The slag backfills of the former sulfuric acid factory of Mortagne-du-Nord (59) are sulfide- and metal-rich (Zn, Pb, Cd). Sulfide oxidation leads to acidification of surficial groundwater and the dispersal of the metals into the environment, by flow towards the rivers which run along the site. The surficial groundwater pHs fall between 2.5 and 6.8 and the metal content is very high, reaching in places 6 000 mg/L of Zn, 2.5 mg/L of Pb and 600 mg/L of Cd. To conceive a remediation scenario, we tried beforehand to understand the phenomena that govern the oxidation of the sulfides. For this purpose, water levels and water compositions have been surveyed every 2 months during 4 years, a geochemical simulation of the alteration/oxidation has been proposed and leaching tests have been performed.

  1. The survey of the water table level and periodical chemical analyses of the groundwater have shown that the slag alteration is reactivated when the water table drops until the sulfide-bearing ≪ fresh ≫ slags are unsaturated.

  2. The analysed waters always show an unbalanced negative ionic charge. Geochemical calculations allow to propose several equilibrium models of the waters and to conclude that the presence of thiosulfates (S2O23) in the original waters most likely explains the observed ionic disequilibrium.

  3. The geochemical modelling of the slag alteration, first by percolation in unsaturated conditions (allowing O2 supply) and then under saturated conditions (without O2 renewal), reproduces satisfactorily the chemistry of sampled groundwaters.

  4. Leaching tests of the slags have been performed in the laboratory both by percolation (unsatured environment) and by cirulation (saturated and closed environment). These tests allowed to obtain alteration solutions comparable to the waters sampled on site, with progressive ≪ aging ≫ of the material, in agreement with the decrease of the dissolved metals observed on site during the 4-year survey. Moreover, the tests confirm the importance of the oxygen supply in the reactivation of the alteration.

The evolution of the groundwater chemistry, the thermodynamic modelling, as well as the leaching experiments allow to determine with some details the alteration/oxidation mechanism and show that: (1) alteration is actived or reactived after a drop of the water table within the sulfide-bearing facies, (2) the oxygen supply by diffusion in the poral air is the driving force of slag oxidation, and (3) maintaining the backfills in saturated conditions practically stops alteration.

The geochemical evolution of the site is directly related to its history, with successive re-profiling of the channels which have lead to a lowering of the water table of about 2 m. The remediation should be focused on in-situ processing (water treatment aimed to lower acidity, active barriers, …) rather than on ex-situ (excavation) solutions.

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