A bioassay battery for the ecotoxicity assessment of soils conditioned with two different commercial foaming products

Alkylethoxysulfate (AES) surfactants are the main component of most commercial products used for soil conditioning in the excavation industry, in particular as lubricants for mechanized tunnelling. Huge amounts of soil debris are produced during the excavation process and their possible re-use as by-products (e.g. land covering) or discharge as waste is a critical question in civil engineering. There can be an environmental impact from the potential ecotoxicity of the residual chemical mixture occurring in the foam-conditioned excavated soil. Chemical analyses are not able on their own to assess the potential hazards for the ecosystem ascribable to inadequate management of this kind of conditioned soil. Ecotoxicological tests, on the other hand, provide useful overall information about the potential effects of conditioned debris on soil and aquatic biota, including synergic ones, whether or not there is a detailed knowledge of the chemical composition of all the components of the commercial foaming products. In this context, the objective of this work was to evaluate the overall ecotoxicity of excavated soil conditioned with two common foaming products containing sodium lauryl ether sulphate (SLES) as the main component, in order to verify their possible re-use as by-products. For this purpose, two soils with different geopedological characteristics were conditioned with the commercial foaming agents at the concentrations utilised for mechanized tunnelling and then used for microcosm laboratory experiments. Soil sub-samples were collected at different times (0, 7, 14, 28 days) of incubation and soil elutriates were produced to assess the effects of the foaming agents on five test species (Pseudokirchneriella subcapitata, Daphnia magna, Danio rerio, Vibrio fischeri and Lepidium sativum). The results of the tests were then compared with SLES residual concentrations in the elutriates, determined with the MBAS (methylene blue active substances) spectrophotometric method. Finally, the data were combined in a battery index, which proved effective at evaluating the overall ecotoxicity in a real-life situation of two different excavated soils conditioned with the two products analysed.