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.)
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?
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:
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
Know W(u) from match = 1 and (h0 - h) = 2.4 ft.
Find Transmissivity
since and
1/u = 1 so u =1 and t = 4.1 min = 2.85 x 10-3 day r = 824 ft (given).
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.
Theis equation example.doc- 1 -