Atrazine, alachlor, and metolachlor mobility through two sandy Wisconsin soils

Pesticide mobility through Sparta sand from the Lower Wisconsin River Valley (LWRV) and Plainfield sand from the Central Sands (CS) of Wisconsin was examined to interpret results from state-sponsored groundwater monitoring programs and to identify fundamental mechanisms important in pesticide transport through the unsaturated zone. The mobility of ¹⁴C-labeled atrazine [2-chloro-(4-ethylamino)-6-(isopropylamino)-s-triazine], alachlor [2-chloro-N-(2,6-diethylphenyl)-N-(methoxymethyl) acetamide], and metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl) acetamide] was monitored through replicated intact soil column-microlysimeters. Soil columns of Sparta sand (sandy, mixed, mesic Entic Hapludolls) from the LWRV and Plainfield sand (sandy, mixed, mesic Typic Udipsamments) from the CS were instrumented in the greenhouse to simulate field conditions. All pesticides were shown to be much more mobile through the Sparta sand as compared to Plainfield sand. Peak atrazine concentration was 15 times greater in the leachate from the Sparta soil than from the Plainfield sand. Percentage loss of applied pesticide was four times greater in the leachate from the Sparta soil than from the Plainfield sand. Similar results were found for metolachlor and alachlor. The relative order of mobility for each soil was found to be atrazine > metolachlor > alachlor. Factors found to contribute to increased mobility of pesticides through the Sparta sand include higher hydraulic conductivity, smaller water-holding capacity, and less atrazine adsorption due to less organic C and clay content. The hydraulic conductivity (K(sat)) of the upper rootzone (0-20 cm) of the Sparta soil is five times faster than the Plainfield soil. The water-holding capacity and the atrazine adsorption coefficient (K(d)) of the Plainfield sand were measured to be 1.6 and 1.8 times greater, respectively, than those of the Sparta soil.