Page 119 - Soil and water contamination, 2nd edition

106 Soil and Water Contamination

Box 5.I Chloride as a natural tracer to calculate evapotranspiration

In infiltration areas, the evapotranspiration rate relative to the precipitation rate can be
-
relatively easily determined by comparing the Cl concentration in rainfall with the Cl -
-1
concentration in groundwater. The evapotranspiration rate (mm y ) can be calculated as
follows:
{ Cl} gw { Cl} rain
E P (5.Ia)
{ Cl}
gw
-1
-1
where E = evapotranspiration rate (mm y ), P = annual precipitation (mm y ), {Cl}
gw
-1
= chloride concentration in groundwater (mg l ), and {Cl} = chloride concentration
rain
-1
in rainwater (mg l ). This calculation only applies for situations where anthropogenic
inputs of Cl and dry Cl deposition are negligible, thus in natural, non-coastal areas. In
areas where such additional inputs are substantial, they should be accounted for in the
calculation:
P { Cl} { Cl} 100 I
E gw rain (5.Ib)
{ Cl} gw
where I = additional Cl inputs from anthropogenic sources (e.g. fertiliser application)
-1
-1
and dry deposition (kg ha y ). The factor 100 is included to correct for the different
-2
-1
-1
-1
units (kg ha y → mg m y ). The additional inputs are difficult to measure and are
often unknown, which makes the calculation of evapotranspiration rates from Cl -
concentrations in groundwater very unreliable.
Chloride is a very soluble and mobile ion and is practically inert (i.e. non-reactive or
conservative), which means that it has very little tendency to react with anything in soil
and water. It does not participate in redox reactions , does not form barely soluble salts, does
-
not form important complexes with other ions unless the Cl concentrations are very large,
is barely adsorbed onto mineral surfaces, and does not play a vital role in the biochemical
cycles (Hem, 1989). Therefore, in soils, most of the Cl is dissolved in the soil solution.
However, in acid soils with kaolinitic clays the pH may be below the PZC (see Section
-
4.2.3) and thus these soils may show some adsorption of Cl , though the amounts are
mostly small. In arid regions, the Cl may precipitate and accumulate in the topsoil due to
excess evaporation .
-
Since Cl is a conservative ion, it is frequently used as a natural tracer for studying the
rates of evapotranspiration , water flow, or dispersion . For example, in infiltration areas
-1
the evapotranspiration rates (mm y ) can be relatively easily determined by measuring
-
-
Cl concentrations in groundwater (see Box 5.I). Furthermore, Cl is sometimes injected
into groundwater or surface water to measure the water flow rates. Other commonly used
-
tracers are other conservative ions that occur naturally in a water system, such as iodide (I )
-
3
and bromide (Br ), environmental isotopes such as tritium (T = H), other contaminants
of all kinds, and dyes that are deliberately added to the water, such as rhodamine and
-
uranine. For more information about the use of Cl as a tracer and tracer experiments, see
hydrological or water quality textbooks (e.g. Thomann and Mueler, 1987; Domenico and
Schwarz, 1996).

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