Grantee Research Project Results
1998 Progress Report: The Role of Colloidal Particles in the Transport of Chemicals Through an Agricultural Watershed
EPA Grant Number: R824772Title: The Role of Colloidal Particles in the Transport of Chemicals Through an Agricultural Watershed
Investigators: Hornberger, George M. , Saiers, James E. , Herman, Janet S.
Institution: University of Virginia , Florida International University
Current Institution: University of Virginia , Yale University
EPA Project Officer: Packard, Benjamin H
Project Period: November 1, 1995 through October 1, 1998
Project Period Covered by this Report: November 1, 1997 through October 1, 1998
Project Amount: $500,000
RFA: Water and Watersheds (1995) RFA Text | Recipients Lists
Research Category: Watersheds , Water
Objective:
This project is taking an integrated approach in evaluating 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 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. Our interest in watershed-scale transport requires consideration of a number of processes occurring at different scales of time and space. We are working to develop a conceptual model of atrazine and hydroxyatrazine transport through the Muddy Creek watershed in the Shenandoah Valley of Virginia by taking the approach of considering geochemical, hydrological, and microbiological processes and their rates in different settings in the subsurface environment.The research is organized to address three questions. (1) What factors control the variability in the concentrations of colloids and agricultural chemicals in the aquifer-stream system at Muddy Creek? (2) How do nonidealities in the aquifer material and colloidal particles impact the movement of agricultural chemicals such as atrazine and hydroxyatrazine? (3) Do laboratory-based observations of colloid-facilitated transport of agricultural chemicals provide a reliable estimate of transport at the field scale?
Progress Summary:
Field data on transport of herbicides and colloids were analyzed to infer mechanisms of transport by looking at correlations of atrazine with other measured constituents. Atrazine concentrations correlated with organic carbon concentrations. This result may corroborate laboratory evidence of atrazine (and other contaminants) transport facilitated by the presence of humic acids. Organic contaminants may become bound to the organic structures of dissolved or particulate organic carbon which may result in enhanced transport. There has been little evidence of this phenomena in situ; these results provide evidence that this may be a real contaminant transport phenomenon in natural systems. Binding of organic pollutants to natural organic matter will have direct implications in determining adsorption and biodegradation characteristics.Experiments were conducted on intact cores under unsaturated conditions to examine kinetics of sorption and desorption. Column experiments with repacked material were run to investigate the transport behavior of colloids, herbicides, and colloid-associated herbicides. We developed and tested several different types of continuous distribution models to describe the sorption of hydroxyatrazine (HA) during transport. We find that a gamma-distributed model is capable of reproducing experimental data on HA transport for varying conditions of pore water velocity, pore water pH, and influent HA concentration. This work indicates that models based on a continuum of adsorption and desorption rates may be necessary to describe the movement of herbicides through natural geologic materials.
The environmental fate of soil-bound residues, operationally defined as chemicals that remain sorbed to soil after exhaustive solvent extraction, was studied in a long-term laboratory study. Ring-labeled atrazine was applied to soil at a concentration of 5.6 umg/g and aged for three months. At the end of the aging period, the soil samples were extracted by one of three methods--methanol-water (4:1) at 75??C, acetone, and artificial soil water--representing a range of extraction efficiencies. After aging, atrazine extractability with the hot methanol-water method decreased from 96% efficiency for soil aged less than one hour to 66% for soils aged for three months. All of the extraction treatments left behind some fraction of atrazine which remained bioavailable on both short and long time scales.
The extent of colloid generation and colloid-facilitated pesticide transport in the unsaturated zone as affected by precipitation rate, antecedent moisture conditions, and pesticide sorptive strength. was examined in a set of sprinkling experiments in the field. Further work on field plots was conducted using ponding experiments. Transport pathways through the soil were found to be dominated by macropores, and variations in active macroporosity contributed to a large degree of physical heterogeneity in the soil. The antecedent moisture level of the soil controlled the water flux during the rainfall events, whereas the rate of precipitation had little effect. The total mass of colloids collected over the course of the experiments was significantly affected by both precipitation rate and antecedent moisture levels. Colloid mobilization increased with antecedent moisture levels, likely from the increase in macropore flow under more fully saturated conditions, and increased with precipitation rate from increased shear stress on the soil. Low recoveries of colloid-associated atrazine suggested that at this field site, under these conditions, atrazine transport was not facilitated by mobilized colloids.
Stream gages were installed at two locations along the upper portion of Muddy Creek. Stage-activated automatic samplers were used to collect samples at regular intervals across storm hydrographs. The samples were analyzed for atrazine, suspended sediment, and major anions and cations. Shallow wells were installed near the stream-gage sites and were sampled regularly. Overland-flow collectors were placed in several locations in fields above the stream gage sites. Preliminary analyses of the data show high concentrations of atrazine in overland flow from fields (an expected result) and also transient high concentrations in ground water (an unexpected result). Atrazine concentrations in the stream peak very shortly after the initiation of sample collection (i.e., shortly after stream stage begins to rise). Over the period from May to October, atrazine concentrations decline at all sample locations showing depletion of the applied herbicide. The different signals from chemographs of different constituents should allow us to decompose the flow paths for transport in the catchment, an essential step for postulating an integrated, catchment-wide conceptual model of atrazine fate and transport.
Journal Articles:
No journal articles submitted with this report: View all 42 publications for this projectSupplemental Keywords:
RFA, Scientific Discipline, Air, Toxics, Geographic Area, Waste, Water, Ecosystem Protection/Environmental Exposure & Risk, Water & Watershed, Hydrology, particulate matter, Ecosystem/Assessment/Indicators, Ecosystem Protection, exploratory research environmental biology, Chemical Mixtures - Environmental Exposure & Risk, Environmental Chemistry, pesticides, State, Chemistry, Fate & Transport, Ecological Effects - Environmental Exposure & Risk, Microbiology, Ecological Effects - Human Health, Southeast, Biology, Ecological Indicators, Watersheds, EPA Region, fate and transport, ecological exposure, contaminated sediments, water resources, colloidal particles, contaminant transport, soil-bedrock-stream system, hydroxyatrazine, Virginia (VA), agricultural watershed, biodegradation, Region 3, agricultural watersheds, chemical transport, colloidal particles and chemical transport, geology, microbial pollution, ecosystem, atrazine, aquatic ecosystems, pesticide runoff, water quality, Virginia, herbecides, particulate organic matter, land useProgress and Final Reports:
Original AbstractThe perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.