Geochemical studies on the contamination and dispersion of trace metals in intertidal sediments around a military air weapons shooting range

Purpose: A number of publications have raised trace metal contamination of soils and sediments within and around shooting ranges used for sport or military training. To our knowledge, however, there is no publication on the contamination of sediments derived from military shooting ranges in a marine environment. Therefore, this work was purposed to assess the dispersion and fractionation of ammunition- and bomb-derived trace metals in intertidal sediment. Materials and methods: Intertidal sediments (n=32) were collected around a small island that has been used as a target for military air weapons shooting training for more than 50 years in the west coast area of South Korea and were analyzed for size distribution, pH, organic/inorganic carbon contents, and trace metals concentrations. Three kinds of the chemical extraction were used for trace metals analysis: 1) total extraction with a 4:4:1 mixture of HF-HNO₃-HClO₄ acid, 2) partial extraction with 0.1 N HCl, and 3) sequential extraction by a modified Community Bureau of Reference (BCR) method. We also compared our total concentration data with values reported for shallow marine sediments off South Korea and worldwide. Principal component analysis (PCA) was also used to identify shooting-related anthropogenic inputs for some trace metals. Results and discussion: Ranges of total trace metals concentrations (mg kg^-1) are: Cd, 0.5-3.7; Co, 16-22; Cr, 46-71; Cu, 10-51; Mn, 340-998; Ni, 12-28; Pb, 18-277; and Zn, 50-89. Using Li as a geochemical normalizer suggests that significant enrichments of Pb and Cd and moderate enrichments of Cu and Zn occur restrictedly in the immediate vicinity (<150 m) of the shooting range. This is also supported by the results of PCA analysis. Sequential extraction shows that Fe-Mn (hydro-)oxides play an important role in controlling (immobilizing) anthropogenic trace metals (Pb, Cd, Cu, and Zn). Pb and Cu are also bound to carbonate minerals, while Cd is also present in the exchangeable fraction. Conclusions: Trace metals (Cd, Cu, Pb, and Zn) derived from weathering of ammunitions and bombs have not been dispersed extensively to marine environment from the target island, due to either adsorption onto and/or coprecipitation with Fe-Mn (hydro-)oxides and carbonates (typically for Pb, Cu, and Zn) or cation exchange with clays (especially Cd). These geochemical processes of trace metal immobilization would rapidly occur with an increase in pH (up to 8.0-8.6) in the intertidal environment. However, we recommend a better isolation of metal-contaminated island soils from seaward movement.