Grantee Research Project Results
Final 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 , Yale University
EPA Project Officer: Packard, Benjamin H
Project Period: November 1, 1995 through October 1, 1998
Project Amount: $500,000
RFA: Water and Watersheds (1995) RFA Text | Recipients Lists
Research Category: Watersheds , Water
Objective:
This project takes 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, the site of a U.S. Geological Survey National Water Quality Assessment study, by taking the approach of considering geochemical, hydrological, and microbiological processes and their rates in different settings in the subsurface environment.Summary/Accomplishments (Outputs/Outcomes):
A quantitative understanding of the transport of chemicals through an agricultural watershed requires knowledge of processes of water flow over and beneath the land surface and of biogeochemical processes that occur in soils. Our work is focused on a field site at a watershed drained by Muddy Creek in Rockingham County, Virginia. Our work has contributed to the understanding of how: colloids can affect the transport of dissolved constituents, chemicals are transported through unsaturated soils, colloids are mobilized in unsaturated soils in the field, atrazine transport in the unsaturated zone is related to colloid transport, desorption and biodegradation processes are linked in soils, various flow and transport processes can be linked to gain an overall watershed perspective on the importance of particles in transporting herbicides in an agricultural setting.Transport of Hydroxyatrazine in Unsaturated Soil. The extensive movement of agricultural chemicals beyond the root zone generally is attributed to transport through preferred pathways (macropores) created by physical heterogeneities in the soil (e.g., cracks, root channels, and worm burrows). The transfer of material between the preferred paths and the (relatively) immobile water in smaller soil pores can have a large influence on the rate of transport of a chemical from the land surface to the water table. We investigated how the degree of soil wetness influenced the transport of hydroxyatrazine, a persistent degradation product of the widely used herbicide atrazine. Our work showed that a simple sorption model was insufficient for describing transport of hydroxyatrazine through a macroporous soil and that a model that simulates a distribution of sorption sites on the soil provides a useful description.
We also have investigated the cotransport of hydroxyatrazine by inorganic colloids in water-unsaturated porous media. A suite of 20 experiments was conducted in 0.75-m long columns assembled from 10 sections of acrylic tube and packed with dry sand. We found that the extent of hydroxyatrazine penetration into the sand beds is sensitive to influent colloid concentration, infiltration rate, and colloid mineralogy.
Mobilization and Transport of Soil Particles. Evidence that fine particles mobilized and transported in soils and aquifers can have a profound influence on contaminant migration has spawned much interest recently in understanding colloid transport in natural materials. Particle mobilization and transport in soils is directly controlled by solution chemical composition as well as rates of water movement through soil. Despite considerable previous work using columns of soil in the laboratory and limited field experimentation, questions remain about the importance of rapid temporal variations in both flow rates and chemical composition of infiltrating water on the mobilization and transport of soil fines in the unsaturated zone.
We conducted a set of infiltration experiments in an agricultural field to address two questions?whether the source of particles is depleted in the soil after an initial "flush" after a rainfall event and whether the transport of fine particles is enhanced by their "sorption" to air-water interfaces in an unsaturated soil. Water was ponded on plots in the field and colloid concentrations were measured in samples drawn from gravity (pan) lysimeters. Some depletion of fine particles was observed over sequential ponding experiments, but the average concentrations of particles collected after infiltration of a total of 25?50 cm of water were within 60?80 percent of those observed in water drained during infiltration of the first 10 cm of water. Particle concentrations were higher by a factor of 2 to 6 near the ascending and descending limbs of the water flux hydrographs relative to the plateau of the hydrographs. The results suggest that colloid-facilitated transport in unsaturated soils is unlikely to be restricted by a lack of fine particles, at least near the land surface.
Atrazine Adsorption and Colloid-Facilitated Transport in the Field. One explanation for unexpectedly widespread groundwater contamination from atrazine may be the occurrence of colloid-facilitated transport. We studied the extent of adsorption of atrazine to bulk soil and to soil colloids and examined the extent of colloid-facilitated transport of atrazine at our field site. Precipitation rate and antecedent moisture conditions were examined as controlling factors. Atrazine was applied as a broadcast spray at a rate of 0.75 kg/ha over 0.25 m2 plots at the field site. The plots were either near saturation (wet) or near field capacity (dry), and simulated rainfall events of 2.5 (low) and 5.0 cm/hr (high) were performed. Gravity lysimeters located at the base of the root zone (25 cm depth) collected infiltrating water that was monitored for colloid concentrations and pesticide concentrations in filtered (0.2 µm) and unfiltered samples. 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. Colloids isolated from the soil were found to have up to three orders of magnitude more extensive adsorption of atrazine than the bulk soil. Nevertheless, only a fraction (about 10%) of observed atrazine transport at our site was associated with fine particles.
Soil-Bound Residues. Aging has been shown to reduce the desorption, bioavailability, and solvent extractability of contaminants in soil, leaving a significant residual fraction of pesticides in agricultural soils. To examine the fate of aged contaminants and explore the relationship between the bioavailability and extractability, we conducted a long-term microcosm experiment to determine the biodegradability of nonextractable, soil-aged atrazine by a bacterial isolate, reported to mineralize atrazine within days. The extent of atrazine mineralization in the freshly applied microcosms approached 45 percent, compared with only 13 percent in the soils aged for 1 year. The data were analyzed using a coupled first-order biodegradation/two-site desorption model. The model parameters indicate that, after initial mineralization of the "available" atrazine, the biodegradability of the residual atrazine is limited by its slow desorption rate.
Analysis of the aqueous phase of the microcosms at the end of the experiment revealed an interesting finding. All of the aged, viable microcosm treatments showed a significantly higher concentration of atrazine in the aqueous phase (passing through a 0.1 µm filter) compared to that of the viable microcosms containing freshly applied atrazine. Although further analyses are ongoing, we hypothesize that during the aging period, atrazine may have diffused into or become complexed by soil organic matter. Then, over the course of the microcosm experiment, some of the atrazine-organic matter complexes became soluble in the soil water, yielding a dissolved, yet non-biodegradable, fraction of atrazine in solution. Such a non-degradable, aqueous complex would have important implications for environmental contaminant transport.
Integrated Soil-Water, Groundwater, Stream Studies. Elevated levels of atrazine and nutrients have been observed in streamwater and groundwater throughout the United States. The fate and transport of atrazine and nutrients following agricultural application in the early spring is still largely unresolved, but is a very important question to resolve for the protection of water resources. We instrumented a 4.5-km2 agricultural catchment in the Muddy Creek watershed. We examined atrazine, dissolved organic carbon (DOC), suspended sediment, silica, and major ion concentrations in the streamwater, overland flow, soil water, and groundwater during the summer of 1998. Sampling was initiated following springtime pesticide and fertilizer application. The results indicate that, in general, about 10 percent of the transport occurred as sediment-associated atrazine. Analyses suggest a relationship between atrazine sorption and elevated DOC and potassium concentrations, but reduced nitrate levels. This signal is interpreted as an indication of water interacting with field-applied cattle or turkey manure. We hypothesize that sorption of atrazine to mobilized manure particles is responsible for the observed sediment-associated atrazine transport. Sediment-associated atrazine is highest in overland flow that contains the highest concentrations of suspended particles. Once the overland flow is diluted by soil water and streamwater, desorption lowers the fraction of atrazine associated with sediments.
Storm chemographs demonstrated different patterns for constituents derived primarily from weathering (silica and calcium), compared to constituents derived primarily from early spring land applications (nitrate, atrazine, DOC, and potassium). During storms, the concentrations of silica and calcium decreased, the concentrations of nitrate, DOC, and potassium increased, while the atrazine response was variable. Concentrations of nitrate, DOC, and potassium usually followed a similar temporal pattern over a storm, although the nitrate signal often lagged several hours behind the other constituents. A concentration-discharge mixing analysis indicated a rapid, short-duration contribution from a water source with relatively high potassium and DOC concentrations, followed by mixing of two other components (presumably soil water and baseflow water) during the remainder of the event. An end-member mixing analysis showed a pattern that can be explained as a mixing of overland flow (dominating early), with soil water (influential later in the event), and baseflow water. Results indicated that rapid overland flow from fields contributed most of the atrazine, potassium, and DOC signal, while soil-water transport was responsible for the delayed nitrate signal.
Journal Articles on this Report : 8 Displayed | Download in RIS Format
Other project views: | All 42 publications | 8 publications in selected types | All 8 journal articles |
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El-Farhan YH, DeNovio NM, Herman JS, Hornberger GM. Mobilization and transport of soil particles during infiltration experiments in an agricultural field, Shenandoah Valley, Virginia. Environmental Science and Technology 2000;34:3555-3559. |
R824772 (Final) |
not available |
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Hyer KE, Hornberger GM, Herman JS. Processes controlling the episodic steamwater transport of atrazine and other agrichemicals in an agricultural watershed. Journal of Hydrology 2001;254(1-4):47-66. |
R824772 (Final) |
not available |
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Johnson SE, Herman JS, Mills AL, Hornberger GM. Bioavailability and desorption characteristics of aged, nonextractable atrazine in soil. Environmental Toxicology and Chemistry 1999;18(8):1747-1754. |
R824772 (Final) |
not available |
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Kauffman SJ, Bolster CH, Hornberger GM, Herman JS, Mills AL. Rate-limited transport of hydroxyatrazine in an unsaturated soil. Environmental Science and Technology 1998;32:3137-3141. |
R824772 (Final) |
not available |
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Saiers JE, Tao G, Hornberger GM. Colloid-facilitated transport of herbicides. Eos, Transactions, American Geophysical Union 1997;78:152. |
R824772 (Final) |
not available |
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Saiers JE, Hornberger GM. The influence of ionic strength on the facilitated transport of cesium by kaolinite colloids. Water Resources Research 1999;35:1713-1727. |
R824772 (Final) |
not available |
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Saiers JE, Tao G. Evaluation of continuous distribution models for rate-limited solute adsorption to geologic media. Water Resources Research, July 2000;36(7):1627-1639. |
R824772 (Final) |
not available |
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Sprague LA, Herman JS, Hornberger GM, Mills AL. Atrazine adsorption and colloid-facilitated transport through the unsaturated zone. Journal of Environmental Quality 2000;29(5):1632-1641. |
R824772 (Final) |
not available |
Supplemental Keywords:
watersheds, adsorption, chemical transport, atrazine, hydroxyatrazine, colloids, hydrology, geochemistry, microbial ecology, Mid-Atlantic, Virginia, VA, agriculture., RFA, Scientific Discipline, Toxics, Air, Waste, Geographic Area, 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.