MESA was calculated (0.151 L/d), the precision of MESA extraction from stored POCIS discs was determined, and the effectiveness of HLB to extract MESA was examined. Results of this work will be used in larger projects to discern the groundwater mean residence times, to evaluate the delivery of nitrate-N from groundwater to surface waters under various soil, agronomic, and land use conditions, and to examine the effectiveness of conservation practices.