Formatted Title

Statistical Analysis, Bioaccessibility and Soil Amendment: Resolving Soil Lead Contamination from Mining and Smelting

Background/Objectives

A former base metal smelter in eastern Australia which processed and refined sulfide ores and concentrates for approximately 100 years was suspected to have caused soil lead contamination in the adjacent residential suburb. As part of site closure and surrender of the operating license, the current owner was required to: assess for the presence of smelter-related soil lead contamination in surrounding residential areas; and carry out remediation where potential risks were identified. The potentially affected area is an older suburb with several arterial roads – meaning other (non-smelter) sources of contamination (including lead-based paints and leaded petrol vehicle emissions) may have contributed to soil lead contamination. Senversa was engaged to complete a multi-stage, five-year investigation program to identify the nature and extent of smelter-related contamination, determine risks posed, and develop and implement remedial strategies.

Approach/Activities

The investigation consisted of an initial review of the historical smelter layouts and operations, environmental setting information and previous investigations in the surrounding suburb – the purpose being to identify the locations and likely nature of smelter sources and the potentially contaminated residential areas. These data were used to inform the scope of subsequent sampling in public open space and select private properties to determine the presence and extent of soil lead contamination. Statistical methods (regression and cluster analysis) were used to differentiate smelter-related lead contamination from other sources. In vitro bioaccessibility testing and blood lead modelling were completed to develop area-specific soil management levels and identify areas where risks to human health exist and remediation is required. Remedial strategies are being developed through engagement with impacted stakeholders (residents), with novel soil amendment (to reduce lead bioavailability) using residues from other nearby industry being trialled at properties where removal of contaminated soils is not possible or not the preferred remedial option.

Results/Lessons Learned

The initial historical reviews indicated: key sources of smelter-related lead contamination were from stack and fugitive emissions (including from uncovered ore, concentrate and waste); the emissions were likely to be in the form of native lead, lead sulfides, sulfates and oxides; and contamination could extend up to 1 km to the south and west of the smelter. The subsequent (multi-stage) public open space and private property sampling (which included collection and analysis of more than 600 samples) and comparison against the generic investigation levels (300 mg/kg) indicated soil lead contamination: was predominantly present in the upper 0.2 m of the soil profile (consistent with airborne sources associated with historical smelter emissions); extended up to 800 m to the south and west of the smelter; and was likely to affect approximately 500 residential properties and schools. Statistical analysis comparing concentrations of copper, lead and zinc (the main metals in ore processed at the smelter) found the majority of contamination was attributable to the smelter, with some notable exceptions. Speciation analysis using near edge x-ray absorption spectroscopy (XAS) found soil lead contamination in the residential areas was also sorbed to humic matter (indicating transformation from source species, likely due to environmental exposure). Given the large extent of the soil lead contamination, in vitro bioaccessibilty testing was completed - with the results used in IEUBK modelling to derive an area-specific clean-up level of 400 mg/kg. Comparison of the soil lead concentrations in the residential areas to the bioaccessibility-adjusted clean-up level reduced the number of houses where risk to human health may exist (and where remediation is required) to 60 - saving the client approximately $15M in further investigation and remediation costs and underscoring the importance of determining bioavailability when assessing risks posed by soil lead. The results of the investigations have been reviewed and approved by an independent third-party expert and EPA.

Remediation is currently being planned, with soil amendment using industrial residues being trialled. These residues variously contain high concentrations of iron, manganese, aluminium, organic matter, phosphorous and clay – which the literature indicates can support reducing bioavailability through sorption, complexation and precipitation within the circum-neutral conditions in the intestines – preventing some of the soil lead solubilized in the stomach from entering the human circulatory system.