The general comments in the editorial suggest a misunderstand- ing regarding a fundamental difference between the approach to this method developed by Hurst and Schmidt , which they called the middle distillate degradation MDD model, and our interpretation based on the basic Monod model for microbial substrate utilization kinetics Galperin and Kaplan, Because of the empir- ical nature of this correlation and the absence of descriptions of certain environmental parameters, no objective criteria can be developed on acceptable values of these parameters i.
Evaluation of the MDD model reveals inconsistencies in its experimental base and theoretical premise Galperin and Kaplan, a. For example, be- cause the Monod model relates the rate of substrate metabolism to the concentration of a single growth-controlling substrate Alexander, , it implies an adequate supply of oxygen and nutrients, so that they do not limit bacterial metabolism.
In contrast, a much slower degradation occurs under predominantly anaerobic conditions that commonly develop in the groundwa- ter saturated zone or in a body of non-aqueous phase liquid NAPL. The unsaturated zone soil water content is also a major fac- tor affecting hydrocarbon biodegradation. In practice, however, soil water content has been found to directly limit biodegradation rates only in very dry desert environments.
Kaplan degradation is severely inhibited. Residual NAPL concentration in the vadose zone soil should be considered as being close to the upper concentration limits of applicability of the model. In order to sustain microbial growth, certain nutrients must be available at minimum levels. Primary nutrients, nitrogen and phosphorus, are required in the greatest concentrations and are the most likely to be limiting, whereas the rest are considered micronutrients and generally are available in excess in soil.
However, because nutrients are not destroyed, but recycled in an ecosystem, un- like O2, a steady input of nutrients is not required Leeson and Hinchee, Attempts to use this model outside of its applicability lim- its lead to mistakes in the calculated age of the fugitive fuel in the subsurface environment Stout et al.
Method for determining the age of diesel oil spills in the soil. Ground Water Monitoring and Remediation 23 4: Zero-order kinetics model for the Christensen-Larsen method for fugitive fuel age estimates. Ground Water Monitoring and Remediation 28 2: Environmental Geosciences 15 2: Environmental Geosciences 12 3: Age dating of environmental organic residues.
Environ- mental Forensics 4 3: Principles and practices of bioventing. Age dating heating-oil releases, Part 2. Assessing weath- ering and release time frames through chemistry, geology and site history. Environmental Forensics 10 2: Invited commentary on the Christensen and Larsen technique.
En- vironmental Forensics 3 1: Environmental Forensics 3 1: