Environmental Engineering Reference
In-Depth Information
Seepage-Meter Installation
Select a location near the piezometer that you installed as part of field activity
1. Wade to the location, making sure to not step on the area that will be covered by
the seepage cylinder. The bed should not be covered by any large rocks or debris
(i.e., waterlogged sticks) that would alter seepage or prevent insertion of the
seepage cylinder. Make sure the rubber plug is removed and the port on the side
of the seepage cylinder is open; this allows water to escape as you are pressing the
seepage cylinder into the wetland sediments. Press the cylinder into the sediment
very slowly, allowing gas and water to escape through the top vent tube. You may
need to twist the cylinder to aid in cutting through a vegetative mat, if one is
present. If aquatic vegetation is very dense you may need to first cut a slit in the
vegetative mat with a long knife to facilitate insertion of the cylinder. The bottom
rim of the cylinder typically needs to penetrate the sediment approximately
5-10 cm to ensure a good seal with the sediment. However, if the bed surface is
uneven, the insertion depth may need to be increased so no gaps are present beneath
the edge of the seepage cylinder. You should probe with your fingers along the
interface between the wetland bed and the seepage cylinder. If you can feel the
bottom edge of the cylinder, then the insertion depth is not sufficient. In this case,
press the cylinder deeper into the sediment until you can no longer feel the bottom
edge of the cylinder. The meter also should be inserted with a slight tilt so that the
vent hole is at the highest point, allowing any gas released from the sediment to
escape. Once the meter is set, place a weight on the meter to counter the buoyant
force of the plastic material. A concrete or masonry brick usually is sufficient. Plug
the vent tube with the rubber stopper. The stopper will be removed later, prior to
seepage measurement, to provide a relative guide for the volume of gas released
from the sediment. If the volume is substantial, you will want to install a vent tube
to release gas to the atmosphere. If unvented, gas released from the sediment will
collect inside of the seepage cylinder, displacing water that will be routed to the
seepage-collection bag.
Install the bag shelter and connect the shelter to the seepage cylinder. You may
also need to place a small weight inside of the bag shelter to hold it in place and
prevent movement in response to waves. The wetland bed has been substantially
disturbed during meter installation and it is common for seepage rates to be larger
than normal following meter installation. It is common practice to wait for hydrau-
lic conditions at and near the bed to stabilize before measuring seepage. If your field
schedule permits, wait until the next day before making the first measurement, or
measure seepage directly after installation and compare those values with
measurements made the following day.
Seepage-Meter Measurement
Since you do not know whether water is flowing into or from the wetland across the
portion of the wetland bed isolated by the seepage cylinder, start your first mea-
surement with the seepage bag approximately half filled with water. Place a known
volume of water inside of the bag. Volume can be determined either with a
