Theoretical Chemodynamic Models for Predicting Volatile Emissions to Air from Dredged Material Disposal

Abstract

Some bottom sediment in both fresh and marine waters are contaminated with hazardous organic chemicals that are classified as volatile and semi-volatile. An example is the New Bedford Harbor and Acushnet River Estuary sediment which contains quantities of the polychlorinated biphenyls Aroclor 1242, 1248 and 1254. Dredged material contaminated with these and other volatile organic chemicals (VOCs) can be released to the atmosphere during and after disposal by volatilization. There is a need for methods to predict these volatilization losses in order to develop design, operating and management guidelines for controlling V0C emissions.

Volatilization rates for hydrophobic organic compounds from a confined disposal facility (CDF) containing contaminated dredged material are presently unknown. The primary purpose of this manuscript was to assess the availability of theoretical models for the evaluation of volatile emissions to air during the process of dredge material disposal in a CDF. The first objective was to identify the primary vapor phase transport mechanism for various CDF designs and stages of filling. This provides the theoretical basis for assessing relative volatilization rates. The second objective was to review available laboratory and field procedures for obtaining information needed to measure volatile losses.

Four V0C generating locales were identified. Emission locales are defined as specific locations within a CDF which exhibit common behavioral or operational characteristics that result in the release/generation of VOCs to air. The four are: the sediment relocation locale, the exposed sediment locale, the ponded sediment locale and the vegetation covered sediment locale. The word sediment is used above in place of the phrase dredged material.

Following a section which considers the thermodynamic basis of chemical vapor equilibrium and contaminated sediment, rate equations are presented and reviewed. These equations represent the quantitative results of models of emission mechanisms for each of the four locales. Computations using the equations will yield the chemical flux in mass per unit time. The rate equations are based on transport phenomena fundamentals and have the further advantage of inputs that require concentrations and surface areas of the contaminated sources. The models are sophisticated in the sense that they contain all the complexities of the physiochemical phenomena but some license is taken in assigning the thermal state, concentration gradients, source terms, geometric dimensions, etc. with simple mathematical approximations. This yields equations that are time and space averaged and the predictions of emission rates are therefore limited to time averages from area sources and not point values for specific time. Models for some locales are very crude and additional research is needed to develop more realistic predictive equations.

Emission rates, in the mass of specific or total VOCs per unit time, are primarily dependent on the chemical concentration at the source, the surface area of the source and the degree to which the dredged material is in direct contact with the air. The relative magnitude of these three parameters provides a basis upon which a tentative ranking of emission rates from the various locales can be given. On this basis the exposed sediment locale ranks first. The ponded sediment locale with a high suspended solids concentration in surface waters ranks second. Low in the rankings are bed sediment below a relatively quite water column such as exist in some ponded sediment locales and the vegetation covered sediment locale.

This report contains preliminary calculations of the emission rates of Aroclor 1242 and 1254 from a hypothetical CDF operation in the Upper Acushnet River Estuary of the New Bedford Harbor. The calculations appear in Appendix B and represent sites in two locales of the CDF.