Contamination of soils with dust fallout from the tailings dam at the Rosh Pinah area, Namibia: Regional assessment, dust dispersion modeling and environmental consequences

摘要

The dust blown out from the tailings dam - and to a lesser extent from the ore treatment plant - appears to be the major environmental threat in the Rosh Pinah town. The amount of suspended dust particles in the air (TSP) monitored on the surface of the tailings dam ranges between 8 and 84 mg m~(-3). The air contains (in ug m~(-3)) high amounts of Zn (median: 792), Pb (median: 412), Mn (median: 163), and Cu (median: 42), and lesser amounts of As (median: 5) and Cd (median: 2). Sampling of topsoil together with reference sampling of subsurface soil at a depth of 80-90 cm revealed that the shape of the dust contamination halo is elongated northwestward according to the prevailing direction of winds during summer. The eastern and northeastern parts of the Rosh Pinah town are affected by dust fallout. The factor analysis of data acquired from topsoil revealed two factors governing the source and the nature of individual components: (a) "tailings specific factor" grouping predominantly CO2 (carbonate), S_(tot), Fe, Mn and As, and (b) "ore processing plant specific ones", grouping S_(tot), As, Cd, Cu, Hg, Pb and Zn. Dispersion modeling of suspended dust particles (TSP) in the air at Rosh Pinah revealed that while the whole surface of the tailings dam was dry in the past due to infiltration and evaporation of water, the average yearly concentrations of TPS in the Rosh Pinah town amounted to 100-300 μg m~(-3), but when two thirds of tailings dam surface has currently been covered with water, the concentrations of TPS markedly decreased to 30-100 μg m~(-3). The results of dispersion modeling fit well with the results of ground geochemical survey. The topsoil metal and arsenic contents are in good agreement with their contents in subaerial part of grass (Stipagrostis geminofolia L). The contents of Pb in 52% of collected samples of grass exceed permissible values for dry fodder. The bioaccessibility of metals and arsenic in topsoils was tested by a US EPA-adbpted in vitro method using a simulating gastric fluid formed of a 0.4 M solution of glycine adjusted to pH 1.5 by HC1 addition. Gastric accessible contents of chemical elements in topsoil increase (in % of their total contents in topsoil) in the ascending sequence: Fe (6.2%) -→As (8.6%) → Cu (23.6%)→Zn (39.3%) → Mn (66.7%) → Pb (85.9%). The evaluation of obtained data on gastric bioaccessibility indicates that a health risk related to soil or dust ingestion should be taken into account only in the case of lead.