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CLOSURE REQUEST 5-31-94 a. re is the equivalent radius of the well taking in account the effect of the gravel pack on the volume of the well . b. re = the square root of ( ( (1-n) x rc2) + (n x rw2) ] , n being the porosity of the gravel pack. This is usually around 30 percent Le = The length of screened well below the water table a. Le usually = Bottom well (bw) -- Depth to water (dw) , but in some cases it may be less than this if the well is screened only over a portion of the aquifer. Lw = The length of casing below the water table . h = Aquifer thickness, the distance from the water table to a aquitard in an unconfined aquifer. y = the draw-down = the depth to water in the bailed well from the original depth of water Re = The effective radial distances over which the draw-down y is dissipated by the formation. Re is not directly determinable . • Bouwer and Rice provided a graph of A, B, and C values for the calculation of ln (Re/rw) from a known value of Le/Rw. The values derived from the chart depend on the well geometries . If Lw is less than h, the following equation is used• In (Re/rw) _ [ (1 1/ln (Lw/rw) ) + (A+Bln ( (h-Lw) /rw] ) / (Le/rw) ] -1 If Lw equals h, the following equation is used ln (Re/rw) = f (1 . 1/ln (Lw/rw) + C/ (Le/rw) ] -1 To find the y' s to use in the formula, the draw-down y was plotted on the log scale of semilog paper against recovery time (t) The straight line portion of the graph is used to determine the yi and y2 with t being the difference between the times of the two Y' s . if there is a problem from gravel pack drainage, the graph will show two straight line portions . The second, less steep straight line is used Once the "K" values were computed, they were used to classify the formation type and to assign an effective porosity to each well (by assuming effective porosity is similar to specific yield, as found on page 8, and by using the table on page 12 of "Hydraulic Conductivity of Selected Rocks" in Basic Ground Water • Hydrology, United States Geological Survey Water Supply Paper 2220, 1983) pagel8