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Work Plan for Refined Plume Definition and Management of Floating Product-7500 W 11th St., Tracy, CA. Page 44 N%"01 b = thickness of confined aquifer, in ft. �+ R — radius of the cone of depression, in ft. r = radius of the well, in ft. As can be seen in the equation, well yield is a function of the Iogarithm of the radius of the well. This relationship explains why, as is well known to groundwater-supply contractors, the diameter of the well has only a marginal influence on the rate at which it can be pumped. In fact, when sizing wells, designers only concern themselves with the minimum casing diameter required to house adequately down-hole pumps and other equipment. However, due 'r to the logarithmic nature of the relationship between well yield and well radius, if the well radius were to be extremely large - say on the order of several feet - its influence would be considerable. This effect can be illustrated by consideration of an example based on the hydraulics of hypothetical wells installed in an aquifer of generally similar dimensions to that beneath the Navarra Site. If the parameters, properties and dimensions of an aquifer and extraction well system were as follows: K is 10 gpd/ft2, H is 15 ft., h is 10 ft., R is 100 ft., then, for a 2-in. diameter well (i.e., r = 0.08 ft.), the well yield would be 0.34 gpm. If the well diameter were doubled in size to 4 in., so that r = 0.17 ft with all other parameters held constant, the well yield would increase only 11.76%, to 0.38 gpm. Whale it is recognized that there are, of course, interdependent relationships between well yield, the radius of the cone of depression and well draw-down, for the purpose of illustration, one can hold the other parameters constant while continuing to increase the diameter of the well. For the same example, redoubling the well diameter to 8 in. (i.e., 0.33 ft.), the well yield rises to 0.44 gpm, an increase of 14.54% over that for the 4-in. diameter well. If the well reached a diameter of 2 ft., the well yield would still only amount to 0.56 gpm. It is not until the "well" diameter increases to the size of tens of feet that the well yield increases to a rate that is sufficiently large compared to that for a 2- in.-diameter well to be of any practical significance. For example, for the same set of other parameters as above, a well diameter of 40 ft. yields 2.38 gpm. Of course, wells of that diameter are not constructed except for heavy civil engineering construction projects when they are called coffer dams. However, the examples cited above do show that significant improvements in extraction rate for a given draw-down, which are, in turn, associated with zones of influence of large dimension, can be achieved by use of large excavations such as pits or trenches, where such i. can be used instead of extraction well systems. 9.3.3.2 Effect of Construction Method on Well and Excavation Hydraulic Efficiency Another factor that tends to limit the effectiveness of extraction systems that are based on skimming LNAPL from small-diameter wells as a means of extracting floating product from groundwater is the condition of the boring wall. When well borings are drilled using the majority of available drilling methods, particularly hollow stem auger technology, in clayey sic