Science of the total environment vol:376 issue:1-3 pages:86-99
In the northern Campine in Belgium, large areas are contaminated by heavy metals such as Zn and Cd due to the (former) nonferro
metal industry. In the sandy soils, the heavy metal adsorption/attenuation in the spodic horizon represents the main retention
mechanism of leached pollutants from the contaminated topsoils. In this study, the pH-dependent behaviour of the elements in these
spodic horizons was tested by pHstat experiments and compared to sandy loam soils. Extractions with CaCl2 0.01 M and EDTA
0.05 M provided a further insight into the binding mechanisms.
The results indicate that organic matter is the main factor responsible for the mobility of Cd, Zn and Ca in the spodic horizons.
The binding of elements is not very strong, however, and highly dependent on pH. A slight decrease in pH can cause a significant
release of metals from the spodic horizons, with up to 60% of Cd and 90% of Zn being released within a 1.5 unit change in pH
(starting from the naturally occurring pH). This pH change can happen rapidly in these soils, due to the low buffering capacity, and
is realistic given the acidification in Flanders. For the sandy loam soils, a pH decrease of 3 units is needed to release 40% of Cd and
20% of Zn, and the acid neutralization capacity is exhausted more gradually, suggesting that slower buffering mechanisms take
place. For the sandy loam soils, Cd retention is mainly governed by organic matter, while for Zn other factors such as the clay
minerals also play an important role. Despite the high potential mobility and pH dependence of the heavy metal retention in the
spodic horizons, the actual risk for groundwater pollution is limited. For the diffusely contaminated areas, where traditional
remediation is not an option, spodic horizons may therefore contribute to a natural attenuation of the soil contamination.