M6 PM Fate and Transport of Pollutants from Dredging and Placement
Monday, 14 November 2005: 1:50 PM - 5:30 PM in 324-326

(THI-1117-836771) PAH volatile emissions from dredged material 2-Model development and interpretation.

thibodeaux, l ¹, fountain, k¹, price, c², ravikrishna, r¹, valsaraj, k¹, ¹ louisiana state university² us army corp of engineers

ABSTRACT- Based on measured PAH chemical fluxes to air data obtained from a pilot-scale size windtunnel containing dredged material(DM)the current state of the art mathematical model used for predicting emissions was re-formulated. A process involving the co-evaporation of water and its effect which changed the physics of the soil surface needed to be included. Based on the windtunnel fluxes three regimes are identified to describe the water dominated evaporation process. Early during the experiment, Regime-1, a standing water layer covers the soil-like DM surface and low to zero chemical flux occurs. In Regime-2 dry soil with less water(i.e., "dry patches")occurs with increasing time, producing rapidly increasing fluxes. A maximum flux occurs at complete dryness. Afterwards Regime-3 is characterized by slowly decreasing fluxes. The key factor in restructuring the model was to include the water evaporation/dry soil patch formtion process of Regime-2. A time-linear patch growth area functon was adopted which is consistent with the well known constant rate-period used for water evavpration from porous solids(It normally proceeds the well known falling-rate evaporation period.). Time duration of the constant-rate period is the one adjustable parameter added to the original model. Upon implementing the modified model it was discovered that the chemical evaporation process occuring during Regime-2 was controlled by and sensative to the numerical magnitude of the air-side mass-transfer coefficient. Careful adjustment of this coefficient within the range of values consistent with wind tunnel operations allowed good model-vs-data fitting during Regime-2. Interestingly, the soil-side resistance process dominates the magnitude and shape of the decreasing flux-vs-time data for Regime-3. Regime-1 was not modeled; it presence in the data sets was an artifact of the windtunnel operation and so is not characteristic of field operations for which the model is being developed. The presentation will conclude with a discussion on using the model for making field emission estimates from DM placed in confined disposal facilities.

Key words: chemodynamic transport model, air toxics from dredged materials, chemical vaporization from surface soils, dredge remediation related air emissions