MINERAL CONTRIBUTIONS TO ATRAZINE AND ALACHLOR SORPTION IN SOIL MIXTURES OF VARIABLE ORGANIC-CARBON AND CLAY CONTENT

A sediment mixing approach was taken to systematically vary the organic carbon (oc) and clay content (cm) of a suite of organic-poor, clay-rich sediments. Organic carbon content ranged from 3.2% to 0.4% and clay content ranged from 24% to 51%. Atrazine and alachlor were shown to sorb to both natural organic carbon and clay minerals. Partition coefficients to natural organic carbon (K(oc)) were found to be 217 and 412 L/kg organic carbon for atrazine and alachlor, respectively. Partition coefficients to the clay fraction were found to be 3.5 and 4.9 L/kg clay for atrazine and alachlor, respectively. When expressed in terms of surface area, the partition coefficients to clay for atrazine and alachlor were 1.80.10(-5) and 2.51.10(-5) L/m2 clay, respectively. Critical cm/oc ratios at which mineral phase sorption accounts for 50% of the total are defined. Implications for the modelling of herbicide movement in the subsurface if mineral phase sorption is ignored is discussed.