Environmental Engineering Reference
In-Depth Information
mains per square kilometer of urban area
(Norin
et al
.,
1999
). Lerner (
2002
) estimated that
there are 50 km of water mains, sewers, and
drains per square kilometer of urban area in the
United Kingdom. Leakage rates of 20 to 25% are
common for water mains (Lerner,
2002
). Norin
et al
. (
1999
) estimated a leakage rate of 26% for
Göteborg; leakage rates of up to 50% have been
observed elsewhere (Lerner,
1986
). In urban
areas without sewers, domestic and indus-
trial discharge may be the dominant source of
recharge. The importance of leaks in water con-
veyance systems is not limited to urban areas;
these leaks can be sources of recharge wherever
water is transported long distances, particularly
in areas of irrigation.
Many of the methods described in this text
can be applied for estimating recharge in urban
areas, but careful consideration is required.
Urban hydrologic systems are generally more
complex than natural systems. As with any
study of natural systems, a recharge study of an
urban area should be framed within the context
of a conceptual model of the system. Developing
a conceptual model is difficult because of the
great number of potential sources of recharge.
Point estimates of recharge will generally be of
limited value because of the large spatial varia-
bility in recharge. Point methods can be useful,
though, for determining representative flux
rates at specific points of focused recharge.
Water-budget methods may be the most
widely used techniques for studying recharge
in urban settings. Water budgets can be deter-
mined for the groundwater system of interest,
the watershed, or even for the water-delivery
system. Lerner (
2002
) described how to con-
duct a water-budget study of a water-supply
network. Basically, portions of the network are
isolated during times of low water demand, and
flow into and out of each portion is directly
measured. Fernando and Gerardo (
1999
) ana-
lyzed the aquifer water budget for the city of
Aguascalientes, Mexico; they determined that
water-main leakage and infiltration of waste-
water accounted for 30% of the total estimated
recharge of 1040 mm/yr.
Groundwater tracers have also been used
to study the contribution of human-induced
urban recharge to the total recharge amount.
Barth (
1998
) used boron isotopes and Seiler and
Alvarado Rivas (
1999
) used stable isotopes to pro-
vide qualitative information on the contribu-
tion of human-induced urban recharge to total
recharge. Other studies have been able to gener-
ate quantitative estimates of recharge by using
tracers. Appleyard (
1995
), Davidson (
1995
), and
Appleyard
et al
. (
1999
) used tritium and CFC con-
centrations in groundwater in conjunction with
a water budget and determined that recharge in
urban areas of Perth, Australia, is equivalent to
up to 37% of precipitation, approximately dou-
ble the amount of recharge in nearby nonurban
settings.
Methods based on surface-water data have
also been used for assessing the impacts of
urbanization on base flow. The effects of urban-
ization on Long Island, New York, were studied
with analysis of streamflow duration curves
(Prince,
1981
) and streamflow hydrograph-
separation techniques (Spinello and Simmons,
1992
). Brandes
et al
. (
2005
) and Meyer (
2005
) used
streamflow hydrograph-separation techniques
to investigate the effects of urbanization in the
Delaware River watershed and in northeastern
Illinois, respectively; both studies reported that
urbanization had little impact on base flow.
Groundwater-flow models are useful for
assessing past and current impacts of urban-
ization on recharge (Lerner,
2002
). Models
can also be used to predict future impacts
and to evaluate the effects of various alter-
native management strategies that might be
employed to minimize adverse effects. Yang
et al
. (
1999
) used numerical groundwater-flow,
solute-transport modeling, and an end-mem-
ber mixing analysis based on chloride, sulfate,
and nitrogen concentrations to determine
that precipitation, water-main leakage, and
sewer leakage accounted for 30%, 65%, and 5%,
respectively, of total recharge for Nottingham,
United Kingdom.
9.5 Final thoughts
Estimating recharge is one of the most diffi-
cult tasks in groundwater studies. Information
