A screening-level in vitro test was developed to evaluate the relative solubility of ingested lead (Pb) from different mine wastes in the gastrointestinal (GI) tract. The in vitro method, modeled after assay methods for available iron from food, used a laboratory digestion procedure designed to reproduce GI tract chemistry and function. The in vitro method was independently calibrated against a rabbit feeding study, demonstrating that only 1-6% of the total Pb in four mine-waste samples with disparate Pb mineralogy was bioaccessible. In vitro method development tests indicated that H+ concentration and Clc omplexation control dissolution of Pb-bearing minerals in the stomach and that both GI tract enzymes and organic acids are necessary to maintain Pb in the soluble form on entering the small intestine. The experimental results indicate that ingestion of Pb-bearing mine wastes results in limited Pb solubility and that the in vitro test provides a screening-level estimate of the maximum available Pb from mine wastes.

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Superfund risk assessments and the resulting soil arsenic (As) cleanup levels selected for mining sites are currently based on the toxicity of soluble As in drinking water. However, Anaconda soils and house dusts contain less soluble smelter-related As phases, consisting primarily of metal−arsenic oxides and phosphates. If accidentally ingested, As bioaccessibility is restricted by the sparingly soluble nature of As-bearing phases, the prevalence of authigenic carbonate and silicate rinds, the kinetic hindrance to dissolution, and the inaccessibility of encapsulated As. These limitations to As disolution explain the lower bioavailability factors observed for Anaconda As-bearing soils.

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Microprobe analyses of 38 soil and 5 mine-waste samples from Butte, Montana, demonstrated that the samples contain predominantly sulphide/sulphate and oxide/phosphates of lead (Pb)-bearing phases associated with mine waste. The sulphide/sulphate assemblage consists primarily of galena altering to anglesite and plumbojarosite, with secondary jarosite precipitating and rinding the Pb-bearing minerals. In addition, galena was encapsulated within pyrite or quartz grains. The oxide/phosphate assemblage consists of pH-neutral soils in which a plausible paragenetic sequence of PbO to Pb phosphates, PbMnO, or PbFeO is proposed, dependent on the activity of P, Mn, Fe, and Cl in the soil. In addition, Pb-bearing grains are occasionally armoured by the presence of a 1- to 3-(μm rind of authigenic silicate. The low solubility of the Pb-bearing minerals resulting from encapsulation in non-Pb-bearing reaction rinds may provide an explanation for the limited Pb bioavailability observed when Butte soils were fed to rats (Freemanet al., 1992). Further evidence of the lack of absorption of lead from these soils is provided by the results of a blood-Pb study indicating very low blood-Pb levels in Butte children. The lower bioavailability of Pb from mining sites, compared to smelting and urban environments, is also due to kinetic limitations that control dissolution rates of Pb-bearing solids relative to the residence time of soil in the gastrointestinal (Gl) tract. When the test soil was fed to New Zealand White rabbits, only 9% of the total Pb was solubilised in the stomach, and therefore available for absorption. Anin vitro assay, developed to estimate maximum available Pb from soil, demonstrates that ingestion of mine-waste-bearing soil results in limited Pb dissolution, and produces results similar to thein vivo testing.

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