Environmental Engineering Reference
In-Depth Information
3.8.1.1 Location of Monitoring Wells, Assumptions, and Errors
The value of a single monitoring well for determining exchange between ground-
water and wetland water is slight. Analysis of sediment and distribution of soil
types based on materials removed during the well installation has local value but the
lateral extent of these properties is unknown. Difference in hydraulic head between
the single well and the wetland can be determined, but little can be known about the
actual direction of flow of the ground water with only a single well and wetland
stage. Two wells provide additional information about the local-scale geology and
hydraulic gradient. If holes augured on opposite sides of a wetland both indicate
similar geology, then confidence is increased that geology surrounding the wetland
is somewhat uniform. However, information still will be insufficient to characterize
hydraulic gradients around the entire wetland perimeter with any certainty.
Unless aquifer-gradient information is known a-priori, the minimum number of
wells required to estimate groundwater exchange with a wetland, at least qualita-
tively, is three. If wells are distributed approximately evenly around a wetland,
contour lines of equivalent hydraulic head (equipotential lines) can be drawn based
on the head values from the wells and the stage of the wetland. Once equipotential
lines are drawn, groundwater flowpaths can be drawn perpendicular to the equipo-
tential lines. Flowpath lines will provide an indication of the direction of ground-
water flow. For flow-through wetlands that both receive groundwater discharge and
recharge water to groundwater, the locations of hinge lines, defined as those points
along a shoreline that separate reaches where groundwater discharges to a wetland
from reaches where wetland water flows to groundwater, can be drawn (Fig.
3.22
).
This rudimentary analysis forms the beginning of a groundwater flow-net analysis,
which is another method for estimating the direction of groundwater flow, described
more completely in Rosenberry et al. (
2008
). Examples are shown in Fig.
3.23
based
on a variety of combinations of monitoring wells. For example, heads from any two
wells selected from the array of wells shown in Figs.
3.22
and
3.23
,withthe
exception of wells A and D or A and E, will lead to an incorrect interpretation of
directions of groundwater flowpaths in the vicinity of the wetland. Data from only
wells C and G will lead to the assumption that flow is to the northeast (Fig.
3.23b
).
Data fromwells C and F will lead to the assumption that the wetland is losing water to
groundwater at least along the majority of the wetland margin (Fig.
3.23c
).
Heads only from wells B, E, and G would result in a correct interpretation of the
direction of groundwater flow (Fig.
3.23d
). However, without data fromwells A or D,
the interpretation would be that far less groundwater exchanges with the wetland.
Groundwater flow to the wetland would occur only along the shoreline represented by
well B. Assuming that segments A, B, and C were assigned to the gradient at well B,
that
b
is 20 m, and that
K
is 30 m/day, inflow would total 25,500 m
3
/day. Outflow,
assuming well E is assigned to segments D, E, and F, well G is assigned only to
segment G, and
b
and
K
remain the same at 20 and 30, respectively, would be
19,964 m
3
/day. This would result in an imbalance of over 5,000 m
3
/day or 20 % of
inflow. If wells were located near the three protruding bays of the wetland (wells A,
