A quantitative assessment of the behavior of metallic elements in urban soils exposed to industrial dusts near Dunkirk (Northern France)

Abstract. In urban and industrialized areas, soil contamination and degradation caused by the deposition of industrial dusts may pose significant health and environmental risks. This problem relates to the vertical mobility and bioavailability of Potentially Toxic Elements (PTE). This study investigates the fate of PTE brought by industrial dusts in urban soils located in the Dunkerque agglomeration, one of the most industrialized areas of France. Four soil short cores were collected in the city of Gravelines (Dunkerque agglomeration) following a gradient from the industrial emitters to the deposition site. The soil cores were cut into discrete 1-cm-sections to study their PTE concentrations (using ICP-AES/MS analyses). Single HCl extraction was performed to evaluate the mobility of PTE in soils and to discuss their specific behavior according to the current soil parameters. For this purpose, the main soil parameters were identified (grain-size distribution, mineralogy, pH, CEC, TOC, calcium carbonates and water contents) in addition to the soil chemical composition (XRF, ICP-AES/MS analyses). The studied soils revealed globally low absorbent capacities for pollutants (CEC averaging 5.3 meq/100g), partially counterbalanced by the buffering effect of calcium carbonates (contents ranging from 8 % to 30 %). We highlighted minor (1<EF<3) to moderately severe (5<EF<10) Enrichment Factors in industrial PTE (Cr, Ni, Mo, Mn, Cd, Zn), in the first 3 centimeters of the soils located near the industrial emitters. The contamination profiles of these soils are concordant with atmospheric inputs of metallurgical dust. Using a relatively strong leaching reagent (HCl 1M), we estimated a low vertical mobility for Cr, Ni and Mo (average leached ratios <25 %) in soils, suggesting that these industrial PTE mainly occur in refractory phases (natural or anthropogenic). Mn, Cd and Zn, related to industrial and/or urban sources, present a higher mobility (average leached ratios > 60 % for Mn and Cd, and averaging 44 % for Zn). Our study points out the stability of industrial PTE in soils under current physicochemical conditions (calcareous soils with a slightly basic pH of 7.8). In this context, the monitoring of industrial PTE in these urban soils is highly recommended, considering (1) the presence of allotment gardens in the vicinity of the emitters and (2) the potential evolution of soil conditions as a result of increased flooding events.