Page 108 - Materials Chemistry, Second Edition

Plume Migration in Aquifer and Soil 91

Use Equation (3.19) to find the storativity:

0.3 (0.3)(18,860)(0.0031)
S = Tt 0 = = 0.00078
r 2 (150) 2

Discussion:
1. At t = 7 min (0.00486 day) and r = 150 ft, u is equal to

2
2
1.87 rS 1.87(150 ft)(0.00078)
u = = = 0.36
Tt (18,860 gpd/ft)(0.00486 day)
2. At t = 50 min, u will be smaller than 0.05.

3.4.3 Distance–Drawdown Method
It can be seen from Equation (3.17) that, at any specific time (t = constant), s
varies linearly with log[(constant)/r ]. Based on this relationship and simul-
2
taneous drawdown measurements in at least three observation wells, each at
a different distance from the pumping well, a semilog distance–drawdown
graph can be constructed. From the plot, the slope, Δs (the change in draw-
down per one log cycle of distance), and the intercept, r , of the straight line
0
at zero drawdown can be derived. The following relationships can then be
used to determine the transmissivity and storativity of the aquifer:

528 Q 0.366 Q
T = (AmericanPractical Units) = (SIUnits) (3.20)
s ∆ s ∆

0.3 Tt 2.25 Tt
S = 2 (AmericanPractical Units) = 2 (SIUnits) (3.21)
r 0 r 0

where Δs is in ft or m, r is in ft or m, and the other symbols represent the
0
same terms as in Equation (3.15).
The three methods described here for analysis of pumping test data are
mainly for confined aquifers. Extraction groundwater from an unconfined
aquifer is more complicated. The extracted water comes from two mecha-
nisms: (1) water from the elastic storage due to the decline in pressure, as in
the case of the confined aquifer, and (2) water from drainage of the declin-
ing water table. There would be three distinct phases of time-drawdown
relations in unconfined aquifers. As time progresses, the rate of drawdown
decreases and flow becomes essentially horizontal (when the effects of grav-
ity drainage become much smaller). The time-drawdown data can then be
analyzed using the three methods described previously [1]. A more practi-
cal approach is to ensure that the duration of the pumping test exceeds the