EA 2003
Ground water and aquifer properties
J. M. Stroh
Thurston County has an abundance of ground water in glacial drift.
What are the important properties of these aquifers for water yield?
Varies by major aquifer type and aquifer properties.
Confined
Unconfined
Porosity
Permeability
Homogeneity
Hydraulic conductivity
Various terms for yield and storage.
Transmissivity is the hydraulic conductivity times the aquifer
thickness.
T = Kb
Storativity is the storage coefficient, the volume of water that a
permeable unit will absorb or expel from storage per unit surface
are per unit change in head (dimensionless).
In confined aquifers yield comes from the compressibility of water and
changes in volume of the aquifer skeleton. The hydraulic head changes
but the aquifer remain full of water.
In unconfined aquifers yield comes also comes from the
compressibility of water and changes in volume of the aquifer skeleton,
but also from drainage from the pores. As the hydraulic head changes
the saturated zone diminishes.
How can the transmissivity and the storativity be measured? Measured
by determining how the hydraulic head changes as water is removed
from the aquifer. (Called a drawdown test.)
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Simplest case, confined aquifer with special properties of the aquifer and
the wells.
Aquifer:
Isotropic and homogeneous with perfect confining layers
Horizontal
Infinite
Darcy's law holds
Water flows to the wells radially, parallel to the confining layers,
laminar flow
Wells
Infinitely small diameter
Completely penetrate the aquifer
Water removed without consumption of frictional energy
Test involves a pumping well of known, constant, discharge and a
monitoring well a know distance away for measuring hydraulic head.
Solution given by the Theis equation.
The assumptions, which are not particularly realistic, yet the technique
often yields realistic values of T and S.
Also note the principles; pumping at a constant rate removes water near
the well from elastic storage, The hydraulic head near the well must
decrease to provide the potential energy difference for water to flow to
the well. Given the Theis assumptions this infinitely small diameter well
(to avoid storage problems) will continue to change the head farther and
farther away from the well. This situation never reaches equilibrium.
Keeping in mind the violation of assumptions; how can T and S be
determined from discharge, drawdown and time?
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No simple, exact, algebraic solution, need to do by a variation of trial
and error, matching a function curve to the drawdown data curve.
Theis equation:
Q
(h0  h) 
W (u )
4 T
u
e
W (u )  
du
u u
u
n
e

(1) n  (ui ) n 

u u du  0.57721566  ln ui  
n  n! 
i 1
r 2S
u
4Tt
Example well pumped 220 gpm from aquifer 48 feet thick. Monitoring
well 824 feet away. Drawdown change in hydraulic head (feet)per unit
time (minutes).
Data plotted on 3 by 5 cycle log paper. Theis type curve (solution to the
well function W(u) matched to the drawdown curve on log paper.
For W(u) = 1 and u = 1 match is drawdown = 2.4 feet and time = 4.1
minutes.
Q = 42400 ft3 per day (from 220 gpm).
Rearrange Theis equation to T 
Q
W (u )
(h0  h)4
Know W(u) from match = 1 and (h0 - h) = 2.4 ft.
Find Transmissivity
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Q
42400 ft 3d 1
T
W (u ) 
 1  1410 ft 2 d 1
(h0  h)4
2.4 ft 4
since S 
4Tut
and
r2
1/u = 1 so u =1 and t = 4.1 min = 2.85 x 10-3 day r = 824 ft (given).
4  1410 ft 2 d 1 1  0.00283d
S
 2.4  105 (no dimension)
2
(824 ft )
Rounding T = 1.4 x 103 ft2/day. S = 2.4 x 10-5, an K = 29 ft/day.
Since K = T/b and b = 48 ft (given) K = (1.41 x 103 ft2/day)/48 ft = 29.4
ft/d.
K = 6.2 x 10-1 cm s-1. Well sorted sand or gravel, could be outwash.
Given the strange assumptions how well does the Theis equation work
for determining aquifer properties? Very well, if the wells are completed
so they penetrate the aquifer.
Works best for determining T and S near the well.
In Thurston County few aquifer tests, but those completed usually show
very large T (hydraulic conductivity). Partly due to test being performed
on large production wells.
Other uses of data include well interference studies. "Hey your pumping
dried up my well."
Main point, aquifer properties can be determined with a properly
designed test.
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