Pesticide sorption and leaching potential on three Hawaiian soils

Authors: HALL, KATHLEEN - University Of Minnesota; RAY, CHITTARANJAN - University Of Nebraska; KI, SEO JIN - University Of Nebraska; Spokas, Kurt; KOSKINEN, WILLIAM - Retired ARS Employee

Submitted to: Journal of Environmental Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/28/2015
Publication Date: 5/27/2015
Publication URL: http://handle.nal.usda.gov/10113/61031
Citation: Hall, K.E., Ray, C., Ki, S., Spokas, K.A., Koskinen, W.C. 2015. Pesticide sorption and leaching potential on three Hawaiian soils. Journal of Environmental Management. 159:227-234.

Interpretive Summary: The environmental risk of applied pesticides to soils is often assessed through mathematical models in order to predict the potential leaching of these chemicals to groundwater aquifers. Often, these methods depend on composite indices, such as the GUS leaching index, to make these decisions. In order to evaluate the potential leaching of 6 commonly applied herbicides, we determined the sorption coefficients on three dominant soils from the island of Oahu. Metsulfuron-methyl, aminocylcopyrachlor, picloram, and DKN are relatively low sorbing herbicides (Koc = 3 to 53 mL g-1), alachlor is intermediate (Koc = 120 to 150 mL g-1), while oxyfluorfen sorbs very strongly to the three soils (Koc > 12,000 mL g-1). Following determination of the Koc, the GUS indices for these compounds were calculated. These GUS indices were used to model the behavior of these pesticides on the Islands. From these results, only oxyfluorfen was determined to pose no leaching risk to groundwater. The other 5 herbicides would typically be classified as likely leaching risks, with the exception of alachlor whch was difficult to fully assess due to the intermediate sorption value. Therefore, this data suggests that the GUS index is a powerful tool that would allow a very rapid assessment of the environmental risks of various herbicides once the sorption coefficients are known for a particular soil. These results are significant to farmers and policy makers and will assist scientists and engineers in developing improved models for assessing agrochemical leaching risks based on mechanistic processes, which should be utilized in developing improved assessments of the fate and transport of applied agrochemicals.

Technical Abstract: On the Hawaiian Islands, groundwater is the principal source of potable water and contamination of this key resource by pesticides is of great concern. To evaluate the leaching potential of four weak acid herbicides [aminocyclopyrachlor, picloram, metsulfuron-methyl, biologically active diketonitrile degradate of isoxaflutole (DKN)] and two neutral non-ionizable herbicides [oxyfluorfen, alachlor], their sorption coefficients were determined on three prevalent soils from the island of Oahu. Metsulfuronmethyl, aminocylcopyrachlor, picloram, and DKN were relatively low sorbing herbicides (Koc 1/4 3 e53 mL g_1), alachlor was intermediate (Koc 1/4 120e150 mL g_1), and oxyfluorfen sorbed very strongly to the three soils (Koc > 12,000 mL g_1). Following determination of Koc values, the groundwater ubiquity score (GUS) indices for these compounds were calculated to predicted their behavior with the Comprehensive Leaching Risk Assessment System (CLEARS; Tier-1 methodology for Hawaii). Metsulfuron-methyl, aminocyclopyrachlor, picloram, and DKN would be categorized as likely leachers in all three Hawaiian soils, indicating a high risk of groundwater contamination across the island of Oahu. In contrast, oxyfluorfen, regardless of the degradation rate, would possess a low and acceptable leaching risk due to its high sorption on all three soils. The leaching potential of alachlor was more difficult to classify, with a GUS value between 1.8 and 2.8. In addition, four different biochar amendments to these soils did not significantly alter their sorption capacities for aminocyclopyrachlor, indicating a relatively low impact of black carbon additions from geologic volcanic inputs of black carbon. Due to the fact that pesticide environmental risks are chiefly dependent on local soil characteristics, this work has demonstrated that once soil specific sorption parameters are known one can assess the potential pesticide leaching risks.