During the period from 1920 to 1928, 200,000 unfired German chemical ammunitions from World War I were broken down and open-burned near the former western Frontline. This destruction resulted in locally severe top-soil contamination by arsenic, heavy metals and dioxins and furans. The main type of shell, called “blue cross shell” were loaded high explosives coating a glass bottle containing solid diphenylchloroarsine (CLARK 1) and diphenylcyanoarsine (CLARK 2) which have released considerable amounts of arsenic oxides during their combustion. The biogeochemical behavior of arsenic is poorly documented in this type of environment. The aim of this study was to explore the arsenic speciation in relation to mineral and microbial compartments in this particular context.. Four soils were sampled presenting different levels of contamination. The concentrations of major and trace elements were analyzed by ICP-MS. In the samples, the concentration range of arsenic was 1937-72,820 mg/kg. Zinc, copper and lead reached concentrations up to 90,190 mg/kg, 9113 mg/kg and 5777 mg/kg, respectively. SEM observations and EDS analyses were performed on different grains. Results showed that the polluted material was composed of inherited minerals (silicates), charcoal and secondary phases, carrier of metals. In metal rich grains, two phases were identified : a crystalline phase, where arsenic was associated with copper and zinc, and an amorphous phase rich in iron, zinc, copper, and arsenic. Four secondary arsenate minerals were identified by X-ray diffraction: adamite and olivenite (zinc and copper arsenates, respectively) and two pharmacosiderites. Arsenic speciation displayed by SEM-EDS and XRD was proven to be directly linked to the thermal treatments of shells. Indeed, the two main As-carriers were identified as vitreous phases, composed of a blend of metals, and secondary arsenates minerals, which were formed in the material’s porosity during its cooling. AsIII and AsV concentrations were 1577 mg/kg and 70,178 mg/kg, respectively : whereas AsV was the major species, AsIII concentration was high. Microbial activities may influence the speciation and mobility of the bioavailable fraction of As, in relation with redox conditions and organic matter. Microbial characterization included total cell counts, respiration, and determination of AsIII-oxidizing and AsV reducing activities. Results showed the presence of microorganisms actively contributing to carbon and arsenic metabolims, even in the most polluted soil, thus potentially driving cycle of biovailable As on the site.