Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: August 16, 2001
Publication Date: N/A
Interpretive Summary: Predicting the risk of herbicide movement from the soil surface to groundwater is accomplished using a variety of simulation models. These models require information about rainfall, water flow, soils, and the herbicides in order to make accurate predictions. While past research has provided much of this information, our knowledge is limited to the soils present at the land surface or to those just below the surface. This research measured the binding (sorption) of the herbicide atrazine to soils, subsoils, and several different geologic sediments below the soil profile in Iowa. The sediments below the soil profile were low in organic carbon, but retained more herbicide than would have been predicted based on past research. In addition, we found that certain till materials were able to retain atrazine quite strongly, which would greatly limit the leaching of this herbicide through these materials. This knowledge should increase our capability in predicting herbicide contamination of groundwater. Understanding how herbicides move through the soil and geologic material will aid in development of practices that protect water resources from contamination. This will benefit both producers in managing herbicides more carefully and the general public through less contaminated waters.
Prediction of herbicide movement in the subsurface environment requires accurate estimates of herbicide sorption in soils and geologic materials, but this information is often lacking for materials below the soil surface. We measured the adsorption and desorption of atrazine in surface soils, subsoils, and sediments from different geologic settings, including loess, till, and alluvial sands. Adsorption of atrazine was principally controlled by organic C, but the relationship of sorption to organic was different in high C surface soils and low C soils and sediments. The Koc of subsurface materials with less than 2% organic C was 474 +/- 76 compared to a Koc of 151 +/- 32 for the seven surface soils. Samples of unoxidized tills contained elevated levels of organic C and adsorbed atrazine at levels equivalent to surface soils. All subsurface sediments retained atrazine during desorption to a greater extent than that predicted from adsorption isotherms. Although sorption of atrazine in subsurface sediments is generally low, there is sufficient retention to warrant consideration of sorption processes in the assessment of atrazine transport in the subsurface environment.