Final Report: Development of Pollutant Fate and Transport Models for Use in Terrestrial Ecosystem Exposure Assessment

EPA Grant Number: R825433C051
Subproject: this is subproject number 051 , established and managed by the Center Director under grant R825433
(EPA does not fund or establish subprojects; EPA awards and manages the overall grant for this center).

Center: EERC - Center for Ecological Health Research (Cal Davis)
Center Director: Rolston, Dennis E.
Title: Development of Pollutant Fate and Transport Models for Use in Terrestrial Ecosystem Exposure Assessment
Investigators: Rolston, Dennis E.
Institution: University of California - Davis
EPA Project Officer: Hahn, Intaek
Project Period: October 1, 1996 through September 30, 2000
RFA: Exploratory Environmental Research Centers (1992) RFA Text |  Recipients Lists
Research Category: Center for Ecological Health Research , Targeted Research

Objective:

The objectives of this research project were to: (1) develop pollutant fate and transport models to be used as tools for assessing exposure in terrestrial ecosystems; (2) investigate the fate of soil-applied chemicals; and (3) estimate potential exposure levels in the atmosphere, soil, and aqueous phases in the shallow vadose zone. To estimate exposure levels, it is useful to have a conceptual model of the physical system of concern. We were interested in terrestrial ecosystems and the soil processes that govern the movement of chemicals to locations where receptor populations are found. Model predictions of chemical concentration profiles can be used to estimate the levels to which the organisms are exposed.

Summary/Accomplishments (Outputs/Outcomes):

We developed models that will provide investigators with tools for evaluating exposure routes of chemicals in ecosystems to the birds, small mammals, and aquatic species being studied. Models were built on existing fate and transport models of chemical movement in soil. The modeling also allowed us to evaluate the most important processes requiring further experimental investigation. This includes research in the area of chemical adsorption kinetics, chemical volatilization processes from soil to the atmosphere, and routes of transport to surface water and groundwater.

Pesticide and Volatile Organic Chemical Transport

We started out simulating the above processes using a mathematical model (RIOCATS) describing the fate and transformation of organic chemicals in soil under transient environmental conditions. In addition, to begin assessing exposure in terrestrial ecosystems, we worked on coupling RIOCATS and PRZM-2 with ECOTOX. However, it soon became apparent that data sets were lacking to test and validate the model. Thus, part of the goal of this research project was to conduct laboratory and field experiments to obtain data to test some parts of the model components.

We measured diazinon (O,O-diethyl-O-[2-isopropyl-4-methyl-6-pyrimidinyl] phosphorothioate) vapor adsorption, desorption, and degradation in soil. The results of desorption showed that the partition coefficient (diazinon content in soil divided by diazinon vapor concentration in soil air) was strongly affected by the soil-moisture content. The partition coefficient increased by two orders of magnitude when soil-water content dropped from 11 percent to 3 percent (by weight). For soils with water content greater than 11 percent, the partition coefficient increased slightly with increasing soil-water content.

One of the first steps in modeling pesticide volatilization was to improve the simulation models in terms of the coupling between chemical volatilization and water evaporation. It was found that water vapor diffusion should be included in any water transport model to simulate the pesticide sorption process in the soil surface more accurately. When upward water flow was insufficient, soil-water content at the soil surface was quickly reduced below a threshold, and diazinon volatilization started to decrease exponentially because of diazinon adsorption on dry soil surfaces.

Emission of Ozone Precursors From Soil

Agricultural soils have been recognized as a significant source of nitric oxide (NO) and nitrous oxide (N2O), which are important trace gases involved in several critical processes in the atmosphere. NO is a precursor to nitric acid, and plays a central role in photochemical reactions, which regulate levels of tropospheric ozone. Laboratory experiments were conducted to examine substrate and process controls over temporal variability of NO and N2O production during nitrification, and to quantify the kinetics of nitrous acid (HNO2)-mediated chemical reactions. Data suggest that even during periods of relatively low nitrite accumulation and rapid overall nitrification, HNO2-mediated reactions may have been the primary source of NO. To provide the capability of predicting NO emission from soils, a numerical simulation model of all of the relevant processes was developed. Although fairly data intensive, this model offers the possibility of more accurately predicting NO emissions from land under various management and environmental conditions.

Vapor Transport and Fate Mechanisms

One of the major exposure routes for pesticides and other volatile chemicals is volatilization from soil to the atmosphere and subsequent transport downwind to ecosystems. Volatilization from soil is dependent on diffusion processes, especially the transport physics and diffusion coefficients, and the microbial ecology of biodegradation. We have conducted both theoretical and experimental investigations of vapor phase transport processes in soils and how vapor diffusion is coupled to biodegradation of organic chemicals. We have derived new equations for the advective and diffusive transport of gases with densities different from air, and we found that these new equations make much better predictions of dense gas transport than the traditional equations. Predicting the soil gas diffusion coefficient from basic soil properties has been a challenge for decades. This project supported a significant amount of both theoretical and experimental research on the development of new equations for predicting the diffusion coefficient from knowledge of the soil-water retention properties of different kinds of soils and porous media.

The ultimate fate of many organic chemicals and pesticides is degradation by microbial communities in soils. Research was conducted on coupling transport of volatile chemicals and biodegradation and the development of simulation models to improve the prediction of rates of biodegradation. Results from coupled transport and biodegradation experiments indicate that Monod and Michaelis-Menten kinetics are required to describe the biodegradation of volatile organic chemical mixtures.

The following activities were accomplished:

• A new steady-state chamber system was developed that is capable of accurately measuring small pesticide volatilization rates from soil to the atmosphere. Most chamber systems that have been used for measuring volatilization of pesticides from soil are plagued with several shortcomings. This new chamber design eliminates several of the shortcomings. If adopted by other researchers, the new chamber design should improve the estimates of pesticide emissions from soils.

• Studies on diazinon volatilization from soil show that when the soil surface becomes dry, diazinon volatilization decreases exponentially because of strong diazinon adsorption on dry soil surfaces. Because dry soil very strongly adsorbs volatile pesticides and then releases the vapors during times when the relative humidity increases, most volatilization measurements generally would miss the peak emissions and greatly underestimate the amount of pesticide volatilized to the atmosphere. Researchers and regulators should be aware of these phenomena when designing sampling schemes and setting air quality standards.

• Water vapor diffusion should be included in any water transport model to simulate the pesticide sorption process in the soil surface more accurately. Most simulation models used by consultants and agencies do not consider water vapor movement as a significant process. For arid region soils where the soil surface becomes very dry, neglecting this process will impact the prediction of volatile chemical emissions. Users of simulation models for predicting the fate of pesticides in ecosystems should consider whether this effect is important for the particular system being modeled.

• Simulations from a numerical model and measured diazinon volatilization fluxes agreed well only if enhanced diazinon vapor adsorption in the dry range was considered. Researchers and regulators should be aware of this phenomenon when designing sampling schemes and setting air quality standards.

• A new numerical model was developed to simulate pesticide transport and volatilization from both wet and dry soils, and the calculated and measured results agreed only if corrections for soil temperature inside the chamber were made. This result again points out the problems of using chambers for measuring the fluxes of gases from soils, particularly for pesticides. This finding will be important to researchers trying to estimate volatilization from soil, and to agency scientists interpreting emission estimates.

• The Stefan-Maxwell equations should be used for predicting steady-state fluxes of gas mixtures, but for transient diffusion, Fick’s Law appears to be adequate. Nearly all of the simulation models for predicting the transport and fate of chemicals in soil use Fick's Law for diffusion. Our research gives guidance on when the use of Fick's Law may result in errors. These results will be of interest to users of simulation models.

• New equations using volume-averaging techniques for the advective and diffusive transport of gases with densities different from air give much better predictions of dense gas transport than the traditional equations. This theory development and intricate experiments represent a major advancement in better understanding the physics of gas transport for gases with densities different than air. The results will be of interest to users and developers of simulation models.

• New equations for predicting the diffusion coefficient from knowledge of the soil-water retention properties of porous media compare well with measurements over a wide range of soils and water contents. Nearly all of the simulation models of diffusion process in porous media use equations that are now shown to be inaccurate. These new equations will provide developers of models with better estimates of solute and gas diffusion coefficients.

• Results from coupled transport and biodegradation experiments indicate that Monod and Michaelis-Menten kinetics are required to describe the biodegradation of volatile organic chemical mixtures. Most models for biodegradation of soil contaminants use simple kinetics such as first order. We now know that in general these simple kinetic models often are not correct and result in errors in biodegradation rates. Because so much emphasis is put on in situ biodegradation as a means of "cleaning up" contaminated sites, this result will be useful to agency scientists and regulators.

• Measurements of the temporal variability of NO and N2O production during nitrification in soils show that gross NO production rates were highly correlated with calculated concentrations of HNO2, which were shown to originate from autotrophic microbial oxidation of NH4 to nitrite. The significance of this finding is that even during periods of relatively low nitrite accumulation and rapid overall nitrification, HNO2-mediated reactions may be the primary source of NO emissions from soils, and that fertilized soils may be a larger source of NO than previously thought. This result should be useful in interpreting the causes of ozone exceedance in air quality studies.

• To provide the capability of predicting NO emission from soils, a new numerical simulation model of all of the relevant processes was developed. Although fairly data intensive, this model offers the possibility of more accurately predicting NO emissions from land under various management and environmental conditions. It should provide additional information needed in setting air quality standards for ozone.

• Tillage causes immediate changes in microbial community structure, but little concomitant change in total microbial biomass. Soil quality is an important factor in ecosystem function. This research supports the hypothesis that microbial community structure is more important to ecosystem function than total microbial biomass. With the development of new molecular tools for studying microbial ecology, these results will be of interest to users of those tools.

Supplemental Keywords:

ecosystem, ecosystem protection, environmental exposure and risk, geographic area, international cooperation, water, terrestrial ecosystems, aquatic ecosystem, aquatic ecosystem restoration, aquatic ecosystems and estuarine research, biochemistry, ecological effects, ecological indicators, ecological monitoring, ecology and ecosystems, environmental chemistry, restoration, state, water and watershed, watershed, watershed development, watershed land use, watershed management, watershed modeling, watershed restoration, watershed sustainability, agricultural watershed, exploratory research environmental biology, California, CA, Clear Lake, Lake Tahoe, anthropogenic effects, aquatic habitat, biogeochemical cycling, ecological assessment, ecology assessment models, ecosystem monitoring, ecosystem response, ecosystem stress, environmental stress, environmental stress indicators, fish habitat, hydrologic modeling, hydrology, integrated watershed model, lake ecosystems, lakes, land use, nutrient dynamics, nutrient flux, water management options, water quality, wetlands., RFA, Scientific Discipline, INTERNATIONAL COOPERATION, Waste, Water, ECOSYSTEMS, Ecosystem Protection/Environmental Exposure & Risk, Aquatic Ecosystems & Estuarine Research, Water & Watershed, Restoration, Aquatic Ecosystem, Fate & Transport, Monitoring/Modeling, Environmental Microbiology, Biochemistry, Terrestrial Ecosystems, Ecology and Ecosystems, Aquatic Ecosystem Restoration, Watersheds, aquatic, fate and transport, ambient particle properties, ecosystem assessment, watershed management, Sacramento River, water circulation, pesticides, sediment transport, modeling, restoration strategies, Clear Lake, hydrology, watershed influences, chemical kinetics, integrated watershed model, wetland restoration, aquatic ecosystems, environmental stress, groundwater contamination, watershed sustainablility, source load modeling, material transport, ecology assessment models, ecological impact, agrochemicals, ecosystem stress, ecological research, watershed restoration

Relevant Websites:

http://ice.ucdavis.edu/cehr/ Exit

Progress and Final Reports:

Original Abstract
  • 1997
  • 1998
  • 1999 Progress Report

  • Main Center Abstract and Reports:

    R825433    EERC - Center for Ecological Health Research (Cal Davis)

    Subprojects under this Center: (EPA does not fund or establish subprojects; EPA awards and manages the overall grant for this center).
    R825433C001 Potential for Long-Term Degradation of Wetland Water Quality Due to Natural Discharge of Polluted Groundwater
    R825433C002 Sacramento River Watershed
    R825433C003 Endocrine Disruption in Fish and Birds
    R825433C004 Biomarkers of Exposure and Deleterious Effect: A Laboratory and Field Investigation
    R825433C005 Fish Developmental Toxicity/Recruitment
    R825433C006 Resolving Multiple Stressors by Biochemical Indicator Patterns and their Linkages to Adverse Effects on Benthic Invertebrate Patterns
    R825433C007 Environmental Chemistry of Bioavailability in Sediments and Water Column
    R825433C008 Reproduction of Birds and mammals in a terrestrial-aquatic interface
    R825433C009 Modeling Ecosystems Under Combined Stress
    R825433C010 Mercury Uptake by Fish
    R825433C011 Clear Lake Watershed
    R825433C012 The Role of Fishes as Transporters of Mercury
    R825433C013 Wetlands Restoration
    R825433C014 Wildlife Bioaccumulation and Effects
    R825433C015 Microbiology of Mercury Methylation in Sediments
    R825433C016 Hg and Fe Biogeochemistry
    R825433C017 Water Motions and Material Transport
    R825433C018 Economic Impacts of Multiple Stresses
    R825433C019 The History of Anthropogenic Effects
    R825433C020 Wetland Restoration
    R825433C021 Sierra Nevada Watershed Project
    R825433C022 Regional Transport of Air Pollutants and Exposure of Sierra Nevada Forests to Ozone
    R825433C023 Biomarkers of Ozone Damage to Sierra Nevada Vegetation
    R825433C024 Effects of Air Pollution on Water Quality: Emission of MTBE and Other Pollutants From Motorized Watercraft
    R825433C025 Regional Movement of Toxics
    R825433C026 Effect of Photochemical Reactions in Fog Drops and Aerosol Particles on the Fate of Atmospheric Chemicals in the Central Valley
    R825433C027 Source Load Modeling for Sediment in Mountainous Watersheds
    R825433C028 Stress of Increased Sediment Loading on Lake and Stream Function
    R825433C029 Watershed Response to Natural and Anthropogenic Stress: Lake Tahoe Nutrient Budget
    R825433C030 Mercury Distribution and Cycling in Sierra Nevada Waterbodies
    R825433C031 Pre-contact Forest Structure
    R825433C032 Identification and distribution of pest complexes in relation to late seral/old growth forest structure in the Lake Tahoe watershed
    R825433C033 Subalpine Marsh Plant Communities as Early Indicators of Ecosystem Stress
    R825433C034 Regional Hydrogeology and Contaminant Transport in a Sierra Nevada Ecosystem
    R825433C035 Border Rivers Watershed
    R825433C036 Toxicity Studies
    R825433C037 Watershed Assessment
    R825433C038 Microbiological Processes in Sediments
    R825433C039 Analytical and Biomarkers Core
    R825433C040 Organic Analysis
    R825433C041 Inorganic Analysis
    R825433C042 Immunoassay and Serum Markers
    R825433C043 Sensitive Biomarkers to Detect Biochemical Changes Indicating Multiple Stresses Including Chemically Induced Stresses
    R825433C044 Molecular, Cellular and Animal Biomarkers of Exposure and Effect
    R825433C045 Microbial Community Assays
    R825433C046 Cumulative and Integrative Biochemical Indicators
    R825433C047 Mercury and Iron Biogeochemistry
    R825433C048 Transport and Fate Core
    R825433C049 Role of Hydrogeologic Processes in Alpine Ecosystem Health
    R825433C050 Regional Hydrologic Modeling With Emphasis on Watershed-Scale Environmental Stresses
    R825433C051 Development of Pollutant Fate and Transport Models for Use in Terrestrial Ecosystem Exposure Assessment
    R825433C052 Pesticide Transport in Subsurface and Surface Water Systems
    R825433C053 Currents in Clear Lake
    R825433C054 Data Integration and Decision Support Core
    R825433C055 Spatial Patterns and Biodiversity
    R825433C056 Modeling Transport in Aquatic Systems
    R825433C057 Spatial and Temporal Trends in Water Quality
    R825433C058 Time Series Analysis and Modeling Ecological Risk
    R825433C059 WWW/Outreach
    R825433C060 Economic Effects of Multiple Stresses
    R825433C061 Effects of Nutrients on Algal Growth
    R825433C062 Nutrient Loading
    R825433C063 Subalpine Wetlands as Early Indicators of Ecosystem Stress
    R825433C064 Chlorinated Hydrocarbons
    R825433C065 Sierra Ozone Studies
    R825433C066 Assessment of Multiple Stresses on Soil Microbial Communities
    R825433C067 Terrestrial - Agriculture
    R825433C069 Molecular Epidemiology Core
    R825433C070 Serum Markers of Environmental Stress
    R825433C071 Development of Sensitive Biomarkers Based on Chemically Induced Changes in Expressions of Oncogenes
    R825433C072 Molecular Monitoring of Microbial Populations
    R825433C073 Aquatic - Rivers and Estuaries
    R825433C074 Border Rivers - Toxicity Studies