Appendix G. Chemical-specific Soil Half-life G.1 Algorithm for Estimating Chemical-specific Soil Half-life

The average concentration of a substance in soil (Csoil) is a function of several different variables, including deposition rate, accumulation period, mixing depth, soil bulk density, and the chemical-specific half-life, as shown in equation G-1 below: Csoil = [GLC (Dep-rate) (86,400) (X)] / [Ks (SD) (BD) (Tt)] (Eq. G-1) where: Csoil = average soil concentration at a specific receptor location over the evaluation period (µg/kg) GLC = ground level concentration from the air dispersion modeling (µg/m3) Dep-rate = vertical rate of deposition (m/sec) (see Chapter 2 for values) 86,400 = seconds per day conversion factor X = integral function accounting for soil half-life Ks = soil elimination time constant = 0.693/T1/2 SD = soil mixing depth = 1 cm for dermal scenario BD = bulk density of soil = 1333 kg/m3 Tt = total averaging time = 70 years = 25,550 days The soil half-life is part of the integral function X determined as below: X = [{Exp (-Ks ×Tf)- Exp (-Ks ×To)} / Ks] + Tt (Eq. G-2) where: EXP = Exponent base e = 2.72 Ks = soil elimination constant = 0.693 / T1/2 T1/2 = chemical-specific soil half-life Tf = end of exposure duration (days); 25,500 for a 70-year exposure T0 = beginning of exposure duration (days) = 0 days Tt = total days of exposure period = Tf- T0 (days) Estimating toxicant soil concentration is necessary for estimating dose from incidental soil ingestion by home raised meat, home raised produce, and dermal absorption via contact with contaminated soil. Since the chemicals that the Hot Spots program is concerned with are emitted into the air and then subject to deposition to the soil, there are only two classes of chemicals considered. These classes are the semivolatilve organic chemicals, such as PAHs, PCBs and dioxins, and toxic metals such as hexavalent chromium, cadmium, lead