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Human Health Risk Assessment <br /> Former Mobil Oil Bulk Plant 04-343 <br /> 500 East Grant Line Road <br /> t; Tracy, California <br /> PHYSICAL AND CHEMICAL PROPERTIES WHICH AFFECT <br /> VOLATILE EMISSIONS FROM SOIL AND GROUND WATER <br /> OPERTIES EFFECT <br /> iffusion Coefficient Compounds with high diffusion coefficients are emitted at higher rates than those with lower diffusion <br /> coefficients via increased relative transport. The overall diffusion coefficient may be comprised of <br /> diffusion through the soil-water interface, water-air interface, pore-water, and pore-gas. <br /> olecular Weight Compounds with lower molecular volume typically have higher volatilization and diffusion <br /> coefficients. Low molecular volume is generally reflected by lower molecular weight. Therefore. <br /> molecular weight is used to determine diffusion rates in some predictive models. <br /> �enry's Law Constant Henry's Law Constant provides a measure of the extent of chemical partitioning between air and a <br /> liquid solution, such as water,at equilibrium. The higher the Henry's Law constant, the more liken <br /> a chemical is to partition to the vapor phase. <br /> rosity Porosity is one of the controlling factors for chemical diffusion through a porous media such as so' <br /> I� p soil. <br /> Emission rates typically increase with increasing soil porosity. Total porosity (i.e., dry soil) may <br /> represent worst-case conditions for predictive models. Air-filled porosity, as opposed to total <br /> porosity, takes into account the natural moisture content of the soil. <br /> 1rption/Absorption <br /> 'erties of Soil <br /> Soil with high sorption properties will increase the amount of chemical adsorbed to soil particles and, <br /> therefore, reduce the vapor density of the sorbed compounds and thus, the emission rate. However, <br /> the model does not account for the adsorptive capacity of soil, and most likely, overestimates the <br /> vapor density in the subsurface. <br /> it Moisture Soil moisture may draw waste constituents to the surface through the soil pores. This process can <br /> increase the emission rate by increasing the concentration of the constituents at the surface. <br /> iXVlck Effect The Wick Effect refers to soil pore continuity. Increased soilore continuity y increases the emission <br /> rate. <br /> 1 <br /> 30-0136-11 <br />