Characteristic Time and Spatial Scales for Persistent Organic PollutantsEPA Grant Number: U915242
Title: Characteristic Time and Spatial Scales for Persistent Organic Pollutants
Investigators: Bennett, Deborah H.
Institution: University of California - Berkeley
EPA Project Officer: Michaud, Jayne
Project Period: January 1, 1997 through January 1, 2000
Project Amount: $102,000
RFA: STAR Graduate Fellowships (1997) RFA Text | Recipients Lists
Research Category: Fellowship - Risk Assessment , Academic Fellowships , Ecological Indicators/Assessment/Restoration
The objective of this research project is to develop methods to quantify spatial range and characteristic time for semivolatile persistent organic pollutants in a multimedia environment. These values are used to calculate a measure for total human exposure. An uncertainty analysis will be completed and will include the effects of spatial and temporal variability.
Persistence depends on the distribution among the different environmental media because chemicals often have different decay rates in different environmental media. In this research, persistence is determined for a four-compartment steady-state system with air, plants, and two soil layers. The characteristic travel distance is derived from a moving Lagrangian air cell and the nonmoving compartments, vegetation, and soil. The concentration is reduced with distance based on degradation in air, and transfer to and subsequent degradation in the vegetation and soil.
The mass balance between air, vegetation, and soil is controlled by the chemical-specific vegetation/air or soil/air partitioning; the kinetics of mass transfers among air, vegetation, and soil; and degradation rates in air, plant tissue, and soil. The steady-state concentrations are determined by the interactions among the four environmental compartments and are based on a fugacity model. Calculating the steady-state mass distribution considers the effect of advective transport between different medium and nonequilibrium effects resulting from the source terms. A steady-state mass distribution is advantageous because it provides sufficient accuracy while being simple enough to calculate on a spreadsheet. This methodology is appropriate for continuous, large nonpoint emissions (such as an urban airshed).
To demonstrate this methodology, case studies are presented. I calculate the characteristic time and spatial range and compare them to results obtained by other researchers using different methods for a range of chemicals and to trends seen in the actual environment. Uncertainty and sensitivity analyses are used to characterize the reliability of spatial and temporal range estimates and to find the importance of each parameter in contributing to range of uncertainty in the outputs.