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i RISKPRO'S SESOIL for Windows User's Guide The biodegradation process is usually a significant loss mechanism to soil systems since soil Ienvironments have a diverse microbial population and a large variety of food sources and habitats (Hamaker,1972). Many environmental factors affect the rate of biodegradation in soil, including pH, moisture content of the soil, temperature, redox potential, availability of nutrients, oxygen content of the soil air, concentration of the chemical, presence of appropriate microorganisms, and presence of other compounds that may be preferred substrates. However, ISESOIL does not consider these factors Biodegradation in SESOIL is handled as primary degradation, which is defined as any structural transformation in the parent compound which results in a change in the chemical's identity It is estimated using the chemical's rate of decay in both the dissolved and adsorbed phases according to the first-order rate equation. Pd =(C•O •kdi +S• pb•k -A•d *At (1 where: Pd = decayed pollutant mass during time step t (µg) kd, = biodegradation rate of the compound in the liquid phase (day) kd, = biodegradation rate of the compound in the solid phase (day) A = area of pollutant application (CMI) d, = depth of the soil sublayer (cm) I At = time step (day) c, 8, s, and Pb are as defined for Eqs (S) and (9) INote that c, B, and s are functions of time in the SESOIL model The use of a first-order rate equation is typical for fate and transport models and generally is an adequate representation of biodegradation for many chemicals However, due to the many factors affecting biodegradation, in some cases a first-order rate may not be applicable to the site field conditions and a zero-order or a second-, or higher-order rate might be more appropnate The biodegradation algorithm in SESOIL that is described by Eq (15) cannot handle these cases. The user is cautioned regarding the use of literature values for the biodegradation rates since I these values are quite variable and in many cases are not applicable to site field conditions. In most cases, biodegradation rates are very site-specific and uncertainty in these rates must be recognized. The user-supplied first-order decay rate constants (for moisture and solids) should Ibe values measured for the pollutant in a soil culture test under conditions similar to the site being modeled. Page 27