Environmental Geochemistry of Potentially Toxic Metals

Environmental geochemistry is the discipline that uses the chemistry of the solid earth, its aqueous and gaseous components, and life forms to assess heavy metal contamination impacts on our planet’s ecosystems. It deals with physical, chemical and biological conditions in an environment such as temperature, state of matter, acidity (pH), reduction-oxidation (redox) potential, bacterial activity and biological oxygen demand (BOD). These factors and others influence the mobilization, dispersion, deposition, distribution and concentration of potentially toxic metals/metalloids which can impair the health of organisms in an ecosystem. As such, environmental geochemical data identify pristine chemical conditions that pose no threats to ecosystem inhabitants, those that may suffer from (natural) chemical intrusion from rock weathering and decomposition, and environments that are at risk from chemical element pollution as a result of human activities.

Chemical elements are mobilized by physical, chemical and biological vectors. Elements move in solution as cations, anions and ionic complexes. They incorporate into solid inorganic phases (e.g., sediment, suspended sediment, particulates from natural or anthropogenic emissions) or are absorbed/adsorbed by them. The same is true for solid, perhaps vital, organic phases (e.g., soft and hard parts of organisms, particulate organic carbon). In these modes the chemical elements are transported to depositional environments on land or in water bodies by water, wind and glacial ice following surface drainage, aquifer flowpaths, and wind driven water and atmospheric currents. Mobilized heavy metals in an element assemblage can be carried to an environment in concentrations significantly higher than natural levels in speciated forms. If these are bioavailable, they will be toxic to life forms if bioaccumulated over a period of time. When this scenario is met, the metals pose a threat to basic links in an ecosystem foodweb as well as to environmental niches and the ecosystem they comprise.

Potentially toxic metals follow natural environmental pathways and cycles through the many ecosystems that provide for the very essence of life: water, food and waste disposal. To some degree they follow the geochemical cycles for the nutrients that sustain life (Figure 4–1): (O₂) supports respiratory metabolism; CO₂ is the source of carbon for photosynthesis; N₂ is an essential element of proteins; S is essential for protein and vitamin synthesis; and P is incorporated into many organic molecules and essential for metabolic energy use. Terrestrial, fluvial/lacustrine, estuarine and oceanic life forms can suffer short-or long-term perturbation if these pathways and cycles are intruded by natural events or impacted by human activities.

Natural geochemical background concentrations have been taken in the past as average crustal contents (Table 5–1). This is not compatible with environmental geochemistry research on specific pollution problems because of the great variation in composition of the rock types comprising the crustal surface (Table 2–1). This leads to variations in soil and other overburden chemistry. Chemical variations among flora and fauna likely reflect their growing and feeding in an area of varying geology. Added to this are the chemical changes rock materials undergo during weathering and from diagenesis. Natural background varies with other factors such as the sample used (Table 6–1), the size fraction or organism part analysed, and the analytical methodology employed.

Critical phases in planning a study to assess the health status of an ecosystem are: 1) determining which samples can be used; 2) understanding what the sample represents in space (area and volume) and time; 3) knowing how chemical elements may be bound in a sample, physically and chemically; and, 4) establishing an ideal scale of sampling with realistic modifications as a function of where a sample type can be collected.

Geochemical analysis of sample types evaluated in environmental research (Table 6–1) should give accurate and precise results at concentrations close to detection limits as well as at high concentrations. Analytical techniques are chosen to obtain optimal results. This improves the basis for assessment and scientific interpretation of geochemical data. Nonetheless, more complete resolutions to environmental geochemistry problems require other data and considerations. A focussed research program starts with sample selection (Chapter 6) and a thorough description of measured and observed physical-chemical-biological parameters at sampling locations. This is followed by care in sampling, proper sample treatment and preservation in the field, and correct sample handling during transport to the laboratory and storage there. Next there must be care in preparing a representative sample for analysis in the laboratory, preparing replicates to monitor precision, and selecting standards matched to sample matrix type(s) and projected concentration ranges of target elements or compounds to monitor accuracy. The July 1994 (Vol. 18) Special Number of the Geostandards Newsletter lists producers of about 400 geochemical reference materials available to the analytical community. It describes the standards and provides data such as “best values” for the concentrations of about 60 chemical elements and also considers problems that might be associated with use of specific standards.

A major role of geochemistry in environmental projects is to assess clean-up possibilities for ecosystems that contain pollutant concentrations of potentially toxic metals that could access a foodweb. This role extends to remediation of environments where, in addition to heavy metals, extreme conditions occur that threaten ecosystem life such as low pH (acidic) waters or waters with limited BOD capacity The targeted cleanup media include solids, liquids and gases from contaminated soils, groundwater and surface waters, sediment (fluvial, lacustrine, estuarine, marine), waste disposal sites and sewage sludge (industrial, agricultural, mining and municipal), and chimney emissions (e.g., smelting and electricity generating facilities).

The essence of environmental sustainability lies with a population’s accessibility to several basic biological, chemical and physical needs. These needs are clean air, clean water, sufficient nutrition, contained and isolated waste disposal, natural resources including wood and fiber for shelter and clothing, metals and non-metals for manufactured goods, sources of energy, and safe living and working space. In some global areas these needs are met but at the expense of creating unsustainable conditions (e.g., from deforestation, from over fishing, from over cropping without addition of nutrients to soil, and from mining without care to acid and metal-bearing drainage). In other areas, with forethought, planning and execution of directives, a sustainable ecosystem (e.g., with respect to forestry and fisheries) can be established. If attained, sustainability can meet the needs of the present population. Ideally this can be achieved without compromising the ability to meet the needs of future generations with larger populations.