Leaching of Contaminants to Groundwater - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 835

contents and water fluxes can significantly influence the speciation, and thus the
mobility and availability, of elements. Decreasing water contents near the soil sur-
face, furthermore, may lower the pH of the soil liquid phase and produce new cation
exchange equilibrium conditions. The upward transport of Cl during summer due to
increased evapotranspiration, and subsequent accumulation of Cl near the soil sur-
face, can cause an increase in the total aqueous cadmium concentration because of
the formation of cadmium-Cl complexes.
The HP1 multi-component transport simulator is used in this section to simulate
the leaching of cadmium and zinc in a dry Spodosol in a sandy region of Northern
Belgium using in-situ measured cadmium and zinc (and additional elements) con-
centration profiles. Soils in the region were contaminated by atmospheric deposition
of cadmium and zinc from non-ferrous industry (Seuntjens 2000 ). Water flow and
the transport of major cations (Na, K, Mg, and Ca), heavy metals (cadmium and
zinc), anions (Cl, Br) and Al were simulated for a 1-m deep multi-layered podsol
soil profile subject to atmospheric boundary conditions for a period of 30 years.
The main focus is on how processes affecting water contents and water fluxes also
influence the geochemical conditions in the soil. Specifically, the effect of cycles of
evaporation and infiltration on pH and cadmium speciation will be discussed.
Interactions between major cations and heavy metals with the soil solid phase
were simulated by means of cation exchange processes assuming local equilibrium
on a single type of exchange sites. Voegelin ( 2001 ) earlier showed that this approach
adequately describes various features of cadmium transport experiments. Although
sorption on specific sites with a high affinity for cadmium may also occur in soils
(Selim et al. 1992 ), this type of binding is unlikely in acid sandy soils (Voegelin
et al. 2001 ). Exchange parameters were calibrated using concentrations measured
in drainage water from a steady-state flow experiment on large, undisturbed soil
lysimeters (1 m long and 0.8 m diameter; Seuntjens et al. 2001 ). The initial compo-
sition of the cation exchange site was measured for each soil horizon (Jacques et al.
2008a ). While the same log( K ) parameters were used for all soil horizons, the size
of the cation exchange complex was assumed to vary between horizons.
Daily values of precipitation ( P ) and potential evapotranspiration ( E p) during
the 30-year simulation period were imposed as climatic boundary condition, from
which the daily actual evaporation rate was calculated. A subset of the P - E p data is
shown in Fig. 18.18 .
Figure 18.19 shows time series between 1972 and 1982 for the water content,
p H and total Cl and Cd concentrations in the liquid phase at two depths. The results
illustrate the relation between water flow and geochemical conditions in the soil.
The alternation between precipitation (wet conditions) and evaporation (dry condi-
tions) as dictated by the atmospheric conditions clearly affected the dynamics of
the water content, with upward water flow during dry periods. The flow dynamics
in turn significantly influenced the geochemistry near the soil surface. As illus-
trated, the most mobile elements (anions such as Cl − and monovalent cations such
as Na + ) move upwards during the evaporation periods, thus causing these ions to
accumulate near the soil surface. The decrease in water content near the soil surface
due to evaporation resulted in higher concentrations and a lower pH. PHREEQC