Environmental fate of monosodium methanearsonate (MSMA)—Part 2: Modeling sequestration and transformation
Authors: Williams W. Martin , Cheplick J. Mark , Cohen Stuart Z. , Eldan Michal , Hoogeweg Cornelis G. , Masue-Slowey Yoko , Vamshi Raghu 
Published: June 2024

Abstract
Monosodium methanearsonate (MSMA), a sodium salt of monomethylarsonic acid (MMA), is a selective contact herbicide used for the control of a broad spectrum of weeds. In water, MSMA dissociates to ions of sodium (Na+) and monomethylarsonate (MMA−) that is stable and does not transform abiotically. In soils characteristic of MSMA use, several simultaneous processes can occur: (1) microbial methylation of MMA to dimethylarsinic acid (DMA), (2) microbial demethylation of MMA to inorganic arsenic (iAs), (3) methylation of iAs to MMA, and (4) sorption and sequestration of MMA and its metabolites to soil minerals. Sequestered residues are residues that cannot be desorbed from soil in environmental conditions. Sequestration is rapid in the initial several days after MSMA application and continues at a progressively slower rate over time. Once sequestered, MMA and its metabolites are inaccessible to soil microorganisms and cannot be transformed. The rate and extent of the sorption and sequestration as well as the mobility of MMA and its metabolites depend on the local edaphic conditions. In typical MSMA use areas, the variability of the edaphic conditions is constrained. The goal of this research was to estimate the amount of iAs potentially added to drinking water as a result of the use of MSMA, with models and scenarios developed by the US Environmental Protection Agency for pesticide risk assessment. In this project, the estimated drinking water concentrations (EDWCs) for iAs were assessed as the average concentration in the reservoir over a 30-year simulation with annual applications of MSMA at maximum label rates. When the total area of suitable land was assumed to be treated, EDWCs ranged from &lt;0.001 to 0.12 µg/L. When high estimates of actually treated acreage are considered, the EDWCs are below 0.06 µg/L across all scenarios. Integr Environ Assess Manag 2024;20:2076–2087. © 2024 The Author(s). Integrated Environmental Assessment and Management published by Wiley Periodicals LLC on behalf of Society of Environmental Toxicology & Chemistry (SETAC).
Key Points
Inorganic arsenic (iAs) potentially added to drinking water from the use of monosodium methanearsonate (MSMA) was estimated with models (Pesticide Root Zone Model, version 5 [PRZM5], Variable Volume Water Model [VVWM], and aerial spray prediction model [AgDRIFT]) and based on scenarios developed by the USEPA for pesticide risk assessment. Sequestered residues (residues that could not be extracted by water or weak acids) were considered as losses to the modeled systems in PRZM5 and VVWM. Rate constants used for modeling sequestration and transformation were derived from empirical data representative of MSMA use environments. The modeled estimated drinking water concentration for iAs ranged from &lt;0.001 to 0.12 µg/L when 100% of the suitable area of a watershed is treated with MSMA and below 0.06 µg/L across all scenarios when the actual area treated with MSMA is considered.