Step-drawdown pump test

(1) The efficiency of a well with respect to en­trance losses and friction losses can be determined from a step-dmwdown pumping test, in which the well is pumped at a constant rate of flow until either the drawdown becomes stabilized or a straight-line rela­tion of the time-drawdown curve plotted to a semilog scale is established. Then, the rate of pumping is in­
creased and the above-described procedure repeated until the well has been pumped at three or four rates. The drawdown from each step should be plotted as a continuous time-drawdown curve as illustrated in fig­ure C-12. The straight-line portion of the time-draw­down curves is extended as shown by the dashed lines in figure C-12, 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 C-13. If the flow is entirely laminar, the draw­down (H-h 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 well-entrance loss He, consisting of fric­tion losses at the aquifer and filter interface through the filter and through the well screen, can be deter-

Step-drawdown pump test

U. S. Army Corps of Engineers

 

Figure C-12. Drawdown versus elapsed pumping time for a step-drawdown test.

1,500

 

2,000

 

WELL DISCHARGE (Q^).GPM

 

Step-drawdown pump test

U. S. Army Corps of Engineers

Figure C-13. Drawdown versus pumping rate for a step-drawdown test.

mined from the drawdown versus distance plots for a step-drawdown pump test as illustrated in figure C-14. The difference in drawdown between the ex­tended drawdown-distance curve and the water eleva­tion measured in the well represents the well-entrance loss and can be plotted versus the pumping rate as shown in figure C-15. 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 step-drawdown test cannot be analyzed.

100

 

DISTANCE FROM WELL (D.^T

 

U. S. Army Corps of Engineers

 

Step-drawdown pump test

Step-drawdown pump test

U. S. Army Corps of Engineers

 

Figure C-15. Well-entrance loss versus pumping rate for a step-drawdown 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 C-16. 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. C-17). This plot should be a straight line and should intersect the zero

24

 

46

TIME, HR

 

72

 

96

 

(Courtesy ofUOP Johnson Division)

 

Step-drawdown pump test

Step-drawdown pump test

20 30 50

RATIO, t’/t’

(Courtesy of UOP Johnson Division)

 

Figure C-l 7. Residual dmwdown versus t’/t" (time during recovery period increased toward the left).

Подпись: T.Подпись: (C-9)

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 C-18, indicates a variance with the assumed conditions.

Step-drawdown pump test

10

RATIO, V/V

(Courtesy of JJOP Johnson Division)

 

h

IL

z*

5

О

Q

5

К

О

J

о

(/)

ш

 

Figure C-18. Displacement of residual dmwdown curve when aquifer conditions vary from theoretical conditions