Assessment of leaching risk of trace metals, PAHs and PCBs from a brownfield located in a flooding zone

Abstract

An old industrial site (brownfield) located south of Paris in a flooding plain and containing demolition disposal as well as a burning zone for metal recovery is being regenerated to satisfy local need for public green space. The main objective of the described study was therefore to assess the risk of remobilisation of trace metals, PAH and PCB present. The research focused on vertical migration due to rainfall (non-saturated flow) and to river flooding (saturated flow). To assess the remobilisation risk, representative soil profiles were reconstituted and eluted in columns with artificial rain and filtered river water for 6 weeks with an equivalent of 25 mm d^-1. Soil analysis showed that both zones are highly contaminated, exceeding the French environmental standards. Though the superficial metal content was much higher in the burning zone with levels of g kg^-1 than that in the demolition zone, most metals showed higher levels in the eluents of the latter. The level of dissolved Zn in the burning zone eluent was 30 μg L^-1, while in the demolition zone, it was 300 μg L^-1, 40 times the admissible level. Zn was thereby correlated to aromaticity parameter HIX, indicating a link with organic matter transformation. The Cu was only significantly released under saturated condition (up to 80 μg L^-1) in the demolition soil, indicating as implicated mechanism manganese and iron oxide reduction rather than organic matter transformation. Despite the high PAH and PCB soil contents, these pollutants were not released. The total PAH content in the effluent was 30 ng L^-1 in average and did not significantly differ between the two zones and the types of hydrology. Only Zn and Cu issued from demolition zone presented an eco-toxicological risk. Crossed statistical analysis of the results showed that the role of the soil type is preponderant in the pollutant release and that temporary flooding condition would induce a lower impact on the groundwater quality than an equivalent amount of fallen rain. Though the burning site was far more contaminated in the upper soil than the demolition zone, it presented little risk compared to the demolition zone, more profound and more permeable. The latter showed therefore significative trace metal release, up to 2.1 kg ha^-1 year^-1 for zinc, doubling the local atmospheric deposition.

Keywords: Column experiments; Environmental impact; Flood; Heavy metals; Organic micropollutants; Pollutant remobilisation; Rain; Saturated and unsaturated flow conditions; Soil contamination.