Supervising Scientist, Darwin, 2005
ISBN 0 642 24395 6
ISSN 0 158-4030
The Environment Protection (Alligator Rivers Region) Act 1978 established the Alligator Rivers Region Research Institute (ARRRI) to undertake research into the environmental effects of uranium mining in the Alligator Rivers Region. The scope of the research programme was widened in 1994 following amendments to the Act. ARRRI was subsequently renamed the Environmental Research Institute of the Supervising Scientist (eriss ).
The work of eriss consists of two main areas:
The focus of this Chapter is the work being undertaken on mining-related activities in the Alligator Rivers region. The research that eriss carries out on tropical river and wetland systems is described in Chapter 5.
Six thematic areas (based primarily on geographic provenance) have been identified by the Alligator Rivers Region Technical Committee (ARRTC) as being the Key Knowledge Needs to ensure the current and future protection of the environment of the Alligator Rivers Region (Appendix 1).
For each of the thematic areas the key research requirements for assessing current status by monitoring, for establishing rehabilitation requirements, and for producing defensible closure (relinquishment) criteria and post closure monitoring programs were identified and prioritised (in both technical need and time dimensions). These research requirements provide the basis for defining the core eriss project activities to be carried out from year to year. The content of the research programme is assessed annually by ARRTC, whose charter and activities are described in detail in Chapter 4 of this Report .
eriss contributes to the addressing of each of the key knowledge needs by applying a broad range of scientific expertise across the research fields of:
A selection of highlights from each of these topics is presented here. Further details on research outcomes are published in journal and conference papers and in the Supervising Scientist and Internal Report series. Publications by SSD staff in 2004–05 are listed in Appendix 3. More information on SSD publications, including the full list of SSD staff publications from 1978 to end June 2005, is available at www.deh.gov.au/ssd/publications.
Magnesium sulfate (MgSO4) is the dominant surface water contaminant associated with Ranger uranium mine (Ranger). Although this common salt is generally considered to be of very low toxicity, aquatic surveys in waterbodies on the Ranger site in the mid-1990s showed correlations between changes in macroinvertebrate community structure and increasing MgSO4. This finding prompted a full ecotoxicological investigation, including: identification of the dominant toxic ion; assessment of Mg toxicity in extremely soft local creek water, in both the laboratory and in the field; and evaluation of the influence of calcium (Ca) on Mg toxicity in the laboratory. Much of this research has been described in previous Supervising Scientist Annual Reports (2001–02, 2002–03). Here, we present research quantifying the influence of Ca on Mg toxicity and the implications of this on a site-specific water quality trigger value for Mg.
The three species (of six assessed) found to be most sensitive to Mg in local Magela Creek water (duckweed, Lemna aequinoctialis; snail, Amerianna cumingi; and green hydra, Hydra viridissima) were used to quantify the influence of Ca on Mg toxicity. The test species were exposed to a constant Mg concentration, being the concentration known to result in a 50% inhibition of response (eg population growth, reproduction) relative to unexposed (control) organisms, at increasing concentrations of Ca. Figure 3.1 shows that as Ca increases (ie as the Mg:Ca ratio decreases), Mg toxicity decreases for all three species.
Figure 3.1 Effect of Ca (expressed as Mg:Ca ratio) on the toxicity of Mg to three species, when Mg concentration was held constant at the IC50 concentration (ie 5 mg/L for Lemna, 10 mg/L for Hydra, 18 mg/L for Amerianna)
For Hydra and Lemna, a full recovery was observed, whilst an approximate 80% recovery was observed for Amerianna. The differences in the extent of recovery between species is possibly due to different mechanisms of toxicity of Mg. Using the relationships from Figure 3.1, a Mg:Ca ratio of 9:1 was considered the best approximation of a ratio below which the likelihood of unacceptable Mg toxicity is very low.
To complete the research, the toxicity of Mg was again fully characterised using six local freshwater species, but this time the Ca concentration was also manipulated such that the Mg:Ca ratio was maintained at, or around, the ‘safe’ ratio of 9:1. The summary results, compared to those obtained for Mg in the presence of only Magela Creek background Ca concentrations (~0.2 mg/L), are shown in Table 3.1. The values presented represent maximum concentrations of Mg above which effects to the organisms might be considered ecologically unacceptable. For all species, Mg toxicity was markedly lower at the Mg:Ca ratio of 9:1. However, the extent to which toxicity was reduced varied between species. For example, the cladoceran and snail were approximately only 2 x less sensitive to Mg in the presence of Ca, whilst the alga and gudgeon were well over 1000 x less sensitive. These differences in relative sensitivity are possibly due to different physiologies of the test species and different mechanisms of toxicity of Mg.
|Mg toxicity (expressed as IC15 values) 1|
Without additional Ca 2
At Mg:Ca ratio of 9:1 3
|72-h cell division rate
|96-h plant growth
|96-h population growth
|16 5||4300 4,5|
1 IC15 : The concentration resulting in a 15% inhibition of response (eg reproduction) relative to unexposed (ie control) organisms.
2 Except where noted, data for each species represent geometric means of IC15 values from 3 independent toxicity tests.
3 Except where noted, data for each species represent geometric means of IC15 values from 2 independent toxicity tests.
4 Data for each species represent values from 1 toxicity test. These data should be considered as interim only.
5 Data for M. mogurnda represent LC5 values; the LC5 being the concentration resulting in 5% mortality relative to unexposed (ie control) organisms. This more conservative value is required because the endpoint for this test represents an acute, lethal response.
Using the BurrliOZ6 species sensitivity distribution approach recommended in the ANZECC and ARMCANZ (2000) Water Quality Guidelines, a site-specific water quality trigger value for Mg in Magela Creek was derived. Given that Magela Creek lies largely within the World Heritage and Ramsar listed Kakadu National Park, a high level of protection is required. Consequently, a water quality trigger value for Mg was calculated that will protect at least 99% of species. Figure 3.2 shows the cumulative probability plots for Mg toxicity at (A) Magela Creek background Ca concentration and (B) the Mg:Ca ratio of 9:1.
Figure 3.2 BurrliOZ species sensitivity distributions for Mg toxicity at (A) natural Magela Creek background Ca concentration (ie ~0.2 mg/L) and (B) a constant Mg:Ca ratio of 9:1. Data points represent the IC15 toxicity values for each species (see Table 3.1) and are plotted as the cumulative frequency. The Burr Type III distribution is represented by the black fitted curve and is the distribution used to calculate the trigger values for Mg (as recommended by ANZECC & ARMCANZ 2000).
Based on the Burr Type III distribution (black fitted line in Figure 3.2), the concentration of Mg that should protect at least 99% of species is approximately 1 mg/L when Ca concentration is maintained at Magela Creek background levels, and 4 mg/L7 when the Ca concentration is manipulated to maintain the Mg:Ca ratio at 9:1.
The above 4-fold difference in the two trigger values is of significance to the management of discharged waste water at Ranger, because the discharged waters contain elevated Ca as well as elevated Mg. Thus, the Ca in Ranger waste waters is able to provide a protective function against potential Mg toxicity in the discharged water, and this needs to be taken into account when developing a site-specific trigger value. Therefore, if the Mg:Ca ratio in Magela Creek downstream of Ranger is maintained at or below 9:1, Mg concentrations of up to 4 mg/L should present very low risk to the local aquatic biota. To illustrate the low risk to aquatic biota to date, based on actual water quality data, Figure 3.3 shows cumulative frequency distributions for Mg and the Mg:Ca ratio as measured at the monitoring point downstream of Ranger, from 1985 to 2005. The Mg trigger value has been exceeded for only approximately 0.5% of the time, whilst the Mg:Ca ratio of 9:1 has not been exceeded. Thus, these data indicate there is negligible risk to the aquatic biota of Magela Creek from Mg in surface water as a result of current mining operations . Had the Mg trigger value of ~1 mg/L, which does not account for the ameliorative effect of Ca on Mg toxicity, been applied, the risk to aquatic biota would have been significantly overestimated.
Although yet to be finalised, the process that derives trigger values for management and regulatory assessment of water quality in Magela Creek will need to take into account both the measured Mg concentration and the Mg:Ca ratio.
Figure 3.3 Cumulative probability distributions for Mg concentration and Mg:Ca ratio in Magela Creek downstream of Ranger, from 1985-2005. The vertical broken lines represent, from left to right, the Mg trigger value of 4 mg/L (when the Mg:Ca ratio is maintained at or below 9:1) and the 'safe' Mg:Ca ratio of 9:1.
7This must be considered an interim value because the full toxicity data set is yet to be completed for several species (see table 3.1).