Environmental Engineering Reference
In-Depth Information
Field Exercises
Field Activity 1: Installation of a Wetland Staff Gage, Water-Table Well,
and Piezometer
With a staff gage to indicate wetland stage and measurement of the depth to water in
a nearby water-table well, a wetland scientist can determine whether groundwater
has the potential to flow to the wetland or whether the wetland is likely to lose water
to the adjacent groundwater system. If we know hydraulic conductivity (
K
) at the
well, and make the assumption that
K
is uniform in the vicinity of the well and the
wetland, we can calculate flow (
Q
) between the wetland and groundwater in an area
for which we think data from the well is representative. Lastly, two additional
measurements of
Q
can be made; one utilizes a seepage meter installed in the
wetland bed and the other makes use of changes in temperature gradients in the
wetland sediments. The temperature method requires installation of sensors at
various depths beneath the wetland bed. Since we have to auger a hole or pound
a pipe a meter or two into the sediment to install these sensors, it also makes sense
to put a well screen at the bottom, in which case we can determine the hydraulic
gradient on a vertical plane as well as
K
based on a single-well test. With that
information, and our measurement of
Q
from the seepage meter, we can use
Darcy's law to calculate
K
of the wetland sediment on a vertical axis. This will
give us an idea of anisotropy, the ratio of horizontal to vertical hydraulic conduc-
tivity. With this small investment of time and money, we will have learned a great
deal about wetland hydrology and hydrogeology at this site.
This first of three exercises near the wetland shoreline will demonstrate the
installation of a monitoring well and a staff gage. Detailed instructions and parts
lists presented here, and also those presented in the other field exercises, represent
the authors' preferences and describe only one of many different ways to achieve
these objectives. Students are encouraged to seek other descriptions and opinions
for accomplishing these tasks and then develop their own impressions and methods
for collecting data in the field.
Wetland Staff Gage
Figure
3.40
shows a wetland staff-gage installation and illustrates some of the
problems that can be associated with their use. First, note that there are two staff
gages in the photograph. In settings where wetland stage changes substantially, it
may be necessary to have multiple staff gages so that when one gage is completely
submerged during periods of high water another situated at a higher elevation can
be read to indicate wetland stage. Secondly, note the substantial angle from vertical
of the staff gage in the distance. This is the result of ice on the wetland surface
having moved at some point during the winter, tilting the staff gage. If the ice
moves enough, the staff gage can be completely removed from the wetland bed and
sometimes transported a considerable distance. The surveyor holding the rod on the
