Atrazine adsorption and colloid-facilitated transport through the unsaturated zone

One explanation for unexpectedly widespread ground water contamination from atrazine (2-chloro-4-ethylamino-6-isopropylamino-s-triazine) may be the oc currence of colloid-facilitated transport, whereby the dissolved herbicide becomes adsorbed to mobile colloids that migrate through preferential now-p aths in the soil zone and into the ground water. The objectives of this stu dy were to determine the extent of adsorption of atrazine to bulk soil and to soil colloids and to determine the extent of colloid-facilitated transpo rt of atrazine at a field site in Virginia during simulated rainfall events . Equilibrium batch adsorption experiments were performed over a concentrat ion range of 0.05 to 10.0 mg atrazine L-1 on bulk soil samples and on collo idal suspensions of 75 mg L-1, a concentration comparable with those observ ed at the field site. Linear partition coefficients ranged from 0.496 to 2. 48 L kg(-1) for the bulk soil and from 70.8 to 832 L kg(-1) for the soil co lloids. In the field, gravity lysimeters were installed at a depth of 25 cm below the surface of six 0.25-m(2) undisturbed plots. Mass recovery of sur face-applied atrazine in the lysimeters was not significantly affected by r ainfall rate and was, on average, 2.7% for plots receiving 25 mm h(-1) simu lated rainfall and 3.6% for plots receiving 50 mm h(-1) simulated rainfall. Of the total atrazine collected in the Lysimeters, the fraction that was c olloid-associated ranged from 4.9 to 30% (mean of 15%), indicating that a m easurable portion of mobile atrazine is transported via association with co lloids.