A SIMPLE METHODOLOGY FOR DETERMINING THE BIOACCESSIBILITY OF AS, CD, AND PB IN SMELTER CONTAMINATED SOIL SAMPLES: A CASE STUDY OF SOILS IN THE DESERT SOUTHWESTERN UNITED STATES CONTAMINATED BY PB-AG ORE SMELTER EFFLUENT

Epoxy mounted soil samples contaminated by effluent from late 19^th century Pb-Ag ore smelters were documented by SEM and electron microprobe to evaluate metal solubilities after treatment by established EPA gastric leach solutions. Soils collected at shallow depths show Pb, Sb, As, and Cd to be enriched 24-60 times that of unweathered rock material.

Metal-bearing material in smelter impacted soils are grouped into two major categories, primary and weathering-related. Primary effluent is characterized by 4-10 micron grains or aggregates derived from the original smelter ore charge. Metal-bearing grains are comprised chiefly of mimetite (Pb₅(AsO₄)3Cl) and miscellaneous Pb, As, Fe, and Sb-bearing oxides, with minor arsenopyrite and pyrite, and rare galena, exsolved melt fragments, and Pb-Zn-Fe condensate droplets. Weathering-related metalliferous material is characterized by soil aggregates or concretions (10-40 microns) cemented by metal-rich, iron oxyhydroxides (>10% Pb, As, Sb, Zn, Cu), or less commonly Mn-Pb oxides. In addition, metal-rich Fe-oxhydroxides line cavities within the soil aggregates. Crystalline morphologies are rarely observed by SEM in the concretion cements. However, void fillings are commonly lined with delicate masses of acicular Fe-oxyhydroxide crystals.

Following immersion of the sample mounts for 1 hour in the simulated gastric solution, previously characterized metal-bearing phases were reexamined by SEM to compare pre-and post leach morphologies. Preliminary results indicate strong dissolution of primary mimetite grains, whereas, metal-bearing Fe-oxyhydroxides appear to be unaffected. Thin Mn-Pb oxide bands in one concretion appear to be partially dissolved by the leach fluid. These results imply that although Pb and As are readily bioaccessible in primary mimetite grains and weathering related Mn-Pb oxides, they may be kinetically inaccessible in the Fe-oxyhydroxides.