Presentation on theme: "Appendix 3 Frank Wania Evaluating Persistence and Long Range Transport Potential of Organic Chemicals Using Multimedia Fate Models."— Presentation transcript:
Appendix 3 Frank Wania Evaluating Persistence and Long Range Transport Potential of Organic Chemicals Using Multimedia Fate Models
What? development of techiques that incorporate multimedia fate models in the process of evaluating candidate POPs for persistence and long range transport potential. Why? because the multimedia distribution of a chemical profoundly affects its environmental persistence and potential for long range transport. Evaluating Persistence and Long Range Transport Potential of Organic Chemicals Using Multimedia Fate Models Frank Wania, WECC Don Mackay, Eva Webster, Trent University Andreas Beyer, Michael Matthies, Universität Osnabrück
overall persistence multimedia partitioning, and thus , is governed by: physical-chemical properties mode of emission environmental characteristics Webster, E., Mackay, D., Wania, F. Evaluating Environmental Persistence. Environ. Toxicol. Chem. 1998, 17, M tot and N Rtot can be calculated using a multimedia environmental fate model such as EQC Evaluating Environmental Persistence
3-compartment level III model used to estimate an overall persistence of an organic chemical in the global environment Overall Global Persistence Calculating Overall Persistence EAEA D RE ·f E Air Soil Water E EWEW D RW ·f W D RA ·f A D LW ·f W D LE ·f E D LA ·f A D AW ·f A D AE ·f A D EW ·f E D EA ·f E D WA ·f W Wania, F. An integrated criterion for the persistence of organic chemicals based on model calculations. WECC Report 1/98.
dependence of overall persistence on physical chemical properties as expressed by log K AW and log K OW. Assumptions: Equal fraction of emissions into air, water and soil. Half-lifes 48 h in air, 1460 h in water and 4380 in soil. Level III. Calculating Overall Persistence
water fraction of emissions into water air fraction of emissions into soil overall persistence water with emission into water only overall persistence soil with emission into soil only overall persistence air with emission into air only air fraction of emissions into air linear additivity of overall persistence = air · air + water · water + soil · soil
Calculating Long Range Transport Potential Assumptions: steady-state between moving phase and stationary phase no dispersion advective transport uni-directional C M0 CMCM C M0 /e distance LMLM Characteristic Travel Distance distance it takes for the concentration in the moving phase (e.g. air) to fall to e -1 or 37 % of its initial value due to degradation in the moving phase (e.g. air) and net transfer to the stationary phase (e.g. soil, water). van Pul et al. 1998, Bennett et al. 1998, Beyer et al. 1999
Reformulation for Well-Mixed (or Box) Systems the distance in well-mixed system over which the concentration in the moving phase falls to half its input value. Then the rate of advective loss equals the total loss by reaction: 0.5 N In = N Out = (N RM + N RS ) air Calculating Long Range Transport Potential soil N In N Out N RA N RS Example: Air Moving Over Soil N AS N SA L A = u·M A / (N RA + N AS ·F) L A = u·V A ·Z A / (D RA + D AS ·F) where F = D RS / (D SA + D RS ) (fraction of chemical retained by soil) characteristic travel distance in air facilitates use of traditional multimedia model for calculation of L Beyer, A., Mackay, D., Matthies, M., Wania, F., Webster, E An evaluation of the role of mass balance models for assessing the long range transport potential of organic chemicals. Report 99:01, Environmental Modelling Centre, Trent University, Peterborough
Relationship Between Characteristic Travel Distance and Overall Persistence L M = u·M M · / M tot L M is distance a molecule travels during the environmental residence time (u· ), multiplied by the proportion of mass in the moving medium (M M / M tot ) Example: Travel Distance in Air for very volatile chemicals M air / M tot = 1, thus L air = u· (maximum possible) for less volatile chemicals M air / M tot is small, thus L air is small It can be shown that the general formulation for the travel distance in moving phase M is L M = u·M M / N Rtot whereas overall persistence was defined as = M tot / N Rtot
half-life in air in hours travel ditance in air in km OCDD aldrin benzene HCB tetraCB heptaCB decaCB dieldrin chlorobenzene DDT -HCH u. air maximum travel distance chemical partitions only into moving phase (air) minimum travel distance chemical partitions completely onto particles and deposition is irreversible Calculating Long Range Transport Potential
travel distance in air in km -HCH TCDD DDT DDE HCB tetraCB hexaCB dieldrin OCDD km overall persistence in days aldrin dieldrin -HCH biphenyl chlorobenzene HCB OCDD DDT benzene tetraCB u.u. travel distance in water in km Calculating Long Range Transport Potential using a multimedia model (EQC) to estimate a characteristic travel distance in air and water (Beyer et al., 1999) u.u. -HCH overall persistence in days
Limitations of These Techniques 1.for many candidate substances, not even the most basic physical-chemical properties are available. 2. overall persistence and travel distance are dependent on environmental characteristics, e.g. temperature. 3. these techniques provide a scale to rank chemicals according to the persistence and LRT potential, but not cut-off criteria, for what constitutes persistence/ non-persistence, and LRT potential/no LRT potential.
Overall Persistence and Global Distribution Overall persistence of -HCH as calculated by a global distribution model during the time period persistence is not fixed value, but dependent on climate and thus on the zonal distribution of a chemical Wania, F., and D. Mackay Global chemical fate of -hexachlorocyclohexane. 2. Use of a global distribution model for mass balancing, source apportionment, and trend predictions. Environ. Toxicol. Chem., in press.
Effect of Temperature on Travel Distance in Air A drop in temperature causes two opposing effects: 1. reaction half-lifes increase, resulting in an increase in persistence 2. partitioning shifts from air into surface media (soil, water, etc.) For chemicals with < 550 days, L air always increases with decreasing temperature. If degradation in environment is fast, a short L air is determined by a short persistence and not by small partitioning into air. If T drops, the persistence of such substances will increase severely and L air will also rise.
There is a need to investigate the influence of zonal ecosystem characteristics (climate, vegetation, soils, etc.) on the multimedia fate of organic chemicals Objective: Comparing various ecosystems with respect to their potential to cause high exposure of POPs to organisms Comparative Environmental Chemistry of POPs fate processecosystem characteristic degradation - clearance potential by degradation partitioning - dilution potential intermedia transfer- clearance potential by export / retention potential - focussing potential within ecosystem bioaccumulation- focussing potential within ecosystem