Environmental Engineering Reference
In-Depth Information
graduated cylinder or by weighing the water and the bag with an electronic scale. If
using an electronic scale, knowing that the density of water is 1 g/cm
3
and that 1 ml
equals 1 cm
3
allows you to measure change in volume by recording change in
weight of the seepage bag. Before making any measurements using an electronic
scale, you should weigh the bag empty, then completely full, so you will know the
range of volume that can be measured with the bag.
Once an initial volume of water in the bag has been measured (or weighed), you
will need to remove all remaining air from inside of the bag prior to connecting the
bag to the seepage cylinder. This is commonly called de-airing the bag. Close the
valve on the bag that contains a measured volume of water, walk out to the bag
shelter, suspend the bag vertically while holding onto the bag fitting, open the valve,
and slowly lower the bag into the water, immersing the bag with the valve
constantly pointing up and always above the water surface. This process will
force air inside of the bag to leave via the open valve located above the water
surface. Once the bag is pulled beneath the surface to the point where water inside
of the bag is at the same level as the valve, close the valve. The bag is now de-aired
and ready for deployment.
Carefully remove the bag-shelter lid and attach the bag to the threaded fitting
inside of the bag shelter. Straighten the bag so the bag material is not twisted and the
bag is oriented in a relaxed position inside of the bag shelter. Open the valve and
record the time of opening. Your measurement has begun. Place the lid on the bag
shelter very slowly to avoid forcing water out of the bag during the measurement.
Now you wait. Since you do not know the seepage rate a priori, the wait time is
somewhat of a guessing game. A half hour to an hour should be sufficient to allow a
change in water volume that is large enough to allow you to know whether water is
flowing to or from the bag. To remove the bag, repeat the process described above
but in reverse. Remove the lid on the bag shelter very slowly, and close the valve on
the bag being careful to not touch the bag. Record the time as you close the valve.
Remove the bag and measure the final volume of water (or determine the final
weight of the bag plus water if an electronic scale was used prior to bag attach-
ment). By the gain or loss in volume or weight, you will know the direction of flow
and have an initial assessment of the relative seepage rate. If the bag is full or empty
upon removal, you waited too long and your next measurement should be
conducted over a shorter period. If there is no measurable change in volume, your
next measurement period should be increased. After one or two iterations, you
should have a good estimate for the amount of time it will take to make a seepage
measurement. Simply divide the change in volume by the time of bag attachment to
get seepage results in ml/min. Divide that value by the area covered by the seepage
cylinder to report your results in flux units (distance per time).
Installation of Temperature Sensors
Accurate measurements of temperature can be made easily with inexpensive
instruments, making its use in quantifying exchanges between groundwater and
surface water particularly attractive. Here we will make use of newer technology
