The Role of Colloidal Particles in the Transport of Chemicals Through an Agricultural WatershedEPA Grant Number: R824772
Title: The Role of Colloidal Particles in the Transport of Chemicals Through an Agricultural Watershed
Investigators: Hornberger, George M.
Current Investigators: Hornberger, George M. , Herman, Janet S. , Saiers, James E.
Institution: University of Virginia
Current Institution: University of Virginia , Yale University
EPA Project Officer: Hiscock, Michael
Project Period: November 1, 1995 through October 1, 1998
Project Amount: $500,000
RFA: Water and Watersheds (1995) Recipients Lists
Research Category: Water and Watersheds , Water
Description:We seek to expand our understanding of herbicide movement throughout the entire soil-bedrock-stream system in an agricultural watershed in order to support intelligent management decisions about land use and its impact on sustainable water supply. We will evaluate the important controls on the fate of a widely used herbicide atrazine from initial occurrence in the unsaturated soil zone in an agricultural area to occurrence in the groundwater of a bedrock water-supply aquifer to appearance in a stream draining the watershed. We will develop a conceptual model of atrazine and hydroxyatrazine transport through a watershed in the Shenandoah Valley of Virginia, the site of a U. S. Geological Survey National Water Quality Assessment study, by considering geochemical, hydrological, and microbiological processes and their rates in different settings in the subsurface environment. We are investigating processes such as transport through preferential flow paths in heterogeneous soils and bedrock; sorption, potentially with hysteresis, to mineral surfaces and particulate organic matter in the soil and bedrock immobile matrix; sorption to inorganic and organic mobile colloids that facilitate transport; and biodegradation of herbicide solutes and bound residues in the unsaturated soil and in the water-saturated deep soil and bedrock. The relative importance of individual processes influencing the fate of atrazine will emerge through comparisons of their time constants. We anticipate that the time scales of the transport, sorption, and degradation reactions change with changing conditions of the hydrological, geochemical, and ecological milieu.
Pan lysimeters to collect soil water were installed at four locations in the catchment. Colloid concentrations, bulk chemical composition, and atrazine concentrations are being measured for monthly samples. Adsorption isotherms have been developed for soils from the site and for ideal mineral colloids such as kaolinite. Intact cores from the field site have been collected. Unsaturated flow and transport experiments on the cores are underway in an apparatus constructed for this project. A sprinkler to conduct field plot experiments has been constructed and tested.
Preliminary results suggest that flow along preferred pathways in the shallow soil is of critical importance in transporting agricultural chemicals vertically downward to the water table. Sorption to the soil matrix is the main retarding mechanism in the transport of atrazine and hydroxyatrazine. Sorption of these herbicides to mineral colloids is much stronger than to bulk soils, but preliminary work indicates that effects of facilitated transport of even the strongly sorbing hydroxyatrazine due to sorption to mineral colloids may be modest at this site. Sorption to organic colloids may be much more important. Continuing work will focus on completion of experiments on intact and repacked cores, sprinkling experiments at the plot scale in the field, and further measurements and their interpretation at the catchment scale.