Geology Reference
In-Depth Information
Exercise 12 • Groundwater Hydrology
201
average groundwater velocity in the vicinity of the eastern
flow line! Show your work.
following formula is used to determine groundwater
velocity in ft/day, where
n
e
is unitless and given as a
decimal (i.e., 10 percent = 0.10), and
I
is in ft/ft.
3
v
(ft/day) =
[K
(ft/day)
I
(ft/ft)]/«
e
Knowing the amount of groundwater moving in
an aquifer under a property may be of interest for
resource development. The quantity of groundwater (Q),
in cubic feet per day (ft
3
/day) or cubic feet per second
(cfs, if multiplied by 0.0000115), that passes through a
cross-sectional area of an aquifer can be determined by
means of
Darcy's Law.
4.
On Figure 12.3, construct a groundwater flow line downs-
lope from each of sites A, B, and C. (See the instructor for
other possible sites).
=3
5. a.
If gasoline were spilled at A, would it discharge with
groundwater directly into the ocean? Explain.
Q
(ft
3
/day) =
K
(ft/day)
A
(ft
2
)
I
(ft/ft)
where
A,
the cross-sectional area through which
flow occurs in ft
2
, is equal to the width of the
aquifer times its saturated thickness. Darcy's
Law shows that the quantity of flow increases
with an increase in
K, A,
or
I.
b.
If gasoline were spilled at B, would it discharge with
groundwater directly into the ocean? Explain.
These two equations, for the velocity and quan-
tity of groundwater flow, are useful for estimating the
movement and potential availability of water in an
aquifer. More sophisticated computer models, which
account for geologic variations in the subsurface, are
used by professionals to predict groundwater flow.
The velocity equation, based on Darcy's Law, may not
apply where: (1) groundwater flows through low
hydraulic conductivity materials at low gradient, and
(2) turbulent flow occurs through large fractures or
openings. The following questions are based on the
conditions shown in Figure 12.3.
c.
If gasoline were spilled at C, would it discharge with
groundwater directly into the ocean? Explain.
6.
In the western part of the aquifer, at and down gradient
from D, the hydraulic conductivity is 100 feet per day and
the effective porosity is 30 percent.
a.
What is the average velocity along the flow line from
D? Show your work.
QUESTIONS 12, PART A
1.
What is the average water-level gradient or slope along
the eastern flow line of Figure 12.3 between the 200-ft and
50-ft contour? Give your answer in ft/ft and ft/mi and show
your work.
b.
If the velocity of the groundwater is assumed to also rep-
resent the movement of the contaminant, what is the time
required for gasoline spilled at site D to travel to the end of
the flow path? Show your work (time-distance/velocity D).
2.
If an aquifer near the floodplain in the eastern part of
Figure 12.3 is 30 ft thick and has a porosity of 10 percent,
how much water is
stored
in a 0.1 mile by 0.1 mile area of the
aquifer? Give your answer in cubic feet and gallons; show
your work (see Appendix A for Conversions).
7. Using Darcy's Law, what is the quantity (Q) of groundwa-
ter flowing horizontally through a 2ft X 2ft square of an
aquifer with
K =
180 ft/day and an hydraulic gradient of
1 ft/1000 ft?
9
b3
3.
If the hydraulic conductivity
(K)
of the aquifer is 150
ft/day, and the effective porosity (n
e
) is 15%, what is the
