Ecological Risk Assessment
Natural resource managers in the Alligator Rivers Region (ARR), and elsewhere in northern Australia, have few tools to determine what environmental assets are at greatest risk from multiple threats, where these range in scale from point source pollutants through to diffuse landscape-scale impacts of invasive species and, on a global scale, the impacts of climate change. However, Ecological Risk Assessment is a powerful analytical tool that allows objective comparison of the relative risk contributed by each specific ‘threat’ to ecological structures being managed. This permits risks from multiple stressors to be evaluated and communicated in a logical, robust and transparent manner. The process therefore facilitates optimum decision making for the management of natural resources through complete use of available information on potential environmental stressors, and through participative consultation with all stakeholders.
Kakadu National Park has Ramsar-listed wetlands and is a World Heritage site, but the mining and milling of uranium has occurred on a mineral lease within its boundaries for 25 years without any major off-site environmental impact. Nevertheless, Kakadu is exposed to other major ecological threats such as invasive species and climate change. For the past five years eriss has been undertaking a quantitative Ecological Risk Assessment of the Magela Creek floodplain, downstream of Ranger mine that encompasses threats identified from:
- point source mining-related risks; and
- diffuse landscape-scale risks.
A high protection level for the biodiversity of aquatic ecosystems was used as the assessment endpoint and, whilst measurement endpoints inevitably varied, they all encapsulate some metric of ‘species affected’ facilitating comparison between different risks. For minesite risks the focus was on three key chemicals (uranium, sulfate & magnesium) in the surface water pathway, and the focus for landscape-scale risks was wetland weeds, feral pig damage and unmanaged fire.
Additionally as part of the Tropical Rivers Inventory and Assessment Project, eriss is undertaking a broad-scale Ecological Risk Assessment of key threats to Australia’s tropical rivers.
The Ecological Risk Assessment process
Ecological Risk Assessment is the term ascribed to the method(s) for determining risk posed by a stressor (contaminant or perceived threat) to the survival and health of ecosystems. Under these procedures risk is defined as the probability that an adverse effect will occur as a result of ecosystem exposure to a particular ‘concentration’ of the stressor. Hence risk is determined by measuring two components:
- the consequences (also measured as effects)of an adverse event; and
- the likelihood or probability of the event occurring (exposure).
Using these criteria, risk is quantified as the probability of an adverse event, or the likelihood of exposure multiplied by the consequences or effects of that exposure (Prisk = Pexposure x Peffects).
Hence, the aim of Ecological Risk Assessment is to estimate the probability of adverse events from identified environmental stressors. Traditionally, Ecological Risk Assessment has been used to investigate the effects of the release of particular chemical pollutants (toxicants) into the receiving ‘environment’. However, Ecological Risk Assessment is now applied more broadly to assess the relative impact potential of multiple ‘threats’ against measured and/or predicted impacts on environmental values (appropriate & measurable monitoring endpoints).
The systematic steps for performing Ecological Risk Assessment, as applied to an identified stressor, are outlined in Figure 1. Here it is important to emphasise the iterative nature of risk assessment in that results are updated periodically based on inclusion of new data and/or monitoring information. Further, risk-reduction strategies are developed from improved understanding of both the risks posed by specific stressors and of the processes contributing to them. In this context Ecological Risk Assessment plays an important role in best-practice natural resource management based on adaptive management principles.
Figure 1 A basic framework for conducting Ecological Risk Assessment (adapted from US EPA1998)
Consider the example of determining the risk of exposure to toxic levels of uranium to biota in Magela Creek waters downstream of Ranger mine. In this case there are two information sources for the assessment:
- Exposure observations from monitoring records of uranium concentration in Magela Ck (eg figure 2 a); and
- Effects observations, where the No-Observed-Effect-Concentration thresholds are measured for six native aquatic species of Magela Ck using ecotoxicity testing procedures
As illustrated in Figure 2 uranium levels for all exposure observations fall well under the effect thresholds and the risk probability for exposure to uranium at detrimental levels in Magela Ck is therefore close to zero.
Figure 2 Cumulative probability for exposure to uranium in Magela Ck, downstream of Ranger mine (1998-2005) and for potential effects (No-Observed-Effect-Concentration) of uranium on six species of native aquatic fauna (from ecotoxicological studies). TV = monitoring trigger value for uranium in Magela Ck.
In this case the assessment endpoint used to evaluate impacts from off-site uranium contamination from Ranger mine is the conservation of the biological diversity of the ARR. Data from water quality monitoring of uranium and from laboratory toxicity testing for uranium using native aquatic species are the practical measurement endpoints used to represent potential exposure and effects, respectively. From this data risk of contamination by uranium via the surface water pathway can thereby be determined.
While risks of hazardous exposure to uranium (and other mine-related products) have been found to be extremely low in Magela Creek, a risk-reduction strategy is nevertheless in place. Objectives for water quality management are based on minimum dilution requirements for mine-related products in receiving waters of Magela Creek. The uranium limit of 6 µg/L recommended by the Supervising Scientist has been derived using local ecotoxicological data in accordance with the Australian Water Quality Guidelines to protect 99% of the species present. In the event that a limit is exceeded, an agreed and practical management response is identified. In this case, therefore, risk-reduction management is intrinsically linked with the regulation and controlled release of mine waste-waters, offsite. When used in conjunction with timely monitoring information, these guidelines provide the basis for the ongoing management and re-evaluation of risks to Magela Creek biota.
Depending on the information available for undertaking Ecological Risk Assessment, there are several approaches used to characterise risk. The classical quantitative approach, based on frequency information for effects and exposure uses null hypothesis testing & likelihood estimation (Figure 2). Where frequency data are not available a Bayesian statistical approach which involves assessing degrees of ‘belief’ using qualitative or semi-quantitative reasoning is often used. In practice, a combination of techniques are used, where semi-quantitative assessments tend to be precursors to quantitative assessment (Figure 3). For example, conceptualising risk pathways to identify how and what risks may arise and to plan targeted monitoring is an important qualitative step from which quantitative data can then be acquired. Alternatively, applying a structured semi-quantitative approach to ranking risks (eg Table 1), which considers uncertainties, is a beneficial way of determining priorities, particularly when assessing risks from multiple stressors. This is often done using an expert technical panel to review available information.
|A (Almost certain)||H||H||VH||VH||U (H)|
|B (Likely)||M||H||H||VH||U (M)|
|C (Possible)||L||M||H||VH||U (L)|
|D (Unlikely)||L||L||M||H||U (L)|
|E (Unknown)||U (L)||U (L)||U (M)||U (H)||U|
VH= Very High; H-High; M=Medium; L=Low; U=Unknown
Figure 3 The different levels of risk assessment (after Deere and Davidson 2005)
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