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Work Plan for Refined Plume Definition and Management of Floating Product-7500 W 11th St., Tracy, CA. Page 40 the hydraulic gradient ,.� A =the cross-sectional area of flow Typically, the natural hydraulic, or groundwater gradient, at a site is very small (e.g., at the ... 7500 West Eleventh Street site, it is 0.003 fl/ft.). Although, as is shown in Equation 1, there is only a linear relationship between the volume of flow and gradient, if, when groundwater or floating product is pumped at an extraction point, the gradient of flow in the direction of the extraction point can be significantly increased and the rate of flow will increase in direct proportion. �- For example, if a floating product extraction system were designed so that groundwater is drawn down 5 feet at the point of extraction and the zone of significant influence of the system extends to a distance of 50 ft., then the mean groundwater gradient in the direction of •- the extraction point would be 0.1 ft/ft., which is some 33 times greater than the natural groundwater gradient at the 7500 West Eleventh Street site. Such a gradient over a zone of influence of that size would overwhelm the unperturbed groundwater flow rate and gradient '* and cause LNAPL to flow to the extraction point at a high rate while, at the same time, capturing migrating LNAPL across a wide front of an advancing plume. 9.3.1.1 Dual Floating Product Extraction/Groundwater Draw-down Systems ✓ Unfortunately, floating product extraction systems based on designs that skim LNAPL from `r the surface of the groundwater by such techniques as installation of floating ejector pumps in extraction wells have very little influence on the groundwater gradient in the vicinity of the extraction point and a very limited zone of influence. This limitation can be overcome by use of a dual system, whereby a significant gradient of flow in the direction of the extraction point over a significant zone of influence can be achieved by pumping groundwater from beneath the LNAPL floating on the water table to lower the groundwater table at the point of extraction while, at the same time, a second pump skims floating product from the water surface. At sites where the LNAPL to be removed are volatile, such as is the case of gasoline, the dual system can be operated in a continuous mode by temporarily storing the evacuated `^ floating product on-site and treating the contaminated groundwater that is pumped to the surface by air-stripping with activated carbon polishing or other established treatment techniques. However, at sites such as that at 7500 West Eleventh Street, where the fuel hydrocarbons affecting the groundwater include significant proportions of non-volatile materials, such as diesel, the arrangement described above for volatile materials is not useful because dissolved components of non-volatile chemicals cannot be air-stripped from groundwater. Where LNAPL is present, the concentrations of those chemicals in the groundwater is high, so that treatment by activated carbon filtering is not economically feasible because the high rate of mass transfer of the fuel compounds quickly exhausts the absorption capacity of the activated carbon. sic