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++ Work Plan for Refined Plume Definition and Management of Floating Product-7500 W 11th St., Tracy, CA. Page 39 Well MW-7, but, for the reasons discussed in Section 9.3, its reappearance in significant thickness may be long delayed. 9.2.5 Cost of interim Measures The interim floating product management measures being performed at present cost approximately $500.00 per round of product purged from Monitoring Well MW-7. With an estimated total of very much less than two gallons of LNAPL removed after four rounds spread over a period of six weeks, that amounts to a cost on the order of$1,000 per gallon or +,. more. In SJC's professional opinion, such an exorbitant unit cost further supports our experience •- that remediation of floating product from small-diameter boreholes is not a practical solution. 9.3 Hydrogeologic and Engineering Design Issues Related to LNAPL Remediation r.. There are several key factors that must be considered when preparing a site-specific design for a floating product removal system. Those factors are discussed below. 9.3.1 Zone of Influence and Groundwater Drawdown Gradient To successfully remove LNAPL floating on the groundwater in the subsurface beneath a site, it is necessary to design a system that is capable of drawing floating product to an extraction point from a sufficiently large area to ensure that the system captures all floating product that is slowly migrating down the groundwater gradient as it floats on the water table. The system must extract floating product from a sufficiently large "zone of influence," (i.e., the area in any direction over which the extraction system causes floating product to flow toward the point at which it is being evacuated) that down-gradient flow of LNAPL does not bypass the extraction system. The zone of influence of an LNAPL-extraction system is affected by the natural groundwater flow rate and gradient and the hydraulic conductivity of the soil at the specific site, as well as the system-induced gradient of flow towards the point of extraction. The rate at which LNAPL can be extracted from the subsurface is also influenced by the magnitude of the induced gradient within the zone of influence. That gradient must be steep `r if LNAPL is to be removed at a satisfactory rate. The importance of generating deep groundwater gradient to invoke flow toward and `+ extraction point can be visualized by consideration of the Darcy equation of groundwater flow (Darcy 1856), viz: Q=KiA Equation I where Q =the rate of groundwater(or product) flow K=the permeability(either hydraulic or product permeability) of the soil sic