Installation — business terrible  1 part
September 8th, 2015
(1) The efficiency of a well with respect to entrance losses and friction losses can be determined from a stepdmwdown pumping test, in which the well is pumped at a constant rate of flow until either the drawdown becomes stabilized or a straightline relation of the timedrawdown curve plotted to a semilog scale is established. Then, the rate of pumping is in
creased and the abovedescribed procedure repeated until the well has been pumped at three or four rates. The drawdown from each step should be plotted as a continuous timedrawdown curve as illustrated in figure C12. The straightline portion of the timedrawdown curves is extended as shown by the dashed lines in figure C12, and the incremental drawdown AH’ for each step is determined as the difference between the plotted and extended curves at an equal time after each step in pumping. The drawdown H’ for each step is the sum of the preceding incremental drawdowns and can be plotted vesus the pumping rate as shown in figure C13. If the flow is entirely laminar, the drawdown (Hh for artesiun flow and H2—h2 for gmvity flow) versus pumping rate will plot as a straight line; if any of the flow is turbulent, the plot will be curved.
(2) The wellentrance loss He, consisting of friction losses at the aquifer and filter interface through the filter and through the well screen, can be deter

Figure C12. Drawdown versus elapsed pumping time for a stepdrawdown test.




U. S. Army Corps of Engineers
Figure C13. Drawdown versus pumping rate for a stepdrawdown test.
mined from the drawdown versus distance plots for a stepdrawdown pump test as illustrated in figure C14. The difference in drawdown between the extended drawdowndistance curve and the water elevation measured in the well represents the wellentrance loss and can be plotted versus the pumping rate as shown in figure C15. Curvature of the Hw versus Qw line indicates that some of the entrance head loss is the result of turbulent flow into or in the well.
b. Recovery test.
(1) A recovery test may be made at the conclusion of a pumping test to provide a check of the pumping test results and to verily recharge and aquifer boundary conditions assumed in the analysis of the pumping test data. A recovery test is valid only if the pumping test has been conducted at a constant rate of discharge. A recovery test made after a stepdrawdown test cannot be analyzed.






Figure C15. Wellentrance loss versus pumping rate for a stepdrawdown test.
(2) When the pump is turned off, the recovery of the groundwater levels is observed in the same manner as when the pump was turned on, as shown in figure C16. The residual drawdown H’ is plotted versus the
ratio of log t’/t", where t’ is the total elapsed time since the start of pumping, and t" is the elapsed time since the pump was stopped (fig. C17). This plot should be a straight line and should intersect the zero







Figure Cl 7. Residual dmwdown versus t’/t" (time during recovery period increased toward the left).
residual drawdown at a ratio of t’/t" = 1 if there is normal recovery, as well as no recharge and no discontinuities in the aquifer within the zone of drawdown. The ratio t’/t" approaches one as the length of the recovery period is extended.
(3) The transmissibility of the aquifer can be calculated from the equation
2640;
As’ where As’ = residual drawdown in feet per cycle of (log) t’/t" versus residual drawdown curve. Displacement of the residual drawdown versus (log) ratio t’/t" curve, as shown in figure C18, indicates a variance with the assumed conditions.





