Impact of Soil Organic Carbon on Arsenic Transformation Following Msma Application to Two Soils Using Modeling Approach.

The continued use of highly effective arsenical pesticides such as monosodium methyl arsenate (MSMA) in the face of regulatory phaseout requires better knowledge of the fate of the arsenic following reaction with soil. The objective of this research was to determine the effect of SOC on temporal trends in total dissolved As and As speciation in a Candor sand and a Cecil sandy clay loam following MSMA application. A laboratory incubation study was conducted, using a crossed three-factorial treatment design with two soil types, four levels of soil organic carbon added as peat, 10 incubation time points, and three replicates. According to our research, total dissolved As decreased both in Candor and Cecil, with greater As removal from solution in the Cecil system (100-fold removal) than in the Candor system (10-fold removal). In addition, total dissolved As decreased by an order of magnitude within 1 day in the Cecil system but within 28 day for Candor system. The SOC content significantly affected the dissolved As in both soils; with an increasing trend of SOC, total dissolved As increased. Another important finding was in a Candor soil with high SOC (5%), As(V) was dominant (94%) species at the end of the incubation. Numerous models were built for each response measured. Additionally, a Risk Index model was created in order to assess the risk of a system by combining the total As amount and As toxicity (LD₅₀) of various species.  These models enabled us to make predictions on how SOC content (from 0% to 5%) and time (0 to 28 d) may impact responses (total dissolved As, As speciation, and Risk Index) on a similar soil type. This research provides data and information in order for the EPA and other regulatory agencies to make appropriate decisions on MSMA reregistration and future use.