Environmental Engineering Reference
In-Depth Information
is more rapid and extensive in these near-shore margins where the unsaturated zone
is thinnest, and may result in the water table being higher than elsewhere, effec-
tively forming a hydraulic dam between the wetland and groundwater farther from
the wetland. Numerous examples of either a water-table trough or ridge between
a monitoring well and the wetland shoreline are reported in the literature
(e.g., Rosenberry and Winter
1997
). If either a trough or a ridge is present between
a wetland and nearby monitoring well, water cannot flow from the well to the
wetland or vice versa. It often is prudent to install two or more monitoring wells at
different distances from shore to determine if transient water-table ridges or troughs
occur, are frequent, or persistent.
Once installed, determining the water level in a monitoring well can be accom-
plished with several methods ranging from something as simple as lowering a
chalked steel tape into the well to immersing a pressure transducer that includes a
self-contained datalogger for collecting time-series data. Details for measuring
water levels in wells are presented in Cunningham and Schalk (
2011
).
3.8.1.3 Methods for Determining Hydraulic Conductivity (K)
Of all the factors that control the degree of exchange between groundwater and
wetland water,
K
is the most spatially variable and often the most difficult to
determine. A complex history of erosion and deposition of organic and inorganic
sediments is commonly encountered in many wetland settings where stage and
shoreline location can vary by a large amount over time. Organic-rich sediments,
typically with small values of
K
, can be situated next to wave-washed sand and gravel
in these dynamic environments, complicating the determination of
K
on a scale that is
relevant to a wetland water budget. Furthermore, determination of
K
is itself scale
dependent (e.g., Rovey and Cherkauer
1995
). Point measurements may represent
conditions within a few meters of a monitoring well, but will not be representative of
a more transmissive portion of the sediments that may route most of the groundwater
to or from a wetland. Most sediment is more permeable to horizontal flow than to
vertical flow. In addition,
K
commonly decreases with sediment depth (Hayashi
et al.
1998
). Reduction in
K
with depth also is particularly common in peat. An
additional complexity of peat is that it is compressible, which also affects
K
(Surridge
et al.
2005
; Hogan et al.
2006
). For these many reasons, determinations of
K
require
careful consideration and several avenues of investigation.
A single-well slug test provides a reasonable indication of
K
at a scale comparable
to the size of the well screen. This method involves recording the water level within a
well, typically with a submerged pressure transducer, while the water level is
suddenly increased or decreased (e.g., Fetter Jr 2001). The rate of recovery of the
water level in the well is proportional to the hydraulic conductivity of the sediments
that surround the well screen. Analysis of the recovery curve assumes that flow to or
from the well is primarily horizontal and requires use of one of several analytical
methods such as Bouwer (
1989
), Bouwer and Rice (
1976
) or Hvorslev (
1951
)to
calculate
K
.
