Polymer and Separations Research Laboratory (PolySep)
Environmental
An important aspect of our environmental research is aimed at the development of predictive models for describing the dynamic partitioning of toxic chemicals in the environment and models for assessing the exposure of human and ecological receptors to chemical pollutants. Projects in this area deal with theoretical studies of intermedia transport processes and their incorporation into multimedia pollutant transport, exposure, and risk assessment models. Computer software packages developed by Dr. Cohen's research group are now in wide-spread use both in the United States and abroad. Also, as the Director of the UCLA Center for Environmental Risk Reduction. Dr. Cohen has been developing multidisciplinary integrated aproaches to risk reduction and pollution prevention.
- Modeling Contaminant Partitioning in the Multimedia Environment
- Intermedia Transport Facors of Chemicals
- Mathematical Model of Bioaccumulation of Organic Toxins in Finned Fish
- Contaminant Transport in the Soil Matrix
- Neural Networks for QSPRs
Modeling of Contaminant Partitioning in the Multimedia Environment
Research Team: Professor Yoram Cohen and Denise
Yaffe.
In response to the needs of the State of California and California
Industry for conducting a credible multimedia risk assessment for air
toxics, the multimedia transport and exposure assessment (MTEA) project
has focused on the assessment and development of practical multipathway
transport and exposure assessment tools. The first component of the MTEA
project focuses on the continuing development of the integrated
spatial-multimedia-compartmental model (ISMCM) to assess the multimedia
environmental partitioning of volatile and particle-bound pollutants in
the environment. The description of the modeling approach and rationale
are described in two recent papers by Cohen and Clay (1994) and Cohen
(1993). The ISMCM is based on a detailed mechanistic description of
intermedia transfer processes. Thus, relative to other existing multimedia
models, the ISMCM can be used to study various scenarios with minimum
parameter input requirements . The ISMCM describes the environmental media
as composed of eight main compartments (air, aerosol, soil, water,
sediment, suspended solids, biota, vegetation). The model incorporates
theoretical and empirical descriptions of transport processes associated
with the gaseous, dissolved and particle phases to reduce the required
user-input. Furthermore, the ISMCM includes the modeling of rainfall
events and the associated intermedia transport processes such as
precipitation scavenging of contaminants from the atmosphere and
contaminant transport in soils due to rain infiltration and runoff. The
ISMCM model was recently expanded by including a dynamic vegetation and
biota compartments. The vegetation compartment includes contaminant uptake
from both the soil and the atmosphere and the biota compartment accounts
for contaminant uptake from both water and via the food chain. The ISMCM
model has been used in the classroom environment in a course on Multimedia
Environmental Assessment.
Intermedia Transport Factors for Toxic Air Pollutants
Research Team: Professor Yoram Cohen and
Professor Arthur Winer
Research Team: Professor Yoram Cohen and Professor Arthur Winer
This project which has been completed focused on the evaluation of
compound-specific intermedia transport factors (ITFs) that are used in the
assessment of exposure of the human population from multiple routes (e.g.,
inhalation, water drinking, showering, ingestion of vegetables and meat).
Intermedia transfer factors were evaluated and compiled for seven
chemicals (benzene, formaldehyde, chromium VI, mercury, benzo(a)pyrene,
methylene chloride and 2,3,7,8-TCDD) and the final report is now available
through the California Air Resources. Based on the above ITF study the
major factors that affect the multimedia distribution and exposure
pathways for selected toxic air pollutants have been identified. This
information provides support for risk analyses required under California
AB2588. An intermedia transfer factor (ITF) predictor was also developed
along with a companion chemical property database. The ITF software is
user-friendly software developed specifically to operate in the PC Windows
environment.
Mathematical Model of Bioaccumulation of Organic Toxins in Finned Fish
When hydrophobic organic toxins enter clean ecosystems, indigenous aquatic biota become contaminated. The internal build up of toxins in fish results in a ecological threat not only to that particular species, but also to species that depend on it as a food source. Such species include higher level food chain predators and possibly humans. In order to predict the degree of bioaccumulation of hydrophobic toxins in finned fish under dynamic conditions, a compartmental dynamic food-chain bioaccumulation model was developed. The model was developed with the intent of minimizing the number of required user-input parameters while maintaining flexibility of describing a wide range of plausible scenarios. The model is shown to be in excellent agreement with more complex models ((Norstrom,1976;Barber 1989,1991) and with available field data. The effect of various uptake mechanisms, morphometric parameters and species diet on toxin accumulation in finned fish was studied via a number of test cases covering a wide set of species. The interfacing of the current bioaccumulation model with the Integrated Spatial Multimedia Compartmental Model (ISMCM) is currently underway.
Contaminant
Transport in the Soil Matrix
Research Team: Yoram Cohen and Jordi
Grifoll
Solute Transport in the Unsaturated Soil Zone
Research Team: Yoram Cohen and Jordi Grifoll
This project focuses on the development of a theoretical model for the transport of chemical solutes contaminants in the unsaturated soil matrix under the action of rainfall, soil drying, and diurnal temperature cycling. In the first phase of the project a theoretical analysis which allows the determination of the significance of a multiphase versus a pseudo-single phase transport analysis of chemical transport was developed. A multiphase soil transport (MST) model was developed to investigate the movement of organic chemicals in the soil vadose zone. The MST model incorporates a soil moisture transport model coupled with a mass transfer model. In the MST approach climatological rainfall and soil drying data for the location of interest are used for the desired simulation period. In the first phase of the study it was found that, for the Los Angeles area, contaminant flux predictions that incorporates the effect of rainfall vary significantly throughout the year (by up to two orders of magnitude). The rate of volatilization was also found to be a function of the period during which the chemical is introduced to the soil matrix, a phenomenon which is linked to the yearly distribution of rainfall. For example, benzene which is introduced into the soil matrix in January is expected to volatilize from the soil at a rate lower by about a factor of three relative to benzene which is introduced into the soil in July.
Another component of this research program focuses on the development
of efficient numerical models for predicting the movement of liquids
(both water and organic liquids) in the unsaturated soil zone. An
efficicient front-tracking algorithm was recently developed for tracking
the movement of liquids in nearly dry soils.
Chemical Volatilization from Shallow Liquid Waste Impoundments and Water Bodies
Research Team: Yoram Cohen, Francesc Giralt,
Xavier Grau, Ildefonso Cuesta
The volatilization of contaminants from liquid waste impoundments under
natural conditions is being investigated using a three-dimensional
turbulent fluid flow/mass transfer model. The three-dimensional Navier-Stokes
equations coupled with the energy and species mass conservation equations
are solved (for both laminar and turbulent conditions) to yield the fluid
flow and concentration fields resulting from chemical volatilization under
the action of wind shear and due to free-convection. Initial simulations
in ponds of depth:length ratio of 1:4, 1:10 and 1:30 revealed that
recirculating regions that are confined to the down-wind end of the liquid
ponds can exist. Thus, the concentration field can be highly non-uniform.
This has significant implications to the design of sampling protocols for
liquid waste impoundments as well as shallow natural water bodies. Initial
simulation revealed that, at low wind speeds, the mass transfer
coefficients can be significantly lower than estimates derived from
standard correlations which are based on experiments in well-mixed water
bodies. Current work focuses on evaluating the effect of pond geometry,
wind speed, solar radiation, and chemical mass diffusivities. While the
initial simulations focused on low wind speeds and 2-D laminar flow in the
ponds future work will be expanded to include 3-D simulations and a future
extension to turbulent flows. This research project will result in an
improved ability to estimate VOC emissions from liquid waste impoundments
and chemical exchange rates at the atmosphere/water interface for shallow
natural water bodies.
Cuesta, I., F. Giralt, X. Grau and Y. Cohen, "Air-water mass transfer of organics from shallow ponds under laminar recirculation", Int.J. Heat and Mass Transfer, 42, 165-179 (1999).
Last update:
01/11/2004
