FLOOD07651

• County: Statewide
• Basin: Statewide
• Title: Groundwater Pumping Tests: Design and Analysis
• Date: 12/1/1997
• Floodplain - Doc Type: Educational/Technical/Reference Information

78 GROUNDWATER PUMPING TESTS
C = (sn/Qn - snjQn-l)/(Qn-, + Qn)

STEP DRAWDOWN TEST ANALYSIS 79

where s" = increment of drawdown for pumping rate num-
ber n, in ft
Qn = increase in discharge for pumping rate number
n, in cfs
s = increment of drawdown for pumping rate num.
n-'
ber n-l, in ft
Qn-l = increase in discharge for pumping rate number
n-l, in cfs
n = pumping rate number (I, 2, or 3)

Table 4.1. Database for Example Problem 4.1

Discharge Rate Drawdown
(gpm) (ft)

100 3.35
151 5.14
199 6.86

C, ~d 2 = (S2/Q, - s,/Q,)/IQ, + Q,) (4.2)

output values of well loss coefficient for steps 1 and 2, 2
and 3, 1 +2 and 3, and 2+3 and 1. In addition, the program
calculates the well loss for the specified discharge rate.
Well loss coefficients are calculated for steps 1 + 2 and .3
and 2+3 and 1 because sometimes production wells are
unstable and clogging or development occurs between
discharge rate changes. When this happens, the value of
well loss coefficient calculated for steps 1 and 2 may be
much greater or less-than that calculated for steps 2 and 3.
Solution of Equation 4.3 may be impossible; however, the
well loss coefficient may be calculated with data for steps
1+2 and 3 or 2+3 and 1.

Increments of drawdown are determined by taking in
each case the difference between the observed water level
in the production well and the extrapolation of the preced-
ing water level curve (antecedent trend). Sn and Sn_' values
must be for the same time interval.
For steps 1 and 2:

For steps 2 and 3:
C2~d3 = (S3/Qa - S2/Q,)/(Q, + Qa) (4.3)

Example Problem

A step drawdown test was conducted with a production
well in stable condition in Example Problem 4.1. Measured
discharge rates and associated drawdowns for three steps
are presented in Table 4.1. Calculate the well loss
coefficient and well loss for the 151 gpm discharge rate
utilizing microcomputer program PTI0.
The average calculated value of well loss coefficient is 2.0
sec2/ft5 and well loss at the 151 gpm discharge rate based on
Equation 4.1 is 0.23 ft.

Values ofC for pumping test production wells are gener.
ally less than 10 sec'/ft' and are often about 2.0 sec'/ft'.
Calculation of the well loss coefficient may be impractical
with discharge rates of a few gallons per minute or less.

Well Loss Microcomputer Program

Well loss coefficients and well loss may be calculated
with microcomputer program PTI0. The interactive input
section of the program prompts the user to enter three
values of discharge rate and associated changes in
drawdown and a value of discharge for which well loss is to
be calculated. The program generates and displays as