Everything you need is in this document! Submit your answers by Friday at noon! I have removed a section since this was posted a little later than I had hoped.
DETERMINING
GROUND-WATER CONTAMINATION
Adapted from PL Garvin Cornell College for AGI by NP Flynn Ph.D., & Laura Toran Ph.D., PG, Temple University
OBJECTIVE:
To determine, by preparing and analyzing contour maps, which way a plume of
contaminated ground water will move, which drinking water wells will be affected, and
how long it will take the contaminants to reach the wells. Determine the influence of a
cone of depression on surrounding wells.
MATERIALS:
Calculator, pencils and 2-3 color pencils.
SKILLS:
Understand how to determine the depth of the water table from well data.
Distinguish the zone of aeration from the zone of saturation.
Understand porosity and permeability and how they relate to the transportation of
ground water.
Recognize the connection between the water table, ground water and surface water
Draw equipotential contour lines from well data.
Using the contour lines determine and draw flow direction lines.
Determine well contamination potential.
Calculate Darcy’s velocity when given hydraulic conductivity.
Modify the Darcy velocity when conductivity and slope vary.
Recognize how factors such as sorption and dispersion influences water/contaminant
movement.
What factors influence hydrologic conductivity?
Create a cone of depression and modify ground water flow and contaminate directions.
Research the different types of contaminants and determine how this would alter the
scenario.
Figure 1. The boundary between unsaturated soil and saturated soil (Ground water)
represents the water table. Note the connection to surface water.
Figure 2. Various levels of porosity in rocks types. a) Well sorted sedimentary with low
porosity. b) Poorly sorted sedimentary with low porosity. c) Well sorted sedimentary
rock with cement. d) Well sorted sedimentary rock with cement. (Note: cementation
lowers porosity in c & d) e) rock rendered porous by solution. f) rock rendered porous by
fracturing (altered after Meinzer, 1923).
PROCEDURE:
Underlying a military base in northeastern Michigan is a shallow sand gravel aquifer, a
subsurface layer that is permeable enough to conduct ground water and to yield water
readily to wells and springs. The water table lies between 10 and 25 feet below the
ground surface. A leak in a buried storage drum has allowed a toxic organic liquid to
enter the aquifer. This contamination is a potential threat to drinking water supplies on
the base.
Table 1. lists the ground-surface elevation and depth to the water table for 55 wells on
the military base. English units are used rather than metric units, because ground
elevation data are given in feet. Conversion to metric units gives fractional elevation
data that are more difficult to use. The wells are drilled for various purposes, and their
locations are shown on the map of the base (Worksheet 2.). Note: There are different
types of wells. This lab focuses on production and monitoring wells. (Figure 3.)
Figure 3. Three types of wells.
PROCEDURES:
1. Using the data in Table 1, calculate the elevation and depth of the water table at each
well by subtracting the depth of the water table from the elevation of the well at the
ground surface (column 2 minus column 3). We have left the first 5 for you to calculate.
The remainder are recorded for time purposes. Record your answers in Table 1.
2. On the map (last page of this document) or on a tracing overlay. Carefully plot, in
pencil, the elevation of the water table at each well. Wells Y5, and R84 have been
recorded for an example. Write small and clear and as close to the dot (represents the
actual well).
3. Using pencil, contour the water table elevation on the map or overlay. Using a
contour interval of one foot each. The contour lines you have drawn are called
equipotential lines and show the general location of the water table. Equipotential lines
must increase/decrease in order and can never cross each other. Once you have
checked for errors, color over the lines with a colored pencil. Contour elevations 604,
603, 602, and 588 have been drawn as an example.
4. The direction of ground water flow is generally perpendicular to the equipotential
lines, moving from higher to lower elevations. They are not necessarily straight lines, as
water will flow in a curved path as long as it is moving generally down elevation. Using a
second contrasting color pencil, draw arrows (flow lines) at several places on the map to
show the directions of the ground water movement. Figure 4 is an example of how
curved groundwater flow lines can look.
Figure 4. Equipotential lines and ground water flow lines perpendicular.
QUESTIONS:
1. Based on the direction of ground water movement, if the storage drum marked with a
DOT is leaking, which of the drinking wells is likely to be contaminated by the plume, or
outflow, from the leaking storage drum?
The leaking storage drum will quickly spread to a width of 500 feet in the vicinity of the
well. Shade in the area of the pollution plume. Assume no interaction (a noted
oversimplification).
2. The velocity of ground water movement can be determined from
Darcy’s Law: V= -Ki/n.
This equation shows that the velocity (V) of ground water is a product of the horizontal
hydraulic conductivity (-K) of the underlying rock material (look up several factors) and
represents the ease with which the water can flow between the spaces, horizontal
hydraulic gradient (i) this is effectively slope, change in elevation over distance, and
effective porosity (n). Hydraulic conductivity is a measure of soil permeability. Velocity
is a function of distance/time. Such as feet/day or cm/sec. and represents the rate at
which groundwater can move through the aquifer.
For this lab activity the hydraulic conductivity (-K) is 100 feet/day. Figure 5. shows the
conductivity of several materials. A porosity of n=0.33 will be used.
3. Determine the horizontal hydraulic gradient between the storage drum and the
threatened well (in feet/mile). Note: 1 mile = 5,280 feet.
4. Calculate the velocity of ground water flow from the storage drum to the well
(feet/day).
5. Using the formula time= distance/velocity, determine how long it will take the
contaminants to reach the well, assume no sorption or dispersion of contaminants.
6. Now let’s vary the factors involved in the velocity of the GW movement. Let’s reduce
(-K) to 10 feet/day. How does this change the velocity? And let’s lower the porosity to
n=0.16. How does this influence the velocity?
Figure 5. Hydraulic conductivity of a variety of materials.