Formation and distribution of 14C-atrazine degradation products in the top 120 cm of soil were determined over 16 mo under field conditions in an Estherville sandy loam. After 16 mo, 78% of applied 14C was still present in the soil. By 2 mo after treatment (MAT), 14C had moved to the 30- to 40-cm depth; however, movement to depths greater than 40 cm was not observed. Greater than 98% of the 14C remaining in the soil profile after 16 mo was in the top 20 cm. Twenty-seven percent of the 14C applied was atrazine 16 MAT. Atrazine was the predominant 14C-compound in soil below 10 cm. Hydroxyatrazine (HA) was the major degradation product in the top 10 cm of soil. The proportion of 14C as HA in the top 10 cm increased from 15% 2 MAT to 37% 16 MAT. Deethylatrazine (DEA) was the predominant degradation product at the 10- to 30-cm depth and accounted for up to 23% of the 14C present in the 10- to 20-cm depth. Deisopropylatrazine (DIA) accounted for less than 6% of the radioactivity recovered at any soil depth. The proportion of DEA and DIA increased while the proportion of HA decreased as soil depth increased, indicating that DEA and DIA are more mobile in soil than HA. Detection of HA at depths greater than 10 cm appears to be due to in situ degradation of atrazine previously moved to that soil depth. The large amount of 14C remaining in the soil 16 MAT suggests that a large pool of atrazine and its degradation products are present in the soil for an extended period following application and have the potential to contaminate ground water.

Formation of 14C-atrazine degradation products and their distribution in the top 90 cm of soil was determined over 16 mo in a Webster clay loam in the field. After 16 mo, 64% of the applied 14C could still be accounted for in the 90-cm soil profile. At 1 mo after treatment (MAT), 14C moved to the 70- to 80-cm depth. Rapid movement of radioactivity could be attributed in part to preferential movement through vertical macropores. Atrazine accounted for 32% of the 14C applied 16 MAT and was the predominant 14C-compound in soil below 10 cm through 12 MAT. Hydroxyatrazine (HA) was the major degradation product in the top 10 cm of soil accounting for 9% of the 14C present 1 MAT and increasing to 24% within 6 MAT. Deethylatrazine (BEA) was the predominant degradation product at depths greater than 10 cm, accounting for 26% of the 14C in the 10- to 20-cm depth 16 MAT. Deisopropylatrazine (DIA) accounted for less than 10% of the 14C recovered at any soil depth. Deethyldeisopropylatrazine (DEDIA) and an unidentified product were detected in soil extracts 1 MAT indicating further degradation past primary metabolites. The proportion of DEA and DIA increased while the proportion of HA decreased as soil depth increased indicating that DEA and DIA are more mobile in soil than HA. The large amount of radioactivity remaining in the soil 16 MAT suggests that a large pool of atrazine and its degradation products are present in the soil for a long period of time, having the potential to move deeper in the soil and ultimately contaminate ground water.

Laboratory studies were conducted to determine the influence of degradation and sorption processes on the dissipation of alachlor in one Colorado soil (Kim clay loam) and three Minnesota soils (Port Byron silt loam, Webster silty clay loam, and Estherville sandy loam) as a function of soil depth. Persistence and movement of alachlor in an irrigated corn production system also were determined on the Kim soil. Laboratory degradation data fit first-order kinetics, and rate constants ranged from 0.0094 to 0.0251 d-1 and varied with soil type and depth. For instance, in 60- to 75-cm-depth Kim soil, alachlor degraded at a slower rate (k = 0.011 d-1) than in surface soil samples (k = 0.022 d-1). Alachlor sorption to the four soils was moderate (Kf = 0.7 to 7.4; Kf,oc = 71 to 470) and concentration dependent (1/n < 1.0). Significant hysteretic desorption of alachlor from soils also was observed (1/n desorption < 1/n sorption). The combined effect of degradation and sorption processes has been used to classify a chemical's potential to leach to groundwater. Based on Kf,oc and dissipation half-life, alachlor would be classified as a “leacher” in Kim, Port Byron, and Estherville soils and classified as transitional between “leacher” and “nonleacher” in the Webster soil. The dissipation first-order rate constant (k) of alachlor in Kim soil in the field was 0.036 α 0.012 d-1. Dissipation was apparently not due to leaching since bromide applied at the same time remained in the top 15 cm during the first 28 d. It appears that laboratory-derived leaching indices may overestimate actual leaching and should be used with caution for predictive or regulatory purposes.