Effect of water table variations and input of natural organic matter on the cycles of C and N, and mobility of As, Zn and Cu from a soil impacted by the burning of chemical warfare agents: A mesocosm study

Abstract

A mesocosm study was conducted to assess the impact of water saturation episodes and of the input of bioavailable organic matter on the biogeochemical cycles of C and N, and on the behavior of metal(loid)s in a soil highly contaminated by the destruction of arsenical shells. An instrumented mesocosm was filled with contaminated soil taken from the "Place-à-Gaz" site. Four cycles of dry and wet periods of about one month were simulated for 276days. After two dry/wet cycles, organic litter sampled on the site was added above the topsoil. The nitrogen cycle was the most impacted by the wet/dry cycles, as evidenced by a denitrification microbial process in the saturated level. The concentrations of the two most mobile pollutants, Zn and As, in the soil water and in the mesocosm leachate were, respectively, in the 0.3-1.6mM and 20-110μM ranges. After 8months of experiment, about 83g·m of Zn and 3.5g·m of As were leached from the soil. These important quantities represent <1% of the solid stock of this contaminant. Dry/wet cycles had no major effect on Zn mobility. However, soil saturation induced the immobilization of As by trapping As V but enhanced As III mobility. These phenomena were amplified by the presence of bioavailable organic matter. The study showed that the natural deposition of forest organic litter allowed a part of the soil's biological function to be restored but did not immobilize all the Zn and As, and even contributed to transport of As III to the surrounding environment. The main hazard of this type of site, contaminated by organo-arsenic chemical weapons, is the constitution of a stock of As that may leach into the surrounding environment for several hundred years.

Microbial community response to environmental changes in a technosol historically contaminated by the burning of chemical ammunitions

“…As concentrations ranged from 102 to 150 g.kg -1 , Zn concentrations from 70 to 107 g.kg -1 , Cu concentrations from 13 to 21 g.kg -1 , and Pb concentrations from 3 to 6 g.kg-1 (Thouin et al, 2016). A mesocosm study, previously described in Thouin et al, 2017 was conducted in a 1 m diameter column filled with 610 kg of this polluted material that had a high organic content (25.9 %), mainly in the form of non-biodegradable coal ash (Thouin et al, 2016(Thouin et al, , 2017. The experimental soil cylinder was divided into four distinct layers (H1, H2, H3 and H4), from H1 (surface), to H4 (bottom).…”

“…after four months), 24 kg of fragmented organic forest litter sampled at the Place-à-Gaz site were added to the top of the surface soil (level H1). The forest litter added was characterized by high organic carbon (30.5 %), nitrogen (1.8 %) concentrations, and high HI/OI (Thouin et al, 2017). Coring was performed in the mesocosm using 5 cm diameter stainless steel pipes at the beginning of the experiment (T0), just prior to the addition of fragmented forest litter (4 months, T4), and at the end of the experiment (8 months, T8).…”

“…This previous study provided results on the processes that affect the fate of As and metals that were published in Thouin et al (2017Thouin et al ( , 2018. Adding litter contributed to the growth of As transforming microorganisms; soil saturation enhanced arsenate-reducing activity (Thouin et al, 2018) resulting in the increase of arsenite concentration in soil solution.…”

“…The layer is highly contaminated with As, zinc (Zn), copper (Cu) and lead (Pb) (which concentrations respectively reach 72.8 g.kg -1 , 90.2 g.kg -1 , 9.1 g.kg -1 and 5.8 g.kg -1 ) forming unexpected mineralogical associations (Thouin et al, 2016). Organic contamination was also observed with the detection of nitroaromatic explosives and their derivatives (Bausinger et al, 2007), dioxins, furans, polycyclic aromatic hydrocarbons (PAH) and diphenylarsinic acid (Thouin et al, 2017). The most contaminated zone of this site has no vegetation and floristic diversity is very low at the periphery.…”

“…However, only global microbial parameters (carbon mineralization, enumeration and activity tests for As-transforming microorganisms) were considered at this stage. Here, we investigated the 5 structure of the microbial communities in the samples from this mesocosm experiment (Thouin et al 2017(Thouin et al , 2018 with the heavily contaminated technosol of the Place-à-Gaz, giving insight, for the first time, into the microbial diversity of polluted sites that are part of the complex legacy of chemical shell destruction. In a second time, the effects of two environmental parameters (namely water saturation events and natural organic matter input) on this diversity were evaluated through analyzes of the bacterial, archaeal and fungal communities in samples from the mesocosm experiment.…”