Selecting chemical and ecotoxicological test batteries for risk assessment of trace element-contaminated soils (Phyto)managed by gentle remediation options (GRO)

During the past decades a number of field trials with gentle remediation options (GRO) have been established on trace elements (TE) contaminated sites throughout Europe. Each research group selects different methods to assess the remediation success making it difficult to compare efficacy between various sites and treatments. This study aimed at selecting a minimum risk assessment battery combining chemical and ecotoxicological assays for assessing and comparing the effectiveness of GRO implemented in seven European case studies. Two test batteries were pre-selected; a chemical one for quantifying TE exposure in untreated soils and GRO-managed soils and a biological one for characterizing soil functionality and ecotoxicity. Soil samples from field studies representing one of the main GROs (phytoextraction in Belgium, Sweden, Germany and Switzerland, aided phytoextraction in France, and aided phytostabilisation or in situ stabilization/phytoexclusion in Poland, France and Austria) were collected and assessed using the selected test batteries. Extractable TE concentrations generally decreased more significantly in soils managed by in situ stabilisation combined with phytoexclusion, phytostabilisation or phytoextraction than in soils only managed with phytoextraction. Pseudo-total TE concentrations did not change in the phytomanaged sites, except for one case, which was attributed to the dilution by the amendments. Among the single chemical extractions, the NH4NO3- and EDTA-extractions showed most frequently the differences in the extracted TE concentrations between the treated and untreated soils, while the most frequent correlations with the biological responses occurred for NH4NO3, followed by NaNO3-extractable TE. Pseudo-total (aqua regia extractable) concentrations showed weak correlation with the biological responses. Out of the bioindicators (plants, earthworms, and nematodes), dwarf beans, especially through root mass, followed by shoot length, and stress enzyme activities, were the most responsive indicators to the soil treatments. Even though the selective chemical extractions did not always show statistically significant changes in TE extractability, dwarf beans and stress enzymes developed a stronger response to the tested GRO options. Generally, the plant growth decreased with higher extractable TE concentrations in soil, while bean stress enzymes reacted in the opposite way, i.e. increased with increasing TE extractability. It is suggested that a minimum risk assessment battery to compare or monitor the sites phytomanaged by GROs might consist of the NH4NO3-extraction and the dwarf bean Plantox test including the stress enzyme activities. Validation of this assessment battery as a potential indicator of successful remediation should be further implemented for a broader range of soils and long-term treatments