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5tantec • • April 13, 2009 Page 11 of 13 Reference: Response to RWQCB March 11, 2009 Letter Similar stoichiometric relationships are developed in the U.S. Environmental Protection Agency guidance document Bioventing Principles and Practice, Volume 11: Bioventing Design, dated 1995. The mass of aquifer contaminants in proximity to well TS/MW-2D can be estimated with conservative assumptions. Assuming a cylindrical volume with a height of 15 to 20 feet, a radius of 20 to 40 feet, maximum recent historical concentrations of TPHg, TPHD, BTEX and MTBE, and a 30 percent porosity, the mass of dissolved contaminants would range from approximately 5.4 to 29 pounds of hydrocarbons. The details of these calculations are shown in Attachment 3. These relations and mass estimates illustrate the feasibility of the pilot study approach. 9 The site is equipped with two ozone generation systems which the STTC claims has effectively reduced downgradient contaminant concentrations. One generator reported a 100 percent runtime for the fourth quarter. STTC needs to explain why use of this technology or the existing system was not presented as an option for the TS/MW-2D pilot study. A number of reasons exist for not including ozone injection in general or the existing system in particular as an option for the TS/MW-2D pilot study. The most critical reasons are system limitations in the design of ozone generators. Ozone is an extremely metastable compound with a half-life measured in seconds. This half life is strongly pressure dependent, decreasing rapidly with depth. A significant amount of energy is necessary to generate ozone and systems are designed with a maximum pressure generation potential of 40 PSI. As discussed above, with an approximate increase in hydrostatic pressure with depth of 4.3 PSI per 10 feet of water column, this correlates to a depth of approximately 93 feet bgs. At this depth, ozone is being created at about the same rate as it degrades into oxygen. This also explains the rapid increase in cost for installation of ozone-sparging systems with increasing depth due to the much smaller effective radius of influence. Aside from this technical limitation, the current number of injection wells in the ozone arrays are essentially at the maximum load the current ozone generators are able to handle. Stantec is currently procuring updated systems with higher well capacities. Stantec may also consider an approach involving a phased implementation of oxygen delivery. This could potentially allow use of bottled oxygen delivery methods until the strongly reducing environment is brought up to aerobic conditions and appropriate microbial communities have developed. The new ozone system being purchased has the capacity to produce and deliver molecular oxygen as well. This approach would require additional piping, trenching, and other implementation costs as well as evaluation of other technical considerations and a cost benefit analysis. 10 STTC needs to specify the criteria that the STTC will use to determine whether the frequency of biodegradation parameter data collection should be decreased from monthly, for the first three months, to quarterly as the test progresses. I:\STTC-Stockton\Reports\Response to CommenWRWQCB Letter 3-11-09\STC Response to RWQCB 3-11-09 Letter Final.00c