Q = Ps, in which Q is the flow per unit area and P is the permeability. Standard
English units for permeability have been gal/ft
2 -d at a slope of 1 ft/ft at 60
F; the
equivalent metric units would be m
3 /m
2 -d at a slope of 1/1 at 15
C. To handle large
areas, a standard field coefficient of permeability is sometimes used in which the
slope is extended to 1 ft/mi. In metric units this translates to 1 m/km. It must be noted
that the permeability P is through a unit area of cross-section. To determine the
coefficient of transmissibility, T = Pt a , in which t a is the thickness of the aquifer.
T then has units of volume/day/unit of area of the aquifer for its entire depth. This is
useful information in terms of determining the amount of water available.
It is useful to know the direction of flow of ground water and the slope. This can
be determined by the use of three or more observation wells. The water level of each
well is measured relative to some reference elevation. Frequently, mean sea level is
used, but this is not necessary. The three (or more) observation wells are plotted in
plan to scale (see Fig. 6.18). Choosing three adjacent wells, a line is drawn between
the well with the highest water level elevation and the well with the lowest water
level elevation. This line is then subdivided into equal increments assuming that for
the relatively short distance between these two points the slope is constant. This line
will then represent equal increments of slope such as meters or feet. A line is then
drawn connecting the intermediate level well to its equivalent elevation on the line
drawn between the highest and the lowest water levels. This is then considered a line
of equal elevation. Any line perpendicular to this line will indicate the direction of
flow from the highest to lowest elevation. Further if this perpendicular line is
extended on the diagram to the location of the well with the highest water level,
the distance from the well to this line can be determined. Knowing the change in
elevation and the distance, the slope can be determined as the change in elevation
with distance. In large areas where more than three wells are located, similar sets of
wells are studied and the direction and slope may be determined for each area. In
Table 6.5 Characteristics of soils
Type
Size
Porosity Permeability
Velocity
in
mm
%
gal/ft
2
-d
Quartzite, granite
0.1
Limestone and shale
5
1
Sandstone
15
700
Gravel
>0.08
1.5–8
25
100,000
1000 mi/yr
Very coarse sand
0.04–0.08
0.8–0.3
30
65,000
600 mi/yr
Coarse sand
0.02–0.04
0.5–2
35
8500
80 mi/yr
Medium sand
0.01–0.02
0.25–0.5
40
2000
20 mi/yr
Fine sand
0.005–0.01
0.05–0.25
45
80
1 mi/yr
Very fine sand
0.003–0.005 0.005–0.05 50
2
100 ft/yr
Silt
<0.003
0.005–0.05 58
1
50 ft/yr
Clay
0.005–0.05 70
0.002
1 ft/10 yr
Colloidal clay
10 Å–0.01
90
2 Â10
À5
1 ft/1,000 yr
268
D. B. Aulenbach et al.
English units for permeability have been gal/ft
2 -d at a slope of 1 ft/ft at 60
F; the
equivalent metric units would be m
3 /m
2 -d at a slope of 1/1 at 15
C. To handle large
areas, a standard field coefficient of permeability is sometimes used in which the
slope is extended to 1 ft/mi. In metric units this translates to 1 m/km. It must be noted
that the permeability P is through a unit area of cross-section. To determine the
coefficient of transmissibility, T = Pt a , in which t a is the thickness of the aquifer.
T then has units of volume/day/unit of area of the aquifer for its entire depth. This is
useful information in terms of determining the amount of water available.
It is useful to know the direction of flow of ground water and the slope. This can
be determined by the use of three or more observation wells. The water level of each
well is measured relative to some reference elevation. Frequently, mean sea level is
used, but this is not necessary. The three (or more) observation wells are plotted in
plan to scale (see Fig. 6.18). Choosing three adjacent wells, a line is drawn between
the well with the highest water level elevation and the well with the lowest water
level elevation. This line is then subdivided into equal increments assuming that for
the relatively short distance between these two points the slope is constant. This line
will then represent equal increments of slope such as meters or feet. A line is then
drawn connecting the intermediate level well to its equivalent elevation on the line
drawn between the highest and the lowest water levels. This is then considered a line
of equal elevation. Any line perpendicular to this line will indicate the direction of
flow from the highest to lowest elevation. Further if this perpendicular line is
extended on the diagram to the location of the well with the highest water level,
the distance from the well to this line can be determined. Knowing the change in
elevation and the distance, the slope can be determined as the change in elevation
with distance. In large areas where more than three wells are located, similar sets of
wells are studied and the direction and slope may be determined for each area. In
Table 6.5 Characteristics of soils
Type
Size
Porosity Permeability
Velocity
in
mm
%
gal/ft
2
-d
Quartzite, granite
0.1
Limestone and shale
5
1
Sandstone
15
700
Gravel
>0.08
1.5–8
25
100,000
1000 mi/yr
Very coarse sand
0.04–0.08
0.8–0.3
30
65,000
600 mi/yr
Coarse sand
0.02–0.04
0.5–2
35
8500
80 mi/yr
Medium sand
0.01–0.02
0.25–0.5
40
2000
20 mi/yr
Fine sand
0.005–0.01
0.05–0.25
45
80
1 mi/yr
Very fine sand
0.003–0.005 0.005–0.05 50
2
100 ft/yr
Silt
<0.003
0.005–0.05 58
1
50 ft/yr
Clay
0.005–0.05 70
0.002
1 ft/10 yr
Colloidal clay
10 Å–0.01
90
2 Â10
À5
1 ft/1,000 yr
268
D. B. Aulenbach et al.