Theoretical models for evaluation of volatile emissions to air during dredged material disposal with applications to New Bedford Harbor, Massachusetts
Thibodeaux, Louis J.
Abstract: Some bottom sediments in both fresh and marine waters are contaminated with hazardous organic chemicals that are classified as volatile and semivolatile. An example is the New Bedford Harbor and Acushnet River Estuary sediment. which contains quantities of the polychlorinated biphenyls Aroclors 1242, 1248, and 1254. Dredged material contaminated with these and other volatile organic chemicals (VOCs) can release these contaminants to the atmosphere during and after disposal by volatilization. Methods to predict these volatilization losses are needed to develop design, operation, and management guidelines for controlling VOC 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 study was to assess the availability of theoretical models for the evaluation of volatile emissions to air during the process of dredged 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 provided the theoretical basis for assessing relative volatilization rates. The second objective was to review available laboratory and field procedures for obtaining the information needed to measure volatile losses. Four VOC emission locales were identified: the sediment (dredged material) relocation locale, the exposed sediment locale, the ponded sediment locale, and the vegetation-covered sediment locale. These were specific locations within a CDF that exhibited common behavioral or operational characteristics that resulted in the release/generation of VOCs to air. Following a section that considers the thermodynamic basis of chemical vapor equilibrium and contaminated sediment, rate equations are presented and reviewed. These equations represent the quantitative results of the models of emission mechanisms from each of the four locales. Computations using these equations yield the chemical flux rate 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. 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 the bed sediment below a relatively quiet water column, such as exists in some ponded sediment locales, and the vegetation-covered sediment locale. The report also contains preliminary calculations of the emission rates of Aroclors 1242 and 1254 from a hypothetical CDF operation in the Upper Acushnet River Estuary (Appendix A). Appendix B presents a detailed derivation of the rivulet and ponded VOC emission model.
Environmental Laboratory (U.S.)
Dredging--Massachusetts--Environmental aspects; Air--Massachusetts--Pollution potential; Confined disposal facility; Dredging spoil; Volatile organic compounds