Page 109 - Materials Chemistry, Second Edition

92 Practical Design Calculations for Groundwater and Soil Remediation

TABLE 3.5
Suggested Guidelines for Pumping Tests of Unconfined Aquifer
Predominant Aquifer Material Minimum Pumping Time (hours)
Medium sand and coarser materials 4
Fine sand 30
Silt and clay 170
Source: [5].

suggested guidelines in Table 3.5 [5]. As shown in the table, the suggested
pumping duration increases with the tightness of aquifer. A minimum of 7
days pumping is suggested for silty or clayey aquifers.

Example 3.15: Analysis of Pumping Test Data Using
the Distance–Drawdown Method

A pumping test (Q = 120 gpm) was conducted on a confined aquifer (aquifer
thickness = 30.0 ft). The distance–drawdown data (at t = 90 min) were col-
lected from three monitoring wells and shown in the following table.
Use the distance–drawdown method to determine the hydraulic conduc-
tivity and storativity of the aquifer.
Distance from the Pumping Well (ft) Drawdown, s (ft)
50 1.55
150 0.90
300 0.50

Solution:
(a) The data are first plotted on a semilog scale. (See Figure 3.3)
From the plot, we find Δs = 1.4 ft.
(b) Use Equation (3.20) to find the transmissivity:

528 Q (528)(50)
T = = = 18,860 gpd/ft
s ∆ 1.4
(c) Hydraulic conductivity can then be found as:
K = T/b = (18,860)/(30) = 629 gpd/ft 2
(d) From the plot, we find the intercept, r = 650 ft
0
(e) Use Equation (3.21) and t = 80 min = 0.0555 day to find the
storativity:

0.3 Tt (0.3)(18,860)(0.0555)
S = 2 = 2 = 0.00074
r 0 (650)