Influence of soil colloids on the migration of atrazine and zinc through large soil monoliths

The role of soil colloids and their potential to co-transport agrochemicals in subsurface soil environments was evaluated in a leaching experiment utilizing large soil monoliths. The monoliths were created by hydraulically driving steel pipe sections (50 cm diameter x 50 cm length) into Maury silt loam (fine, mixed, mesic Typic Paleudalf) and Loradale silt loam (fine, silty, mixed mesic Typic Argiudoll) soils. Water dispersible colloids fractionated from the Bt horizons of the above soils were spiked with 3 mg L-1 atrazine (2-chloro-4-ethylamino-6-isopropylamine-s-triazine) and 10 mg L-1 zinc (Zn), and after a twenty-four hour equilibration were applied into the monoliths at eight hour intervals using 500 mL pulse applications. Solutions containing atrazine and Zn without added colloids were applied to separate monoliths from each soil to represent control treatments. Colloid, atrazine, and Zn recoveries in the eluent varied greatly with respect to soil type. Colloid recovery in the Loradale monoliths averaged 65.1 +/- 26.5%, with maxima approaching the input level, while in the Maury monoliths the average recovery was low (5.7 +/- 6.2%) and never exceeded 25% of the input level. Atrazine eluted from the two monoliths averaged 40.3 +/- 12.5% (Loradale) and 29.0 +/- 20.0% (Maury), with considerable enhancement in the presence of colloids, especially in the Loradale soil. In contrast, the elution of Zn averaged 3.0 +/- 3.2%, in the Loradale monoliths and rarely exceeded control concentrations in the Maury monoliths, suggesting a stronger retardation of Zn over atrazine within the soil matrix, especially when colloid transport was deterred. Settling-rate experiments at varying pH and electrical conductivity (EC) values suggested that the transport of Maury colloids may have been hindered due to flocculation within the monoliths, while the Loradale colloids remained stable throughout the leaching experiment. Although the presence of colloids enhanced atrazine elution in all monoliths, the actual amount of atrazine transported bound to either colloid type was minimal, suggesting mainly physical exclusion transport processes. In contrast, stronger chemisorption of Zn to colloid surfaces than the soil matrix appeared to enhance the transport of Zn by both colloids.