Comparison of sorption and solute transport behaviour of several herbicides in an alkaline agricultural soil

2,4-Dichlorophenoxyl acetic acid (2,4-D), atrazine, glyphosate, and metribuzine, have been used extensively as herbicides, which can lead to contamination of soil and groundwater. The objective of this study was to quantify and compare the sorption and solute transport of these commonly used herbicides in alkaline agricultural soil since there is little transport data for all these contaminants under comparable conditions and for the same soil, and there is also limited information for sorption of herbicides to alkaline soils. The amount of sorption as average distribution coefficient (K-d) obtained from water saturated-soil-column tracer experiments varied between these herbicides, and included 0.01, 0.06, 0.44, and 2.14 L/kg for 2,4-D, metribuzine, atrazine, and glyphosate, respectively. These miscible-displacement, tracer experiments indicated linear and rate-limited sorption for all herbicides studied, and they also showed that the sorption amount, or K-d, for 2,4-D and metribuzine was low and comparable, whereas increased sorption K-d was observed for atrazine and glyphosate. Hydrophobic attraction to the soil organic fraction was the dominant mechanism for the non-ideal sorption, because K-d increased with K-oc and herbicide pK(a) values were below the soil and water pH. Two-site sorption and transport modelling, including linear sorption and rate-limited sorption, matched all of the observed herbicide breakthrough curves. The sorption of 2,4-D and metribuzine onto alkaline agricultural soil was nearly negligible, which confirms increased contamination risk of groundwater resources. Whereas, atrazine and glyphosate exhibited significant sorption to the alkaline soil with decreased leaching risk compared to 2,4-D and metribuzine.