A global distribution model for persistent organic chemicals

Abstract

A nonsteady-state, multi-compartmental mass balance model of organic contaminant fate is presented in which the global environment is represented by nine sequentially arranged climatic zones. Each zone has an air, ocean water, fresh water, fresh water sediment, and two soil compartments, connected by advective and intermedia transport processes. Degradation can take place in every compartment and zone. The time, magnitude, and medium of chemical discharge is specified for each climatic zone. The seasonal variability of some key parameters such as air and ocean water temperature is taken into account. The mass balances for each of the compartments result in a system of 54 differential equations, solved numerically to yield estimates of concentrations, masses, transport fluxes, and reaction rates as a function of time. A series of illustrative calculations studies the major factors governing the global dispersion of persistent organic chemicals: (1) temperature, (2) chemical degradability, (3) environmental descriptors such as transport rates and organic carbon contents, (4) location and amount of chemical discharge and (5) physico-chemical properties. The calculations confirm that condensation at low temperatures can result in elevated contaminant concentrations in the polar regions and that chemicals show distinct global distribution patterns based on their physico-chemical properties. Location and medium of discharge into the environment and several environmental parameters strongly influence the calculated global fate of contaminants. The factors that primarily influence pollutant enrichment and behavior in polar regions should be studied more thoroughly with a view to obtaining more accurate parameter values and improved equilibrium and rate expressions.