Environmental Engineering Reference
In-Depth Information
3.8 Groundwater Inflow and Outflow
Determining exchanges between wetland water and groundwater can be a surpris-
ingly complex task for wetland hydrologists. Low-permeability, organic-rich soils
are often situated beneath and adjacent to wetlands; they reduce rates of exchange
and can greatly increase residence time of water in pore spaces, enhancing geo-
chemical processes. Flow across the sediment-water interface is variable both in
space and time on multiple scales. Directions of flow between groundwater and
surface water can reverse seasonally or in response to individual precipitation
events or evapotranspiration (e.g., Doss
1993
; Rosenberry and Winter
1997
).
A variety of tools and methods are available to quantify exchange between
groundwater and surface water, the selection of which should include strong
consideration of the appropriateness of the scale of the measurement method
relative to the scale of the goals of the study. On the larger end of the scale
spectrum, suitable methods include watershed-scale rainfall-runoff modeling,
groundwater-flow modeling, quantifying changes in streamflow along stream-
reach segments (commonly called a seepage run), and making use of aerial imagery
to locate areas of focused groundwater discharge. On a local scale, appropriate for
smaller wetlands or specific shoreline segments or riparian reaches, methods
include measurement of hydraulic properties using piezometers and water-level
monitoring wells, use of seepage meters to quantify flow across an isolated portion
of submerged bed sediment, and measurements of temperature to determine quan-
titatively or qualitatively distribution and rates of groundwater discharge to specific
portions of wetland beds. Several of the most-commonly utilized methods are
described below.
3.8.1 Darcy Flux Method
Use of the Darcy equation to determine flow through porous media is one of the
core concepts of hydrogeology and is commonly employed to estimate exchanges
between surface water and groundwater in wetland settings. The Darcy equation
(e.g., Freeze and Cherry
1979
) can be expressed as
KA
h
1
h
2
Q
¼
(3.34)
l
where
Q
is the volume of water that flows across the bed of the wetland to enter or
leave the wetland,
K
(hydraulic conductivity, m/s) is a proportionality constant that
represents the ease with which water can flow through porous media,
A
is the area of
the sediment-water interface through which water flows to enter or leave a wetland,
h
1
h
, is the difference between hydraulic head measured at a nearby
monitoring well and the wetland surface, and
l
is the distance from the monitoring
h
2
,or
Δ
