Environmental Engineering Reference
In-Depth Information
Errors of interpretation and incorrect assumptions regarding directions of
flowlines can be greatly reduced through the use of one of a variety of commonly
available flow-modeling techniques. Perhaps the simplest and oldest is the previ-
ously mentioned hand-drawn flow-net approach. However, many analytical and
numerical computer-based models can provide a quick analysis of likely ground-
water flowpaths and estimations of volumes of exchange between groundwater and
water in the wetland. The influence of upper and lower bounds of
K
and
b
on
volumetric exchange also can be determined with numerical simulation.
3.8.1.2 Location and Installation of Monitoring Wells
The interpretation of groundwater-surface-water exchange, whether by segmented-
Darcy, flow-net, or analytical or numerical models, depends on data from monitor-
ing wells. Fortunately, installation of wells for the purpose of measuring the
elevation of the upper extent of saturated sediments (the water table) is often
relatively simple and inexpensive. Shallow water tables and small depths to water,
not to mention soft and often nearly saturated sediments in near-shore wetland
margins, allow monitoring wells to be installed manually rather than with a drilling
rig. Although wells can sometimes be driven to depth with a post driver, sledge
hammer, or hydraulic-push rig, it usually is better to auger a test hole, collect, describe,
and analyze the sediments removed from the hole, and then install the well in the test
hole. This may be difficult in some sediments, either because the sediments are poorly
consolidated and slump back into the hole, or because sediments contain a large
fraction of cobbles or larger particles, making hand auguring difficult or impossible.
Two types of monitoring wells are used for wetland-hydrology investigations.
A water-table monitoring well is designed to indicate the level of the top of the
aquifer, where total pressure is equal to atmospheric pressure and below which all
the pores in the soil are filled with water; in other words, the water table. Because
the water table can fluctuate over a range of several meters in some wetland
settings, a water-table well needs to have a well screen that is long enough that it
intersects the water table whether the water table happens to be high or low at the
time. Note the long well screen for the water-table well shown in Fig.
3.24
that
extends above the water table. The other type of monitoring well is often termed a
piezometer. A piezometer is designed to represent hydraulic head at a single point
in an aquifer. Ideally, such a well would just have an opening at the bottom of the
well casing to represent pressure. Because many piezometers also are designed as
water-quality sampling points and need to produce some water for sampling, they
often have short screened intervals. In such cases, the mid-point of the screened
interval is the depth to which pressure head indicated by the piezometer is generally
associated. Two piezometers are indicated in Fig.
3.24
. One represents a piezometer
installed near a wetland and will provide a pressure head to compare with the
adjacent water-table well. The other is installed in the sediments beneath the
wetland bed and is designed to provide a hydraulic gradient on a vertical axis as
