Analysis of metolachlor ethane sulfonic acid (MESA) chirality in groundwater: A tool for dating groundwater movement in agricultural settings
Graphical abstract
Introduction
Under the 1972 Clean Water Act, many US waterways are listed as impaired often due to nitrate from agricultural nonpoint sources, resulting in total maximum daily load (TMDL) restrictions (USEPA, 2009). Estimating these loads is done using models that require numerous inputs including knowledge of groundwater contributions. However, significant uncertainties exist in this process especially concerning the residence time of nitrate-N in groundwater (Lindsey et al., 2003). A number of approaches have been utilized to improve these estimates, including isotopic ratio changes in nitrate or water, fortuitous detection of soluble anthropogenic compounds, or gaseous analysis (Lindsey et al., 2003). No selective dating markers exist for groundwater processes in an agricultural setting.
Metolachlor (2-chloro-N-(2-ethyl-6-methylphenyl)-N-(1-methoxypropan-2-yl)acetamide) is a major-use, pre-emergent herbicide for maize and soybean production and has been registered in the US since 1976 (Fernandez-Cornejo et al., 2014). After application, metolachlor is readily transported into the soil where it is transformed into several compounds, the most abundant being, metolachlor ethane sulfonic acid (2-[(2-ethyl-6-methylphenyl)(2-methoxy-1-methylethyl)amino]-2-oxoethanesulfonic acid, MESA) (Baran and Gourcy, 2013, Bayless et al., 2008, White et al., 2010). MESA can be found in groundwater wherever metolachlor has been used and it appears to be quite stable in groundwater (Baran et al., 2004, Barbash et al., 1999, Bayless et al., 2008, Denver et al., 2010, Domagalski et al., 2008, Huntscha et al., 2008, Kalkhoff et al., 2012, Krutz et al., 2006, Rebich et al., 2004, Hancock et al., 2008).
MESA is also more mobile than metolachlor due to its much greater water solubility (MESA: 2.12 × 105 mg L− 1; metolachlor: 530 mg L− 1) (Bayless et al., 2008) and smaller organic carbon partition coefficient (Koc) values (MESA Koc: 13.8 ± 2.4 L kg− 1; metolachlor Koc: 172 ± 25 L kg− 1, Baran and Gourcy, 2013). All these properties taken together suggest that MESA under the right conditions can be a good indicator of nitrate loadings from agriculture (McCarty et al., 2014). For example, concentrations of MESA were relatively high in the unconfined groundwater of Morgan Creek in Maryland (United States), and MESA loads were well correlated with nitrate-N loads (Domagalski et al., 2008). Most recently, MESA was used to examine the fate of nitrate-N in the Choptank River (Maryland) both upstream and in the downstream estuary (McCarty et al., 2014).
Another interesting feature of metolachlor and MESA is that they both exist as a set of four stereoisomers, Fig. 1. As with many biologically-active compounds, one enantiomer is often more active than the other, and this is true for metolachlor, where the S-form is more active than the R-form (Blaser et al., 2007). In the US, metolachlor applications were switched to the more active S-metolachlor form after 1988. Prior to this time, metolachlor was applied as a racemic mixture (rac), equal amounts of S- and R-enantiomers. Several studies based on analysis of chiral forms of metolachlor have made use of this unique time marker produced by the abrupt switch to the more-efficacious S-metolachlor. The chiral ratios of metolachlor were calculated for samples obtained from Swiss lakes in 1998 and 1999, where use of S-metolachlor began in 1997 (Buser et al., 2000). A rapid environmental response to the changed structural-composition of the herbicide was observed in surface waters that received fresh input from runoff enriched in the S-enantiomers, whereas, the deeper water in the lakes retained ratios closer to the racemic composition. A similar approach was used to describe several processes involved in distributing metolachlor in southern Canada (Kurt-Karakus et al., 2010b). There is only one report (Klein et al., 2006) for chiral analyses of MESA in groundwater, and there are no reports for chiral analysis of metolachlor in groundwater.
Although some research has examined the change in chirality of metolachlor, little research has been carried out exploiting the chiral forms of MESA which has a much longer persistence and is observed at larger concentrations in the environment than metolachlor (Baran and Gourcy, 2013, Graham et al., 1999, Huntscha et al., 2008, Kalkhoff et al., 2012). The transformation of metolachlor to MESA is a biotic process involving substitution of the chlorine with a sulfonic moiety (Field and Thurman, 1996), and this occurs without any rearrangements in the chiral carbon centers (Klein et al., 2006). These authors examined enantiomer ratios for metolachlor while also monitoring MESA to establish this non-enantioselectivity; however, others have just examined the disappearance of enantiomers of metolachlor, and there are contradicting reports for chiral selectivity. Ma et al. (2006) showed preferential degradation of the S-enantiomers and Maillard et al. (2016) showed preferential degradation of the R-enantiomer while Aboul Eish and Wells (2008) found no enantioselectivity. Since there is controversy on this point and no conclusive evidence to contradict us we assumed for this paper that enantiomeric changes in metolachlor commercial products should be reflected in similar chiral shifts in MESA, i.e., the applied product of 88% S-metolachlor will produce S-MESA that has the same enantiomeric composition.
In this paper, we use the chiral ratios of MESA and metolachlor to describe the groundwater flushing times in a riparian buffer adjacent to cropland that was under continuous culture of Zea maize and was treated annually with metolachlor. Using the enantiomeric ratio, the groundwater mean residence time was determined for this riparian area. This approach should be useful in dating the groundwater in other areas and in assessing the effectiveness of conservation measures such as installing riparian buffers or wetlands.
Section snippets
Materials
Standard reference material for racemic (rac) MESA (CGA-354743) and R- and S-MESA (SYN-502271 and CGA-380168, respectively) was obtained from Syngenta (Greensboro, NC). Racemic metolachlor was obtained from Chem Services (West Chester, PA) 98.8% pure. Internal standards for quantitation were [13C] 2, 4-dichlorophenoxyacetic acid, obtained from Cambridge Isotope Laboratories Inc., (Cambridge, Massachusetts) and terbuthylazine, obtained from Dr. Ehrenstorfer, GMBH (Augsburg, Germany). The
Description of sample results
In general, total MESA concentration values were much larger, by up to two orders of magnitude, than total metolachlor concentration values, except for some stream samples where metolachlor concentrations were sometimes larger just after metolachlor was applied. In the stream, the mean ± SD for total MESA concentration was 8.9 ± 5.0 μg L− 1 (range: 0.010 μg L− 1 to 16 μg L− 1, n = 30), whereas the mean total metolachlor concentration was 3.4 ± 12 μg L− 1 (range: less than the LOD, 0.001 μg L− 1, to 69 μg L− 1, n = 29).
Disclaimer
Mention of specific products is for identification and does not imply endorsement by the US Department of Agriculture to the exclusion of other suitable products or suppliers.
Abbreviations used
- Koc
organic carbon partition coefficient
- LOD
limit of detection
- MESA
metolachlor ethane sulfonic acid
- rac
racemic
- SF
S-enrichment factor
- TMDL
total maximum daily load
Acknowledgement
This work was supported financially by US Department of Agriculture, Agricultural Research Service intramural projects in National Programs 211, Water Availability and Water Management, and 212, Soil and Air.
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