Environmental Engineering Reference
In-Depth Information
The numerator on the right-hand side of
Equation (
7.10
) represents total chloride deposi-
tion at land surface; under natural conditions
this includes chloride in precipitation and dry
fallout. Rates of both wet and dry deposition
can be measured with modified precipitation
collectors. Unfortunately, however, only limited
historical data on these rates are available, an
important source of uncertainty for applying
the CMB method. The National Atmospheric
Deposition Program (https://nadp.isws.illinois.
edu; accessed December 13, 2009) provides
isopleth maps of annual average chloride con-
centrations in precipitation across the US for
1994 through to the present (data for some sites
in the network are available from as early as
1978). No such data network exists for chlo-
ride deposition in dry fallout, so these rates
are largely unknown. Chloride concentrations
in precipitation display a distinct spatial trend
of decreasing concentrations with increasing
distance from oceans. Dry deposition rates are
more spatially variable because of local sources
such as wind-blown dusts and salts. In addition,
winds can remove dust and salts from an area,
so net deposition may differ from measured
total deposition. Dettinger (
1989
) found dry and
wet deposition rates to be similar in Nevada.
In southern California, dry Cl deposition can
be up to four times that of wet Cl deposition
(J. A. Izbicki, US Geological Survey, pers. comm.,
2003). Among the many CMB studies that have
been conducted, there appears to be little con-
sistency in approaches for estimating dry chlo-
ride deposition rates. Simple assumptions are
often made, such as dry deposition is negligible
(Edmunds
et al
.,
2002
) or dry deposition is equal
to wet deposition (Dettinger,
1989
; Nolan
et al
.,
2007
; Gates
et al
.,
2008
; Healy
et al
.,
2008
).
As an alternative to direct measurement of
chloride deposition, a one-time profile sampling
of soils and measurement of natural pore water
36
Cl/Cl concentration ratios (i.e. ratios that are
not affected by nuclear weapons testing) can
be used to estimate long-term chloride deposi-
tion at a site. The
36
Cl approach for estimating
chloride input involves dividing the natural
36
Cl
fallout at a site, which varies according to lati-
tude and can be obtained from Phillips (
2000
)
or Moysey
et al
. (
2003
), by the measured natural
36
Cl/Cl ratio. This approach has been used at
several sites in the United States (Phillips
et al
.,
1988
; Scanlon and Goldsmith,
1997
). Scanlon
(
2000
) found uniform natural
36
Cl/Cl concentra-
tion ratios in unsaturated-zone pore water over a
wide range in depths in different regions in the
southwestern United States. The implication of
these results was that chloride deposition rates
have remained fairly stable over time.
Sediment samples for chloride analyses are
generally collected from boreholes. Water con-
tent is determined and chloride is extracted
with deionized water as described in
Section
7. 2 .1
. Chloride analyses can be conducted with a
specific ion electrode or ion chromatography. In
areas where electrical conductivity (Ec) is highly
correlated with chloride concentrations, Ec
measurements can be conducted and calibrated
against chloride measurements. Chloride con-
centration per unit of dry sediment mass is con-
verted to chloride concentration in pore water
by dividing by the gravimetric water content
and multiplying by the pore-water density.
Numerous applications of the unsaturated
zone CMB method are found in the literature
(Dettinger,
1989
; Phillips,
1994
; Flint
et al
.,
2002
); these were mostly conducted in arid and
semiarid regions. Gates
et al
. (
2008
) applied the
method with data from 18 boreholes to examine
spatial variability in potential recharge rates
in desert terrain in northern China; average
potential recharge was estimated to be about
1 mm/yr. Edmunds
et al
. (
2002
) applied the
CMB method to sites in northern Nigeria; esti-
mated potential recharge rates were between
14 and 49 mm/yr. Nolan
et al
. (
2007
) applied
the method in humid and subhumid regions of
the eastern United States, requiring consider-
ation of additional chloride sources, as given in
Equation (
7.9
).
Land-use change, such as replacement of
native vegetation with agricultural crops, may
result in a change in drainage rate through
the unsaturated zone. Such an occurrence vio-
lates the steady-state assumption of the CMB
method, although after a period of time a new
steady-state system may develop. In southern
Australia, Walker
et al
. (
1991
) found that the
