Name__________________
Groundwater Transport
Simulation Apparatus for the Laboratory
The groundwater simulation apparatus is a terrarium with input ports at the upslope end to simulate recharge, and outflow ports on the down-slope end to simulate
discharge (Fig. 1). The terrarium is filled with a layer of fine-gravel (aquifer) confined
by two clay layers (aquitards) to simulate stratigraphy. There is an injection well at the
top of the simulator and a field of twelve monitoring wells in the down-flow direction.
Wells are plastic tubes with screened ports and placed at regular spacings and depths in
the aquifer layer. Water based dye is poured into the injection well so that it can
permeate the strata and move down the gradient towards the monitoring wells.
The apparatus is meant to simulate a real life hazardous waste spill through an
injection well. The area of the terrarium would equate to several tens of acres. The
affected area would be the shallow aquifer and the process of tracking the spill would
take several weeks to several years. It is important to keep the real-life representation in
mind during the experiment.
Fig. 1) Top view of the simulation apparatus showing the location of the injection well,
sampling wells, and stream.
Laboratory Procedure
1.)
Your instructor will dump a slug of dye into the injection well after water
flow in the tank has reached equilibrium. The stop watch will be started at
the same time. When the time is called (~ minutes), students will
withdraw water samples for each of the monitoring wells using a transfer
pipette. The sample will than be transferred to a vial. The concentration
of the dye is then estimated using a color tone calibration chart by visually
matching the color. Be sure to record the time, well, and concentration of
dye for each sample taken. This process is repeated at regular intervals
until all of the dye has passed through the well field. The data will be
recorded on the board, and each student needs to record the data in the
attached table.
2.)
The recorded color data can be plotted as a series of contour diagrams for
each time step. To make the contour plots, you need three variables: “X”
and “Y” for the position and a third value for the quantity to be contoured.
In topographic maps, the third variable is elevation. In this lab, the third
variable is the color “concentration” at each well. Plot a contour map for
each of the sampling times (Conc. 1, Conc. 2, etc.) by hand in the boxes
on the attached page. Your instructor will tell you which time intervals to
plot. See Fig. 2.
3.)
Answer the questions below.
a.) Calculate the velocity of he groundwater flow. To do this, measure how far the slug
has moved from one sampling time to the next sampling time as it passes through
the well field. Pick features of the field that are identifiable (same concentration)
from one contour plot to the next. Some estimating is involved because a feature
of the contour map may only be visible on one or two plots and then other
features will have to be used. Velocity (V) = distance/time.
Concentration _____, at well (a) _____, at time (a) _____ sec
Same concentration, at well (b) _____, at time (a) _____ sec
Distance between wells (a) and (b)_______cm / time between (a) and (b)________sec =
V (velocity) _______ cm/sec
b.) Calculate the slope of the aquifer. First measure the top of the aquifer unit at both the
up and down slope ends, as well as the length of the aquifer, and calculate the
slope of the tank (slope = Δh/l).
Δh (change in height) _____cm = (height of aquifer up slope)_____cm – (height down slope)_____cm
l = length of the aquifer ______cm
Slope _______ = Δh (change in height) ______cm / l (length) ______cm
c.) Calculate the hydraulic conductivity (K), using Darcy’s law;
K = Velocity / Slope
K (hydraulic conductivity) ________ cm/sec = V (velocity) _______cm/sec / Slope _______
d.) What happens to the water in the stream, shortly after the slug is poured into the
injection well? Why do you think this happens?
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Fig. 2. Contour maps of dye density during a
typical laboratory exercise with wells
and density values posted. Dye density
distribution (pollutant concentration) and
contours (contour interval = 2); T1) after
240s, T2) after 480s, T3) after 720s, T4)
after 960s, and T5) after 1,200s.