Simulated Fate and Transport of Metolachlor in the Unsaturated Zone, Maryland, USA

Abstract

An unsaturated-zone transport model was used to examine the transport and fate of metolachlor applied to an agricultural site in Maryland, USA. The study site was instrumented to collect data on soil-water content, soil-water potential, ground water levels, major ions, pesticides, and nutrients from the unsaturated zone during 2002-2004. The data set was enhanced with site-specific information describing weather, soils, and agricultural practices. The Root Zone Water Quality Model was used to simulate physical, chemical, and biological processes occurring in the unsaturated zone. Model calibration to bromide tracer concentrations indicated flow occurred through the soil matrix. Simulated recharge rates were within the measured range of values. The pesticide transport model was calibrated to the intensive data collection period (2002-2004), and the calibrated model was then used to simulate the period 1984 through 2004 to examine the impact of sustained agricultural management practices on the concentrations of metolachlor and its degradates at the study site. Simulation results indicated that metolachlor degrades rapidly in the root zone but that the degradates are transported to depth in measurable quantities. Simulations indicated that degradate transport is strongly related to the duration of sustained use of metolachlor and the extent of biodegradation.

Predicting Unsaturated Zone Nitrogen Mass Balances in Agricultural Settings of the United States

“…Initial versions of the models at C22 and M21 were predicated on manually calibrated models from previous studies (Bayless et al, 2008; Domagalski et al, 2008) and were altered as necessary to confine estimates of N mass balances to the unsaturated zone. We specified a unit gradient boundary condition for the lower boundary at all four sites to allow free drainage and keep the water table below the simulated profile.…”

Metolachlor metabolite (MESA) reveals agricultural nitrate-N fate and transport in Choptank River watershed

“…Like nitrate-N, MESA is very soluble (2.12*10 5 mg/L) (Bayless et al, 2008), has a low sorption coefficient (calculated log K oc = 1.13) (Bayless et al, 2008), and has been classified as highly mobile Domagalski et al, 2008;Huntscha et al, 2008). In contrast to nitrate-N, once MESA enters ground water, it is very stable (Phillips Table 1 Conceptual model of landscape characteristics that influence transport, processing, and delivery of nitrate-N and MESA to headwater streams of the Choptank River (Ator et al, 2005;Bachman and Phillips, 1996;Denver et al, 2010, in press;Phillips and Bachman, 1996;Phillips et al, 1993 Steele et al, 2008); MESA persists over decadal time scales (Denver et al, 2010).…”

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

“…MESA formation has been shown to occur in the upper aerobic-layer of the soil column (Baran and Gourcy, 2013;Bayless et al, 2008;Denver et al, 2004;Denver et al, 2010;Domagalski et al, 2008;Huntscha et al, 2008). In a multi-year field and modeling study, Bayless et al (2008) found that metolachlor was very transient with a simulated half-life of 10 days and that MESA was rapidly formed in the first meter of the soil and was leached to the water table within 1-2 years. This rapid transformation of metolachlor to MESA led to high concentrations of MESA in the water table beneath the field.…”

“…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).…”