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Environment Australia, 2001
The Environment Protection (Alligator Rivers Region) Act 1978 established a research institute, the Environmental Research Institute of the Supervising Scientist (eriss), to undertake research into the environmental effects of uranium mining in the Alligator Rivers Region, and into other environmental issues elsewhere as appropriate.
eriss research is organised into two major programmes:
In addition eriss carries out general environmental research that meets specific needs identified by the Australian Government.
The programme of eriss is reviewed annually by the Alligator Rivers Region Technical Committee (ARRTC).
The objective of the eriss programme on the environmental impact of mining is to provide advice, based on research and monitoring, to the Supervising Scientist and stakeholders on standards, practices and procedures to protect the environment from the effects of mining, particularly uranium mining in the Alligator Rivers Region.
In 2000-01, the majority of research projects carried out within this programme were focused on the following areas:
Full reports on the results of research projects are provided in Internal Reports and in the peer reviewed reports in the Supervising Scientist Report series. In the following sections, brief reports are provided on a limited number of research projects carried out during 2000-01.
This collaborative research between eriss and the Northern Territory University aims to establish a GIS-centred approach to assessing the long-term geomorphological impacts of the Jabiluka mine proposal (see fig 3.1). GIS offers a means by which the data collected during the assessment of possible mining impacts can be stored and analysed. GIS has been linked with a significant number of erosion and hydrology models and used in a limited number of impact assessment studies. However, this study differs from many previous studies by adopting a GIS-centred approach to the management and analysis of data generated by a geomorphological impact assessment. Benefits of this approach include the simplification of data maintenance, revision, and update, as well as facilitating availability and access for users. The design of the GIS has focused on three areas: 1) the development of a rapid erosion assessment technique; 2) the integration of GIS and a complex landform evolution model; and 3) the development of a suite of GIS based tools for the geomorphometric analysis of landform evolution.
Figure 3.1 A hillshaded version of the Ngarradj (Swift Creek) digital elevation model
Illustrating the types of data that can be rapidly accessed through the customised
GIS interface (where SC, ET and UM are stream gauging stations)
A GIS based rapid erosion assessment method has been developed and evaluated. The method allows the user to quickly acquire and evaluate existing data to assist in the planning of more detailed monitoring, modelling and erosion assessment programmes. The rapid erosion assessment method is based on a simplified version of the revised universal soil loss equation (RUSLE), and allows the rapid parameterisation of the model from widely available land unit and elevation datasets. The rapid erosion assessment method has been evaluated through the investigation of the effects of elevation data resolution on erosion predictions and field data validation.
More detailed, quantitative risk assessment can be conducted using a combination of landform evolution modelling and basin analysis in a GIS framework. A landform evolution model ('SIBERIA') has been linked to the GIS through the development of a programme extension named 'ArcEvolve'. Linking SIBERIA with a GIS provides benefits not available in one or other of these environments. SIBERIA has been successfully parameterised using field data from the Ngarradj catchment, which will be the first catchment to be affected should any impact occur as a result of mining operations at the Jabiluka site. Landform evolution modelling with SIBERIA represents a suitable technique for assessing physical impact within a medium scale catchment. Qualitative assessment of the simulations showed little change under natural conditions in the long term.
Combining the GIS-SIBERIA interface with tools for geomorphometric analysis strengthens erosion risk impact assessment as landform change can be assessed quantitatively. A number of tools for the geomorphometric analysis of digital elevation data (the primary input and output of SIBERIA) have been developed or included as part of ArcEvolve. The tools allow the direct comparison of two SIBERIA simulated datasets by the calculation of the denudation rate and volumetric difference between two surfaces. The geomorphic statistics that can be calculated are the width function (see fig 3.1), hypsometric curve, cumulative area diagram and area-slope relationship. These statistics can be rapidly derived from a digital elevation model and have been shown to be important measures of catchment geomorphology and hydrology. These descriptors have also been successfully used to quantify and compare simulated landscapes with natural landscapes. Quantitative analyses of the initial model application to the Ngarradj (Swift Creek) catchment indicate that all the geomorphometric measures, except for the area-slope relationship, could be applied to this very complex catchment for basin analysis. The geomorphometric measures confirm that little change occurs in the undisturbed catchment upstream of the Swift Creek gauging station over periods of about 1000 years. Future work in this project will use field data from disturbed areas of the minesite to test the GIS and assess possible mining impacts.
This study used mathematical models to determine erosion rates on the rehabilitated Nabarlek minesite. These rates can be used to predict increases above natural, background sediment concentrations in the nearby Cooper Creek (see fig 3.2).
Figure 3.2 Catchments affected by the former Nabarlek Uranium Mine
Nabarlek uranium mine is 270 kilometres east of Darwin at the western edge of Arnhem Land (see fig 3.2). Water from the minesite drains into the East Alligator River catchment. The mine is located in a pristine environment and it is important that erosion of the site is understood to effectively manage potential environmental impact.
The Nabarlek uranium deposit was found in 1970 by Queensland Mines Limited (QML). It was mined, nine years after its discovery, in 143 days and the ore stockpiled and progressively processed until 1988. Rehabilitation of the site in 1995 involved levelling all the buildings and structures, covering the area with waste rock and planting seeds. The final shape of the area was designed to minimise erosion on the site.
Model input parameter values were derived for the rehabilitated surface condition at Nabarlek minesite using soil samples and vegetation cover measurements collected at areas within the minesite. Using these input parameter values, the mathematical model was used to estimate how much soil was being moved off the Nabarlek site by erosion. The mathematical model was also used to calculate the natural sediment concentration in the streams draining Nabarlek. Using predicted soil loss values from natural, undisturbed areas surrounding the Nabarlek minesite and predicted stream flows in Cooper Creek it was possible to estimate the background sediment concentration in Cooper Creek. These estimated values were similar to observed values.
Average erosion rates on the rehabilitated Nabarlek site are only slightly higher than natural erosion rates. As would be expected, some areas of the site have erosion rates much higher than natural levels that are balanced by other areas with erosion rates notably lower than natural rates. The predicted total sediment loss from Nabarlek under the current rehabilitated surface condition is only slightly higher than that predicted from Nabarlek under pre-mining, natural surface conditions. As the infiltration rate is higher on ripped and vegetated areas of the rehabilitated site, the reduced runoff from these areas would suggest that the sediment concentration in small ephemeral streams (see fig 3.2) draining Nabarlek has increased.
The predicted sediment concentration in Buffalo Creek may increase above natural background levels the most, with the sediment concentration almost doubling (see fig 3.3). Most areas of the Buffalo Creek catchment have been disturbed and therefore have higher amounts of soil being removed from them. This results in a high concentration of sediment in Buffalo Creek. Erosion on the rehabilitated landform may result in sediment concentration increases in the small creeks immediately draining Nabarlek (see fig 3.3), but reduced flows in the small creeks mean that sediment concentrations downstream of their confluence with Cooper Creek will not be elevated significantly above natural levels (see fig 3.4). This means that, from an erosion and stream sediment concentration perspective, the Nabarlek mine should have little impact on the lower reaches of Cooper Creek, and therefore the East Alligator River, if current management strategies and erosion rates are maintained.
During 2000-01, eriss undertook a series of sample and dataset collections in the upper South Alligator River valley. This work was carried out to assist Parks Australia North with its plans for rehabilitation of old uranium mine and mill locations in the area, and to help in the assessment of any radiological risk to people arising from the presence of these sites.
A number of remotely sensed data sets were acquired in order to gain a synoptic view of the state of abandoned uranium minesites in the upper South Alligator River valley, and in particular, to identify any radiological risk within the valley. The primary data set was a high resolution 50 m line spaced airborne gamma survey, which resulted in the collection of airborne radiometric data (eU, eTh, K and total count rates), magnetic intensity, and digital elevation data. Parks Australia North and eriss jointly funded the acquisition of the airborne gamma survey. A colour infrared (CIR) digital airborne camera (ADAR) was flown over the valley in order to provide high spatial resolution imagery to enhance the location of minesite features and field survey identification. In addition, a hyperspectral data set was captured as part of the MASTER series during September 2000, at 10 m spatial resolution.
Figure 3.3 Stream sediment concentration in small creeks draining the Nabarlek minesite
Figure 3.4 Stream sediment concentration in Cooper Creek downstream of the confluences of creeks draining the Nabarlek minesite
The airborne gamma survey was commissioned because the eU signal represents a measure of radium-226 in surface soil, which can be expected to be elevated in uranium mine and mill material such as waste rock and tailings. Figure 3.5 shows the area flown, with those areas of highest eU count rate (>500 counts per second) shaded black. These areas all correspond to known mine or mill locations, including Rockhole, El Sherana, Scinto V, Scinto VI, Saddle Ridge, Palette and Coronation Hill, as well as the tailings area of the old mill site near the Rockhole Mine area. Currently, the data are being used primarily to identify areas for further investigation on the ground. Until this work is complete, conclusions based on the airborne survey alone must be regarded as preliminary.
In the cases of the former minesites, the survey eU signals were relatively contained geographically, implying that materials such as waste rock which remain on-site are reasonably stable. There were indications of some dispersion and deposition of activity from the tailings area, and of deposition of activity in the Rockhole Mine Creek near to its confluence with the South Alligator River. Since this area is within the main South Alligator River valley and relatively easily accessed by members of the public, and since radium concentrations in mill tailings are higher than those of mine waste rock, it is this area which has been the focus of initial ground-truthing survey work.
Figure 3.5 Airborne gamma survey of the upper South Alligator River valley
Based on the findings so far, the use of high resolution airborne gamma data, particularly the eU channel, has proven useful in giving an overall geographic 'picture' of the state of historic mine areas within the valley and in targeting areas for further ground investigations.
In November 2000, a set of 174 freshwater mussels (Velesunio angasi) were collected from the South Alligator River at five sites in the vicinity of the confluence with the Rockhole Mine Creek and of the known location of uranium mill tailings (see fig 3.5, points 1 and 2). The effect of inflows from the creek were included in this study because acid mine drainage from an exploratory adit at the former Rockhole Mine is known to contain elevated uranium series activity.
The flesh of the mussels is being analysed for a number of elements (including heavy metals) and radionuclides. The aim of this project is to provide information to Parks Australia North and to Aboriginal Traditional Owners regarding the safety of aquatic animals in the area for human consumption. In particular, it is known that Velesunio angasi accumulates radium in its edible tissue.
Previous research in the Alligator Rivers Region has shown that the average concentrations of radium in mussel tissue is age-dependent. Consequently, the mussels were aged by counting of (annual) dark bands in the shells before analysis. The flesh from mussels of the same age from each location was then combined for analysis by gamma spectrometry.
Table 3.1 shows the radium isotope results obtained for 3-year old mussels. Sites 1 and 2 are control sites for which no influence from the tailings area and Rockhole Mine Creek inflows would be expected, site 3 is downstream of the tailings area, while sites 4 and 5 are upstream and downstream of the confluence with the Rockhole Mine Creek, respectively.
|Site 1: 0-10 m upstream to 40 m downstream RMC confluence, south bank of SAR (unexposed)||
266 ± 9
0.74 ± 0.05
|Site 2: Well upstream of tails, downstream of bridge over SAR (Gunlom Road)||
272 ± 9
0.61 ± 0.04
|Site 3: Downstream of tails at gully into north (roadside) bank of SAR||
356 ± 5
1.11 ± 0.04
|Site 4: 0-10 m upstream RMC confluence, north bank of SAR||
350 ± 7
0.84 ± 0.04
|Site 5: 0-10 m downstream RMC confluence, north bank of SAR||
350 ± 6
0.91 ± 0.04
All results are for dried tissue following bulking of a number (n) of mussel flesh samples. The uncertainties are analytical uncertainties only.
The concentrations of radium-226 measured in the edible flesh of mussels from sites 3, 4 and 5 were about 30% higher than those for the two control sites. The radium-226/radium-228 activity ratios were also higher for mussels from sites 3, 4 and 5 than for sites 1 and 2. This ratio is a sensitive indicator of the origin of the radium in mussels, a higher ratio indicating the influence of a uranium-mineralised source. Overall, the results imply that there is an effect from the tailings as well as possibly from the Rockhole Mine Creek inflow on radium-226 levels in these mussels, but that this effect is not large. The resultant dose estimates for people are quite low. For example, the committed effective dose to a 10-year old child who eats 2 kg of mussel flesh from the affected area is about 0.07 mSv. This estimate is much lower than the relevant annual dose limit of 1 mSv yr-1.
From its construction in 1980 until 1998, the concentration of uranium in Retention Pond 1 was generally below 2 µg/L. During the 1998-99 Wet Season, a rapid increase in the concentration of uranium was observed, with a maximum value near 70 µg/L measured in early 1999. Elevated concentrations have been recorded in the subsequent Wet seasons (1999-2000 and 2000-01), but on a declining trend (peak observed values of approximately 40 µg/L and 20 µg/L respectively). The water chemistry of the pond over the past three years has otherwise been normal.
These observations suggest that the excess uranium burden during these years did not originate from the pond itself (remobilisation due to changing physical or chemical conditions), but was due to events in the pond's catchment. Nevertheless, the margins of Retention Pond 1 are only seasonally inundated. This means that alternate cycles of flooding, deposition and drying could result in uranium mobilisation on an annual basis. This mechanism could have operated in years subsequent to the first abnormal observations, when a proportion of the uranium from the 1998-99 episode may have been adsorbed at the pond margins, then desorbed during the following year's flooding. However, this mechanism cannot apply to the first year because there was no previous abnormal uranium load.
The most likely source of the additional uranium is leaching from waste rock derived from Pit 3. This rock has been progressively deposited in the catchment of Retention Pond 1 since before 1998, which accords with the observation of elevated uranium. Since the initial episode, ERA has undertaken substantial engineering works in the catchment aimed at remediating the loss of uranium from the catchment. The evidence from the last two Wet seasons suggests that this has been partially successful. Mass balance calculations using the concentration of uranium in the water of a sediment trap upstream from Retention Pond 1 were performed. The water from this trap flows unimpeded into Retention Pond 1. The information reviewed included concentration and flow data for the 1999-00 Wet season. These calculations revealed that uranium import from the catchment could probably completely explain the uranium flux in the pond for that year.
A examination of Retention Pond 1 sediments revealed that sediment uranium content was much greater than could be accounted by recent catchment episodes. The detailed sediment examination performed subsequently by eriss revealed that the sediments were enriched in uranium to a maximum depth of about 10 cm. This is consistent with the deposition of sediment from the catchment over the life of Retention Pond 1.
A key objective of the eriss study was to determine whether uranium could be mobilised from Retention Pond 1 sediments under chemical conditions that might occur naturally. The most likely scenario for desorption of uranium would be an acidification event consequent on pond drying and refilling.
The investigation established that approximately 50% of the uranium present in Retention Pond 1 sediments is readily mobilised using 1M HCl, and is predominantly bound to the organic fraction of fine (<63 µm) sediments. Parallel investigations have demonstrated that the labile fraction of uranium in Retention Pond 1 sediments is rapidly desorbed as the pH is lowered from about 3.0 to 2.5. This is consistent with theoretical expectations for organic-bound uranium, and also for extrapolated desorption measurements from eriss experiments at higher pH, as depicted in figure 3.6.
Figure 3.6 A comparison of observed and calculated concentration of uranium for desorption from Retention Pond 1 sediments as a function of pH
The lowest pH ever recorded in Retention Pond 1 was a value of 4.3 observed in January 1991. This occurred during refilling after the pond was drained in 1990 to support mining activities. At this time, Retention Pond 1 was dry for almost several months. Under these conditions, accumulated sulphide minerals would be expected to partially oxidise to sulphuric acid due to the action of aerial oxygen on moist sediments. This episode could be considered to be a fairly rigorous test of the potential to induce an acidification event in Retention Pond 1. At pH 4.3, the concentration of uranium in Retention Pond 1 at that time was about 2 mg/L, which compares with a long-term median value for the concentration of uranium of 0.9 mg/L (1981-1998; n = 412). The measured concentration of uranium value at pH 4.3 is lower than the current Water Quality Guidelines recommendation for the concentration of uranium, and is therefore thought to pose minimal environmental risk, even before dilution with Magela Creek water.
The sulphide burden of Retention Pond 1 is currently about twice the value estimated for 1990. An acidification episode now under similar conditions to the 1990-91 event would probably yield a minimum pH nearer to 4.0 (because of the logarithmic nature of the pH scale). This outcome should likewise not lead to an unacceptable release of uranium from the sediments of Retention Pond 1.
The evidence to date suggests a small probability of unacceptable uranium loss from the sediments of Retention Pond 1. This risk will be lower still if effective catchment management practices are introduced for the pond. These would include minimising the export of uranium from waste rock stored in the catchment, and ensuring that the water balance is such that the pond does not dry completely, even during abnormally dry climatic intervals.
Changes in egg production of freshwater snails observed over time in creekside monitoring
In section 2.2.3, Ranger creekside monitoring results for egg production in freshwater snails (Amerianna cumingii) are described for the period 1992 to 2001. A feature of these data, shown in figure 2.2(A), is the overall decline in egg production over time. This decline has occurred equally at upstream and downstream sites and so has not interfered with the ability to use snails to detect and assess possible effects arising from dispersed Ranger mine waste waters. Higher egg production observed in 1992, 1993 and 1994 may partly be explained by the larger size classes of snails that were used in those Wet seasons. Egg production of snails is a function of snail size. From 1995 onward, changes to this creekside protocol included the selection of a narrower and smaller size range of snails. Nevertheless, a decline in egg production is still evident from 1995 onward and so an investigation was undertaken in 2000-01 to determine the possible cause of this decline. In particular, if it was associated with inbreeding and a decline in the genetic vigour of this snail stock, a self-sustaining stock cultured continuously and without replenishment since 1992, a replacement stock would be required.
To test the hypothesis that the genetic vigour of the snail stock was in decline, new wild stocks of snails were collected from Magela floodplain in April 2000 and cultured at eriss, independently of the 'old', original snail stock, over the ensuing Dry season. In the 2001-01 Wet season, several creekside tests were then conducted using old and new snail stocks side-by-side at both of the creek sites to compare performance.
The results of the parallel tests are shown in figure 3.7. Clearly, there is no difference in egg production observed for the two snail stocks over the Wet season period (January and February 2001), indicating that the decline in egg production observed since 1995 is not attributable to inherent genetic features of the original snail stock.
Figure 3.7 Comparison of egg production in freshwater snails cultured from old and new stocks in creekside trials conducted in Magela Creek during the 2000-01 Wet season
A possible explanation for the decline may be that water-borne food sources, especially nutrients and dissolved and particulate organic matter, for snails held in the creekside tests have declined over time. This appears to be responsible for the decline in egg production observed within Wet seasons (1995, 1996, 1997, 1999, 2001; fig 2.2) as the catchment is gradually flushed of nutrients and creek waters become more dilute. It is possible that this same phenomenon operates at larger (interannual) time scales as well. As noted in section 2.2.2, rainfall for seven out of the past eight Wet seasons has been above average. The consequences of this may be reduced accumulations of organic matter in the creek channels, as well as reduced periods in which the creek channels dry out completely. Dry season desiccation of areas normally wetted in the Wet season is known to be a major driver of Wet season primary and secondary production in seasonal freshwater systems.
In 2000-01, the Wetland Ecology and Conservation programme, through its two sub-groups, 'Ecology and Inventory' and 'Risk Identification and Assessment', and also through the National Centre for Tropical Wetland Research (nctwr), continued to undertake research, offer services and provide advice to the community and other clients.
The primary objective of the Wetland Ecology and Conservation programme is to provide advice, based on research and monitoring, to key stakeholders on the ecology and conservation of tropical wetlands. This research is of direct relevance in assessing the environmental impact of mining in the Alligator Rivers Region since the parts of the environment most at risk from mining are aquatic ecosystems, including wetlands.
As part of a larger assessment of environmental flow requirements for the Daly basin being conducted by the Department of Lands, Planning and Environment, eriss was commissioned in March 2000 to undertake an inventory and risk assessment of water dependent ecosystems in the Daly basin. The specific aims were to :
The Daly basin (19 382 km2 in size) occupies approximately 36% of the catchment of the Daly River. The latter has the largest flow of all Northern Territory rivers and, due to the vast underground aquifers supplying the river, reliable flows of good quality water and areas of high potential soils, serves as an example of a region where water resource and agricultural development is being given serious consideration. Given this situation, and in line with national water reform processes, the potential environmental effects of any such development are also being considered.
The wetland inventory included the development of a GIS platform and the collection of data from field surveys and remotely sensed imagery, and available reports and maps. For the purpose of mapping the distribution of wetlands in the Daly basin, heavy reliance was placed on the availability and accuracy of existing information. This included 1:50 000 topographic maps compiled by the Australian Defence Force and the land unit maps of the Daly basin compiled by the NT Department of Lands, Planning and Environment. The two datasets were found to complement each other and, when used in combination, to provide a fairly reliable source of information about the location of wetlands in the area (fig 3.8). The wetland features mapped were then reclassified on the basis of their landform and water regime. To assist in the classification process, a ground-truthing exercise and a basin-wide, low-level aerial survey were conducted.
Figure 3.8 The distribution of wetlands in the Douglas River (one of the 11 sub-catchments in the Daly basin)
An overview of the biophysical features of the Daly basin was also presented. The area is characterised by three types of karstic rocks known as Tindall limestone, Oolloo limestone and the Jinduckin Formation. All three rock types contain extensive unconfined aquifers that serve as the primary groundwater resources in the area and, as a result, they are responsible for providing the base flow for the majority of the rivers and creeks in the Daly basin. Most wetlands are located in groundwater discharge areas in which the fluctuation of water level and soil moisture are important controlling factors. There is also a proliferation of sinkholes and streamsinks in areas underlain by Tindall limestone. The outflows from the aquifers associated with these features commonly occur in the form of springs that make a significant contribution to the perennial flow of the rivers in the area. The distribution of wetlands and the overall extent of the major wetland types in the Daly basin were described using the discharge data for the Daly River and its major tributaries.
This information was then used in a risk assessment to demonstrate the value of such procedures for land and/or water use planning in the basin. The risk assessment was undertaken using a framework formally agreed by the Ramsar Convention on Wetlands and was based on a Land Use Concept Plan for the basin. The plan was generated through consultation with staff from governmental agencies and from information publicly available on the internet. The scenarios contained within the Land Use Concept Plan enabled the application of the formal risk assessment technique to outline potential risks to wetland in the basin. In this respect the risk assessment can be used to provide guidance to land use planners when a formal land use plan is developed and endorsed.
The analysis phase of the risk assessment considered the possible threats posed by the various land and water uses outlined in the Land Use Concept Plan on the wetlands of the Daly basin. The focus was on threats to the water regime. The plan and the GIS were then used to determine and predict the wetlands most at risk from various threats and the activities that pose most threat.
The existing and projected land uses in the Daly basin that pose the most risk to the wetlands in terms of altering the water regime are considered to be agricultural activities, such as cropping, horticulture and pastoral activities utilising improved pastures, and urban development. Unless appropriately managed, the threats associated with these activities have the potential to result in negative impacts to a range of wetlands on both a local and basin-wide scale. Dam construction, if it were to proceed, would represent a threat to many of the downstream channel and channel-dependent wetland habitats. However, land clearance and surface and groundwater extraction for intensive agriculture and urban expansion are probably of more immediate concern. Based on the land clearance and/or water extraction scenario presented in the Land Use Concept Plan it was found that 5 to 15% of the major wetlands of the Daly basin would experience changes in their water regimes. Under the same scenario less than 20% of the wetlands of the Daly basin would be contained within conservation reserves and therefore at low risk from such activities. Although a large proportion of the river channels would exist within conservation areas, they represent the wetland type that is most vulnerable to threatening activities elsewhere in their catchments and may actually be at greater risk. The remaining wetland habitats would be at low to moderate risk of altered water regimes due to land clearance and water extraction. Floodplain and river channel habitats within the catchments for which large dams have been proposed would be at risk from altered water regimes due to water impoundment, while those downstream of these catchments would be at moderate risk. Wetlands independent of channel flows would be at low risk of experiencing altered water regimes due to water impoundment.
Natural fish kills involving the sudden death of large numbers of fish are common events in the floodplain wetlands of the Top End. The relatively recent expansion of human infrastructure in the Alligator Rivers Region has introduced the possibility of fish kills resulting from unnatural causes such as water pollution. Traditional Owners of the region sometimes harvest dying or freshly killed fish during, or soon after, fish kills. Consequently, they have expressed concern about the possibility of chemical contamination of the fish affecting the safety of the fish for consumption and the loss of the resource. The aim of this study was to enhance methods of diagnosis of these events in order to distinguish between natural and anthropogenic events. The clues to causes come from measurements of water quality, fish behaviour, pathology and bioassay of fish tissue. The project involved documenting fish kills reported to eriss since 1980. Most reports have come from Aboriginal residents in Kakadu National Park and park rangers. When it has been possible eriss has visited the site to record the fish species involved and water quality information that might indicate the cause.
Lack of oxygen is the most common cause of fish kills. However, earlier studies by eriss have shown that in Kakadu natural leaching of acid-sulphate soils on the floodplains can produce toxic levels of aluminium that can cause fish kills. In the diagnosis of a fish kill a simple dissolved oxygen measurement may sometimes suffice to indicate the cause of fish death, however, it does not indicate the cause of the oxygen depletion. Also oxygen depletion of the water may occur for only a very brief period and may not exist at the time of inspecting a fish kill. More complex measurement of the physico-chemical structure of the water body can sometimes indicate the likelihood of a pulse of anoxia having occurred and the processes involved. Similarly, chemical contamination of the water may be a transitory, pulsed event but there may also be evidence of this for a while after the event in slow moving floodplain waters.
Natural fish kills most commonly occur at the beginning and the end of the Wet season and are frequently associated with storm events. The storms can cause oxygen depletion in a number of ways: by washing in terrestrial material that increases biochemical oxygen demand, washing in oxygen depleted water from adjacent swampy areas and mixing of stratified water masses with anoxic deeper layers of water. Storms are also responsible for the washing in of leachates from acid-sulphate soils. Oxygen depletion can result from increased consumption of oxygen by nocturnal respiration of plants and detritus as a result of increasing temperatures and declining water levels in the 'build-up' period of the early-Wet season and at the end of the Wet season as flood waters recede from the floodplains. In some situations the chain of events causing a fish kill can be more complex and involve other biota. This was well demonstrated in fish kills at Gindjela (Goose Camp Billabong) on Nourlangie Creek.
Early storms in October 1996 washed water from the adjacent floodplain into the billabong. The floodplain water had a low pH (3.5) and contained very high levels of iron, manganese, aluminium and elevated levels of sodium, chloride, sulphate and magnesium. The ratio of magnesium to chloride was identical to sea water indicating the history of sea water intrusion in this area which was possibly a result of buffalo damage in the 1970s. The ratio of sulphate to chloride was much lower than for sea water indicating there has been a reduction of sulphate to sulphide in the sediments. Oxidation of that sulphide was the most likely cause of the low pH and the mobilisation of metals into the water on the floodplain. Oxidation of the sediments was clearly assisted by aeration resulting from extensive digging activity of large numbers of magpie geese that gather there each year to forage on the corms of water chestnut (Eleocharis dulcis). Physico-chemical measurements along the billabong showed that the potentially toxic floodplain water extended only half-way along the billabong (fig 3.9). As a result fish in the billabong could avoid the potentially toxic water and did so by gathering at the downstream end of the billabong. Nevertheless, many fish died at dawn over a period of a week at the uncontaminated end as a result of oxygen depletion caused by nocturnal respiration of the dense submerged macrophyte vegetation, and possibly also the increased respiration of fish (fig 3.10). This was shown by the longitudinal profile of dissolved oxygen levels along the billabong at dawn and midday.
An almost identical event in terms of water chemistry occurred two years later. This time, however, the downstream end of the billabong was covered by the floating aquatic weed Salvinia molesta and the shading by this had almost eliminated the submerged macrophytes. As a result there was no oxygen depletion at dawn and only one species of fish, Sleepy cod (Oxyeleotris lineolata), was observed to die from this contamination. On this occasion some fish were also observed swimming in the acidic metal rich waters suggesting that the toxicity of aluminium in natural waters may be very complex.
Figure 3.9 Longitudinal profile of water chemistry parameters in Goose Camp Billabong showing the relatively uncontaminated refuge for fish at the outflow end of the water hole
Figure 3.10 Longitudinal dissolved oxygen profiles in Goose Camp Billabong
Oxygen depletion occurred at dawn at the outlet end of the billabong causing some fish to die but increased to adequate levels for fish as a result of photosynthesis during the morning.
The Goose Camp fish kills illustrated the value of increasing the amount of information about water quality associated with fish kill events, especially those examined at or close to the time of the event, for diagnosing the causes of fish kills. They also enhance scientific understanding of the function of natural aquatic ecosytems improving the ability to diagnose, predict and manage changes that may occur in the future.
Uranium mining in the Magela Creek catchment, Northern Territory, has occurred for over 20 years. The revised Australian and New Zealand Guidelines for Fresh and Marine Water Quality (2000) recommend a trigger value for uranium of 0.5 µ/L. The trigger value is considered to be of low reliability, due to an inadequate toxicity database and the subsequent derivation of the value using the less preferred 'safety factor' approach. Given that the Magela Creek catchment is considered of high conservation/ecological value, a low reliability trigger value is inadequate, and site-specific assessment was considered essential. In order to derive a high reliability, site-specific trigger value for uranium, chronic toxicity data for at least five local species from at least four taxonomic groups were required. However, until recently, appropriate data were limited to four local species belonging to three taxonomic groups, namely the cladoceran, Moinodaphnia macleayi; the green hydra, Hydra viridissima; the purple-spotted gudgeon, Mogurnda mogurnda; and the chequered rainbowfish, Melanotaenia splendida inornata. During 2000-01, the chronic toxicity of uranium to a local green alga, Chlorella sp., was assessed, providing information on a fifth local species and enabling the derivation of a site-specific trigger value for uranium. As can be seen in table 3.2, no-observed-effect concentrations (NOECs) for the five local species ranged from 18 to 810 mg µ-1.
Using the new statistical extrapolation method recommended in the Australian and New Zealand Guidelines for Fresh and Marine Water Quality (2000), the site-specific trigger value (to protect 99% of species) for uranium was 5.8 µg L-1. This value is slightly greater than the historical site-specific guideline value (ie Maximum Allowable Addition) for Magela Creek of 3.8 µg L-1, and is about two orders of magnitude above natural background concentrations. Thus, a revised quality guideline value for uranium in Magela Creek downstream of 5.8 µg L-1 was recommended.
|Species||Test endpoint||NOEC* (µg L-1)||Reference|
Cell division rate
Hogan et al (in prep)
eriss unpubl; Semaan et al (in press)
Hyne et al (1992)
|Melanotaenia splendida inornata||
* NOEC: no-observed-effect concentration
The cane toad, Bufo marinus, arrived in Kakadu National Park in January 2001. Prior to its arrival, a science-based risk assessment was undertaken in collaboration with Parks Australia North, to predict key habitats and species most likely to be at risk from cane toads. From this recommendations for new monitoring programmes could be made, the relevance of existing programmes evaluated, and some management options identified. Major information gaps were also identified. The risk assessment analysed information from published and unpublished, scientific and anecdotal reports, while a number of relevant cane toad, native fauna and wildlife management experts from around Australia were consulted. Discussions were also held with community members in the Borroloola and Mataranka regions to gain an indigenous/cultural perspective of the cane toad issue.
Cane toads will occupy almost all the habitats within Kakadu National Park, although the saline regions and open water habitats were identified as being of less concern. Habitat preference will vary with season, with floodplains and sheltered habitats on the margins of floodplains and shallow billabongs providing ideal cane toad habitat during the early- to mid- Dry season, permanent water being sought during the late-Dry season, and the drier but sufficiently moist woodlands and open forests being preferred during the Wet season. Breeding activity will be concentrated during the Wet season, but could also occur during the Dry season, particularly after rain.
Where possible, key predator, prey and competitor species potentially at risk from cane toads were identified. However, the overall lack of reliable information on the effects of cane toads to species existing in Kakadu National Park made the assessment difficult.
A total of 154 predator species or species groups were listed as potentially at risk from cane toads, although varying degrees of risk and priority were assigned depending on the quality and quantity of available information. Ten species were assigned to risk category one (ie likely to be at risk) based on information of population declines due to cane toads. Of these, the northern quoll was assigned highest priority due to its diminishing range, while the nine remaining species, including a number of varanid lizards (Gould' goanna, mangrove monitor, northern sand goanna, spotted tree monitor), three snakes (western brown snake, king brown snake, northern death adder), and one mammal (dingo) were assigned high priority. Another group of species were identified as being of high priority based on recorded deaths of individuals, and information on habitat, feeding ecology, behaviour and status. These included some small carnivorous mammals, the remaining six species of varanid lizards, pythons and some other snake species, ghost bat, black-necked stork, freshwater crocodile, and a range of native frogs. Some species, such as the majority of fish, and a number of birds and mammals were assigned a low priority status based on relevant ecological, feeding, habitat or behavioural information. However, the risks to the remaining predator species identified are essentially unknown due to a lack of information.
Quantitative data on impacts to prey species are scant, and very little could be concluded about the species or species groups at risk. However, the prey groups identified as most likely to be impacted were the termites, beetles and ants. Similarly, risks to potential competitor species were unclear, although potential effects to some native frog species and insectivorous lizards may be of concern.
A great deal of uncertainty surrounded the prediction of risks. Contributing to this was a lack of understanding or quantitative data on: i) both short- and long-term impacts of cane toads on animal populations; ii) populations, distributions and general ecological information on Kakadu fauna; and iii) potential cane toad densities within Kakadu.
It was concluded that historical or current monitoring programmes within Kakadu National Park needed to be supplemented with additional data to provide a suitable baseline for the assessment of cane toad impacts. The assessment identified the critical monitoring and research requirements based on those species assigned high priority and the many information gaps. Cane toad control options appear extremely limited. At the best, sustained measures may prove effective in localised areas (eg townships, caravan parks), but broad scale control is not possible at this stage.
It was recommended that Parks Australia North manage the invasion of cane toads initially by: i) ensuring that monitoring efforts are underway to assess impacts of cane toads to Kakadu; ii) investigating measures by which cane toads can be managed on a localised basis; and iii) conducting cane toad awareness programmes for Rangers and local communities.