Review of biological indicators for metal mining effluents : a proposed protocol using earthworms

There is growing concern about the need for unified criteria on bioindicators used to evaluate the impacts of mine effluents or accidental spills. Toxicological tests based on lethal concentrations (e.g. LC50) have been extensively used to assess effluents, although the biological impact of mining activities cannot always be attributed to acute lethality alone. Geochemical methods, such as sequential or selective chemical extraction, have also been widely used to provide indirect evidence of the availability of metals to organisms. However, the relationship between geochemical parameters, metal uptake, and biological effects is frequently not clear due to complicating interactions between variables. In order to comprehensively characterize risks from mining related discharges, concerns with existing, commonly employed methodologies must be resolved and protocols to assess the effects of sub-lethal or chronic exposure must be established. This paper reviews current understanding of bioaccumulation and bioavailability of heavy metals associated with mining effluents and it explores the concept of the bioaccumulation factor (BAF) and its applicability to mining impacted sites. Existing protocols for assessment of mining related discharges are also compared and a simple, low-cost methodology using earthworms for the evaluation of metal bioavailability in tailings and effluents is proposed. Earthworms are particularly suitable for the assessment of contaminant bioavailability as they are proven metal accumulators and are in full contact with the substrate they consume. As well, they are well studied, easily bred and participate in many food chains and, unlike fish, can be used to assess a variety of media. INTRODUCTION In response to escalating public concerns about the increased frequency and magnitude of mining operations throughout the globe, several methodologies have been developed to assess the risks to ecological or human health from exposure to mining-related discharges. Although protocols for acute lethality are the most common current practice, it is becoming increasingly apparent that exposure to small doses of contaminants at key stages of development or for extended periods of time may also have serious repercussions. As these adverse effects may not be overtly measurable, or may affect only subsequent generations, assessing risks from "low-level" exposure is extremely difficult but will undoubtedly be necessary in the next stage of biological impacts research. The objective of this review is to expose deficiencies and limitations of the existing protocols used to assess metal bioavailability and propose a simple, low-cost alternative that can be applied to various media (i.e. sediments, tailings, water).