Creased-Plastic Rock Permeameter for Hydrogeology Students Page: 185
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flat sides. In some cases it may be easier to trim an irregu-
larly shaped sample into several flat sides.
Feedback from Students - Creased-plastic perm-
eameters were useful for teaching hydraulic conductivity.
Students commented that using creased-plastic perm-
eameters was more meaningful than being given a hy-
draulic conductivity value in a problem statement. Build-
ing permeameters engaged students. They closely
examined rock samples, associating texture and appear-
ance with measured hydraulic conductivity values.
Student feedback was used to modify the steps in the
methods section of this article (to its present form). They
found that seeing a completed device helped consider-
ably. Therefore, the author recommends that instructors
build a creased-plastic permeameter, and let students use
it as a model. Verbalizing the 10 construction steps ahead
of time was also helpful to students.
Food coloring was helpful for visualizing flow
through rock samples. It was easier to see than clear water.
Many students were surprised how slowly the rocks
transmitted water. The experiment worked best in groups
of two. This format allowed students to exchange ideas
and expedited permeameter construction. For example,
one student could prepare plastic sheets while the other
trimmed a rock specimen.
Attributes of Creased-Plastic Permeameters -
Creased-plastic permeameters have several attributes that
make them useful for a hydrogeology class. Transparent
plastic allows students to visualize flow, by injecting dye
such as food coloring into the permeameter tube. Dye
emerges at the top of the sample, giving students a visual
impression of its ability to conduct water. Flow rates can
also be calculated, by measuring discharge through the
permeameter outlet and dividing by the area of the outlet.
Discharge decreases over time as the water level drops in
the tube.
All of the supplies needed to construct a
creased-plastic permeameter can be obtained from local
office supply or hardware stores. The permeameter is easy
to build, simple to use, and very inexpensive. A complete
permeameter costs less than $2. This cost includes the tub-
ing, transparency sheet, electrical adapter, and silicone
(fraction of a tube). Rock cutting equipment is more ex-
pensive, but readily available in many geoscience depart-
ments. Moreover, the cutting equipment can be used to
build several permeameters. For departments not having
rock saws, a carbide hacksaw blade costs about $3, and a
masonry rotary blade costs between $2 and $20. Alterna-
tively, a coring device could be used to extract the rock
sample. For a cylindrical sample, the creasing procedure
in Steps 2 and 3 could be skipped.
Creased-plastic permeameters can also accommodate
repeated tests on a single rock sample. However, numer-
ous tests on a single sample may eventually produce min-
eral deposits that clog pores and reduce hydraulic
conductivity, or induce leakage along the edges of a speci-
men. Precipitation of mineral deposits is especially likelyif local tap water contains a high concentration of total dis-
solved solids.
Several permeameters can be built at the same time, in
an assembly line fashion, to measure more than one sam-
ple. Furthermore, several tests can be run at the same time
by attaching separate tubes to a single meter stick. The
permeameter does not impose tight constraints on the
shape of a rock. It accommodates polygons with flat sides.
Plastic sheets could also be wrapped around cylindrical
samples.
Precautions and Potential Modifications - While the
construction procedure outlined above can be modified to
accommodate a particular application, certain steps are
necessary to attain good results. Silicone was the most ef-
fective sealant tested in the study. Other sealants such as
latex caulking developed leaks, which lead to erroneously
high estimates of hydraulic conductivity. Any alternative
sealant should be rated for aquariums or marine applica-
tions. Moreover, the sealant should be given at least 24
hours to dry before running water through the
permeameter.
Blowouts are most likely at the base of the
permeameter. The tabs provide extra contact area for seal-
ing the permeameter to the base sheet. Not using tabs, or
folding them outward instead of inward, practically en-
sures leakage. Contacts between the rock sample and
permeameter are also subject to leaking. Cutting a sample
into flat or smooth edges facilitates sealing where it con-
tacts the permeameter.
A 0.5-in (1.3-cm) falling head tube worked well in this
study. Air bubbles and capillary effects are potential prob-
lems with smaller-diameter tubes. The tube should al-
ways be filled slowly to avoid air entrapment.
Transparent tape or plastic cable ties work well for secur-
ing the tube to the meter stick, because they do not sub-
stantially obscure markings on the stick.
Common mistakes made when building creased-
plastic permeameters included folding tabs outward in-
stead of inward at the base of the structure, not creating
continuous beads of silicone (between plastic parts, or be-
tween the plastic and rock), and not allowing adequate
drying time between steps. When conducting a hydraulic
conductivity test, filling the tube too quickly entrapped
air, rendering a false result.
The simplest application of the creased-plastic
permeameter is to solid rocks, where it operates in a fall-
ing-head mode. It could be retrofitted for a constant-head
test (on material such as loose sand having a high hydrau-
lic conductivity) by placing screens inside the chamber,
above and below the sample. A constant-head test would
also require that a reservoir be attached to the top of the
permeameter tube, and that the water level in the reser-
voir be maintained at a fixed level. Moreover, the hydrau-
lic gradient would need to be less than approximately 0.5
to maintain laminar flow. In a retrofitted, constant-head
permeameter, hydraulic conductivity could be estimated
by rearranging variables in Darcy's equation,Hudak - Creased-Plastic Rock Permeameter for Hydrogeology Students
185
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Hudak, Paul F. Creased-Plastic Rock Permeameter for Hydrogeology Students, article, March 2001; Northfield, MN. (https://digital.library.unt.edu/ark:/67531/metadc848593/m1/4/?q=mineral+wells: accessed June 13, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT College of Arts and Sciences.