Natural attenuation of contamination by hexachlorocyclohexanes in the soils around the Bailín landfill site (Sabiñánigo, Huesca province, NE Spain)

INQUINOSA company dumped waste from lindane production in Sabiñánigo (Huesca, Spain). Lindane is the γ-isomer of hexachlorocyclohexane (γ-HCH), a persistent organic pollutant listed in the Stockholm Convention in 2009. The uncontrolled dumping at two poorly managed landfills (Sardas and Bailín) has become one of the most serious contamination cases in Europe. In 2014, after discovering the underground migration of liquid HCH towards the Gállego River, more than 400,000 tonnes of contaminated materials were relocated from the old Bailín landfill to a new security cell. During the transfer operations, a small fraction of ∼100-200 kg HCH was dispersed in the area. The aim of this study is to assess the spatio-temporal dynamics of HCH-isomers around the Bailín landfill. Data from five soil sampling campaigns have been used, spanning the period from 2014 to 2022. Notably, HCH concentrations decreased from 170 kg in 2014-2.4 kg 2022, with the highest contamination levels concentrated near the road used by trucks during the removal operations. Several potential sinks for topsoil pollution were identified and their contribution quantified. For instance, the amount of HCH stored in vegetation (< 1 kg) is negligible, as is the downward movement of HCH through the vadose zone into the water table. Surface runoff does not appear to be a significant mode of HCH loss, as it would have occurred rapidly during the relocation operation, likely reaching the Gállego River. Therefore, volatilisation of HCH into the atmosphere and biological degradation are the two primary sinks for HCH in the area.