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(Td To is the time value at which the normalized displacement data is equal to a <br /> value of 0 37 This value is determined by drawing a horizontal line across the graph <br /> ' from a point on the y-axis equal to 0 37 The initial data points on the graph <br /> represent the response of the filter pack to the slug test, and not the aquifer Once <br /> the normalized drawdown versus time curve is intercepted, a perpendicular to the line <br /> ' for which y = 0 37 is drawn, intersecting the x-axis The time value of the intercept <br /> on the x-axis is the time value for To The graph of MW-9 Rasing Head 2, <br /> demonstrates the practice of determining the basic lag time <br /> If the length of the screen is more than 8 times the radius of the well screen, <br /> then the following formula applies <br /> r2 In L X 1,440 min/day <br /> K = 2LTo <br /> Where <br /> ' K = hydraulic conductivity (ft/day) <br /> r = the inside radius of the riser pipe or screen <br /> To = the basic lag time, as described above <br /> L = the length of the screened section below the water table <br /> R = the effective radius of the screen <br /> [r2 + p(rw2 - r2)11/2 <br /> ' p = the porosity of the sandpack <br /> rw = the radius of the sandpack <br /> 1 <br />