Environmental risks are typically characterized in the risk assessment framework by comparing exposure concentrations and critical effect concentrations. In OECD countries, critical effect concentrations for metals are based on Predicted No Effect Concentrations (PNEC), which are typically derived from long-term laboratory ecotoxicity tests performed shortly after amending “clean” standard soils with highly soluble, almost completely dissociated metal salts.
Under these test conditions most of the metal is usually present in the most bioavailable and toxic form and resulting toxicity thresholds are near or below the background Ni concentrations in naturally occurring soils. Research has demonstrated that when considering the bioavailability of nickel (and other metals) in soils, the following factors are the most important in determining the ecotoxicity to soil organisms:
- Metal Form: Ni can enter the soil environment in different forms, such as soluble (associated with a high bioavailability, e.g., soluble salts) or sparingly soluble compounds (associated with a low bioavailability, e.g., oxides);
- Ageing: Laboratory Ni spiked soils often exhibit greater toxicity than field contaminated soils with the same Ni concentration. The greater toxicity of Ni in spiked soils compared to corresponding field contaminated soils can be partly attributed to the time between the addition of nickel to soils and the measurement of toxicity. The bioavailability and toxicity of nickel in spiked soils tend to decrease with time in a manner that is dependent on soil pH (see Correction for Leaching and Ageing Effects); and
- Soil Characteristics: The toxicity of nickel is highly dependent on soil characteristics. Specifically, Ni toxicity to plants, invertebrates, and microbial processes decreases as the Ef-fective Cation Exchange Capacity (eCEC)1 of the soil in-creases (see Normalization for Soil Types).
Practically speaking, this means that Ni toxicity can vary consid-erably between laboratory-spiked and field-contaminated soils and among soils with different physico-chemical characteristics. It also means that toxicity tests with the same terrestrial species that are performed using different types of soil can result in different toxicity conclusions. Consequently, a generic PNEC may be largely over or under protective depending on the soils and procedures used for generation of toxicity data. Hence, there is a clear need for bioavailability models to account for these differ-ences in order to generate site-specific PNECs for the terrestrial environment.
This fact sheet provides a summary of the development of Ni bioavailability models for the terrestrial compartment, as well as clear guidance on how to perform and implement bioavailability correction for these systems.
1. eCEC: effective cation exchange capacity = CEC measured at the native pH of the soil (in contrast to CEC measured at a fixed, buffered pH). This is a measure for the sum of exchangeable cations plus extractable acidity held on or near the surface of negatively charged material, such as clay or organic matter, at native soil pH. It is usually expressed in centimoles of charge per kilogram of exchanger (cmolc /kg).