Do amended, polluted soils require re-treatment for sustainable risk reduction? — Evidence from field experiments

The use of in situ amendments for chemical stabilization of inorganic pollutants requires some indication of the longevity and sustainability of their effects. The efficiency and residual effect of different amendments for chemical stabilization of trace elements was evaluated in a contaminated soil under field conditions, based on single or repeated applications. Three amendments, two organic (biosolids compost and leonardite) and one inorganic (sugarlime), were applied to a soil contaminated by the tailings dam spill at Aznalcóllar (S. Spain), together with an unamended control. Amendments were applied in 2002 and 2003 to replicated test plots. In 2005 each plot was halved: one half received further amendments in 2005 and 2006 (4 treatments, D4) and the other received no further treatment (2 treatments, D2). The efficacy of the different treatments was evaluated in surface (0–15 cm) and sub-surface (15–30 cm) soil horizons. Both organic amendments significantly increased pH and TOC contents of amended soils, accompanied by reduced trace element extractability (CaCl2 extraction). The inorganic amendment reduced extractable trace element pools and maintained alkaline conditions, but did not affect TOC. The need for repeated amendment applications to sustain these conditions depended on the type of amendment. Two consecutive applications, D2, of the inorganic amendment showed sustained efficacy. Both organic amendments required repeated applications for full effectiveness. These results demonstrate that there are clear differences in the potential long-term sustainability of in situ amendments in field conditions and that treated soils require an effective monitoring programme as part of any post-treatment management.