Analysis of metolachlor ethane sulfonic acid (MESA) chirality in groundwater: A tool for dating groundwater movement in agricultural settings.
To better address how much groundwater contributes to the loadings of pollutants from agriculture we developed a specific dating tool for groundwater residence times. This tool is based on metolachlor ethane sulfonic acid, which is a major soil metabolite of metolachlor. The chiral forms of metolachlor ethane sulfonic acid (MESA) and the chiral forms of metolachlor were examined over a 6-year period in samples of groundwater and water from a groundwater-fed stream in a riparian buffer zone. This buffer zone bordered cropland receiving annual treatments with metolachlor. Racemic (rac) metolachlor was applied for two years in the neighboring field, and subsequently S-metolachlor was used which is enriched by 88% with the S-enantiomer. Chiral analyses of the samples showed an exponential increase in abundance of the S-enantiomeric forms for MESA as a function of time for both the first order riparian buffer stream (R(2)=0.80) and for groundwater within the riparian buffer (R(2)=0.96). However, the S-enrichment values for metolachlor were consistently high indicating different delivery mechanisms for MESA and metolachlor. A mean residence time of 3.8years was determined for depletion of the initially-applied rac-metolachlor. This approach could be useful in dating groundwater and determining the effectiveness of conservation measures.



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.
Sustainable Agricultural Systems Laboratory, Beltsville Agricultural Research Center, US Department of Agriculture, Agricultural Research Service, 10300 Baltimore Avenue, Beltsville, MD 20705-2325, United States. Electronic address: clifford.rice@ars.usda.gov.
Clifford P
CP
Rice
Hydrology and Remote Sensing Laboratory, Beltsville Agricultural Research Center, US Department of Agriculture, Agricultural Research Service, 10300 Baltimore Avenue, Beltsville, MD 20705-2325, United States.
Gregory W
GW
McCarty
Sustainable Agricultural Systems Laboratory, Beltsville Agricultural Research Center, US Department of Agriculture, Agricultural Research Service, 10300 Baltimore Avenue, Beltsville, MD 20705-2325, United States.
Krystyna
K
Bialek-Kalinski
Department of Civil and Environmental Engineering, University of Maryland, College Park, MD 20742-0001, United States.
Kara
K
Zabetakis
Department of Civil and Environmental Engineering, University of Maryland, College Park, MD 20742-0001, United States.
Alba
A
Torrents
Hydrology and Remote Sensing Laboratory, Beltsville Agricultural Research Center, US Department of Agriculture, Agricultural Research Service, 10300 Baltimore Avenue, Beltsville, MD 20705-2325, United States.
Cathleen J
CJ
Hapeman
D000081:Acetamides / Q000032:analysis*; D000383:Agriculture; D004784:Environmental Monitoring / Q000379:methods*; D004980:Ethane / Q000032:analysis; D060587:Groundwater / Q000737:chemistry*; D006540:Herbicides / Q000032:analysis*; D013451:Sulfonic Acids / Q000032:analysis*; D014874:Water Pollutants, Chemical / Q000032:analysis*
D000081:Acetamides; D006540:Herbicides; D013451:Sulfonic Acids; D014874:Water Pollutants, Chemical; D004980:Ethane; C051786:metolachlor
Chiral separation; Groundwater dating; Metolachlor ethane sulfonic acid; S-metolachlor; Stereoisomer fraction