Northwest Territories Geological Survey

Ecological and human health risk assessments at active and abandoned metal mines require accurate geochemical data for earth materials, including soils, sediments, dusts, and mine wastes. Reliable data on metal(loid) concentrations are also important for establishing geochemical baselines and to support environmental monitoring activities. However, the reported concentration for an element in a given sample can be affected by many factors, including mineralogy, grain size, laboratory digestion conditions (reagents, temperature, time), and the instrument(s) used for elemental analysis. In particular, aggressive multi-acid digestion methods used to provide “near-total” data for some metals of environmental interest (e.g. chromium (Cr)) may result in the loss of other elements (e.g. antimony (Sb), arsenic (As), sulfur (S)) through volatilization prior to analysis. When comparing data from different studies or sampling periods, it is crucial to ensure that the analytical protocols used are consistent and that changes in element concentrations over time are not simply due to variations in sample processing or laboratory procedures.

The purpose of this study is to compare common digestion protocols used in the analysis of geological materials and provide recommendations on the most appropriate techniques for use in environmental risk assessments. We analyzed samples of lake sediments, soils, and mine tailings collected at metal mine sites across Canada following digestion using two of the most commonly employed procedures in exploration and environmental geochemistry: modified aqua regia digestion (equal parts HNO3:HCl:H2O at 95°C for one hour) and a 4-acid digestion (HF-HClO4-HNO3 heated to fuming and taken to dryness; residue dissolved in HCl). All solutions were analyzed using ICP-ES/MS, and certified reference materials (CRMs) and duplicate samples were used to monitor analytical accuracy and precision. The concentration of elements (e.g. Cr) hosted in relatively insoluble mineral phases are consistently higher following 4-acid digestions as compared to digestions using aqua regia, but generally lower than total values in CRMs. In contrast, the concentrations of As are consistently higher in all samples following aqua regia digestions and in good agreement with certified values. The behaviour of other elements (e.g. Sb, copper (Cu), lead (Pb), S, zinc (Zn)) was more variable and for many samples, the results following 4-acid and aqua regia digestions were statistically indistinguishable. Variations in the mineralogy of different samples play a key role in determining the fraction of different metal(loid)s released by these digestion techniques. This presentation will highlight some of the advantages and disadvantages of using each of these digestion protocols for risk assessment and environmental monitoring purposes, and provide recommendations for using geochemical data to help guide environmental decision-making at both active and abandoned metal mines.