What's in a Number? Interpretation of Geochemical Analyses for Environmental and Human Health Protection

For environmental and human health protection, geochemical properties of earth materials establish the measure of risk. Geochemical properties originate in mineralogy, and their interpretive value for risk assessment is determined by the combination of geological factors and analytical protocol.

Mineralogy and mineralogical variation reflect geological provenance, process, and past. Provenance indicates the bedrock origins of minerals; process refers to physical actions of wind, water, ice and gravity that redistribute and modify mineral particulate at the earth's surface; past represents the geological record of change in process over time record.

Protocol establishes how geochemical analysis results are obtained: for earth materials, protocol is most simply summarized in terms of grain size and strength of decomposition. Environmental risk assessment is commonly based on either the <2 or the <0.063 mm grain size fractions, and on either total (e.g., HF-HClO₄-HNO₃) or strong acid (e.g., HCl-HNO₃) mixtures; such protocols originate with historical practices in agriculture and mineral exploration.

How geological factors and protocol combine to affect geochemical properties and the interpretation of risk may be expressed by mineral partitioning and grain size, and by mineralogy and decomposition strength.

Mineral partitioning describes the non-uniform distributions of mineral species among grain size fractions. In transported overburden, it results from geological processes that separate minerals according to physical properties. Where geochemical properties differ among grain size fractions, grain size affects the determination of environmental conditions associated with exposure pathways. For particulate ingestion and inhalation, risk may be best defined by properties of clay-sized minerals.

Wet chemical decompositions are selective for mineralogy; with decrease in strength, analytical results are increasingly restricted to elements in minerals having an increased potential for biosphere transformation. As measures of risk, threshold values defined solely by concentration cannot be uniformly applied because their interpretation is dependent on mineralogy, grain size, and decomposition strength.