Environmental Engineering Reference
In-Depth Information
is semiarid to subhumid. Annual precipitation
is 1400 mm/yr in Miami, Florida; 1600 mm/yr
in New Orleans, Louisiana; and 1030 mm/yr in
Baltimore, Maryland.
Recharge is predominantly diffuse, occur-
ring in response to local precipitation, mainly
in interstream areas where soils are perme-
able. Spring is typically the time of highest
recharge rates, but recharge can occur through-
out the year. Groundwater discharge is to
streams and the sea, to extraction by humans,
and, in areas with shallow water tables, to
evapotranspiration.
Recharge rates vary depending on weather,
soil properties, and land use. Many methods for
estimating recharge have been applied; stream-
flow hydrograph analyses are widely used
because most streams are gaining streams.
Stricker (
1983
) applied streamflow hydrograph
analysis to data from 35 streamflow gauging
sites in South Carolina, Georgia, Alabama,
and Mississippi; estimates of base flow ranged
from 25 to 500 mm/yr. Base flow estimates for
seven sites in North Carolina ranged from 300
to 320 mm/yr, as determined by the recession-
curve displacement method (Coes
et al
.,
2007
).
Inverse groundwater-flow modeling techniques
have been used to estimate recharge in studies
of large aquifer systems (Faye and Mayer,
1997
;
Payne
et al
., 2005). Fisher and Healy (
2008
) used
a water-budget approach to estimate recharge
at 315 mm/yr (32% of precipitation) in an agri-
cultural field on the Delmarva Peninsula in
Maryland. Using a soil-water budget model,
O'Reilly (
2004
) determined that recharge at a
site in central Florida was highly episodic, with
essentially no recharge on most days and esti-
mated rates of up to 80 mm/day in response
to heavy spring rainfalls when the soil was
initially saturated (
Figure 3.4
). Stewart
et al
.
(
2007
) applied chemical streamflow hydrograph
separation on the basis of specific conductance
at 10 sites (
Section 4.6.1
). Historical groundwater
tracers, such as CFCs and SF
6
, have been used in
a number of studies (Dunkle
et al
.,
1993
; Szabo
et al
.,
1996
; Plummer
et al
.,
1998
).
Voronin (
2004
) describes a groundwater-flow
model of the New Jersey coastal plain aquifer
system in which recharge occurs in areas of
aquifer outcrop. Recharge was treated as being
constant in time but allowed to vary in space.
Recharge estimates were obtained from a series
of water-budget studies conducted in six subar-
eas of the model domain. Because steady-state
conditions were assumed, net recharge, or
recharge minus groundwater evapotranspira-
tion, was used for model input. Values of net
recharge ranged from 150 to 500 mm/yr for
the aquifers (approximately 13 to 42% of annual
precipitation).
9.4.11 Recharge in urban settings
Urbanization of an area can greatly affect
recharge rates, locations, processes, and even
sources of water. Installation of storm and sani-
tary sewers on Long Island, New York, produced
a decrease in base flows to streams (Prince,
1981
; Spinello and Simmons,
1992
) and pre-
sumably a decrease in recharge. But the long-
held belief that increased impervious areas
produced by buildings, roads, and parking lots
lead to increased surface runoff and decreased
recharge relative to natural conditions has
been dismissed (Lerner,
2002
). A number of
studies have found that urbanization results
in increased amounts of recharge (Appleyard,
1995
; Fernando and Gerardo,
1999
; Yang
et al
.,
1999
), particularly for cities that import water
from outside of the local groundwater system.
Leaking water mains and sewers, storm-water
drains, and surface runoff diversion and stor-
age structures are all features that contribute
to enhanced recharge. Increased runoff from
built-up and paved areas may be funneled
through a retention basin or infiltration gal-
lery directly to the subsurface, resulting in a
relocation of recharge areas and a transition
from a slow, diffuse recharge process to a rapid,
focused process. The large number of these
features and the difficulty of measuring flows
associated with them illustrate the complex-
ity involved in developing a conceptual model
for recharge in urban settings and in selecting
appropriate estimation methods.
The large number of water conveyance
pipes associated with urbanization represents
a significant potential source of recharge. In
Göteborg, Sweden, there are 16 km of water
