Abstract

The industrial waste land in Mortagne-du-Nord has been occupied for over 60 years by a zinc smelter unit linked to a sulfide roasting unit for sulfuric acid production and for a few years a lead smelting unit. The waste land has been systematically explored by drillings and pits ; 58 of these drill holes and pits have been equipped as piezometers and surveyed for 4 years.

The site has been back-filled with slags and smelting crucible to a thickness of about 3 meters. The mineralogy of these materials is highly varied. More than 30 heavy metal-bearing (Zn, Pb, Cd, Cu) mineral species have been identified, these are sulfides, sulfates, carbonates, oxides and silicates. High substitution rates characterize these minerals, especially the sulfides and the oxides.

Field descriptions and mineralogical analyses lead to distinguish two main types of materials: materials called ≪ active ≫ that are characterized by the presence of sulfides and secondary sulfates,, and materials called ≪ non-active ≫ that look clearly weathered and in which the heavy metals are mainly present as carbonates and oxides. The ≪ active ≫ materials with sulfides bear an average of 7 % of heavy metals (Zn + Pb + Cd), whereas the oxidized and altered ≪ non-active ≫ materials contain less than 3 % of heavy metals. The whole stock of heavy metals in the waste land backfills can be assessed at over 15 000 tons.

The hydrochemistry of the surficial groundwater contained in the backfills is directly related to the mineralogy of the material and shows very steep heterogeneities, with heavy metal concentrations increasing 10 or even 100 times over a distance of less than 50 m. The groundwaters adjoining the ≪ non-active ≫ backfills have moderately acidic to neutral pH and are relatively weakly mineralized: SO4 =700 mg/L, Zn++=10 mg/L, Pb++=0,2 m/L et Cd++=0,1 mg/L. On the contrary, the groundwaters adjoining the ≪ active ≫ backfills are acidic (2,5 < pH < 6,3) and are heavyly mineralized: SO4 > 4 000 mg/L, Zn++ > 1 000 mg/L, Pb++ > 1,5 mg/L et Cd++ > 50 mg/L. These groundwaters discharge in the water courses alongside the waste land, with in places heavy metal concentrations equivalent to those within the ≪ active ≫ sulfide bearing backfills.

The deeper regional aquifer contained in the Sables d’Ostricourt Formation is separated from the surficial groundwater by organic- and pyrite-rich clayey alluviums. Because of this, the deeper groundwater is relatively protected, but nevertheless contaminated. The alluvium acts as both a hydrological and a geochemical barrier, and, leads to the precipitation of a part of the dissolved heavy metals as sulfides.

The hydrological modelling of the surficial groundwater allows to calculate the annual fluxes towards the watercourses alongside the waste land. Applying the average concentrations measured in the surficial groundwater to this flux, we can estimate the annual tonnage of heavy metals exported towards the regional watercourses at about 14 000 kg/year of Zn++, 2 kg/year of Pb++ and 25 kg/year of Cd++, merely for the site of the former sulfuric acid plant where the ≪ active ≫ backfills are widespread.

This heavy metal export to the watercourses constitutes the site major environmental harm. Regarding the volume of the polluted materials, it is inconceivable to excavate the backfills. A confining and/or processing method of the polluted waters has to be considered instead.

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