Supervising Scientist, Darwin, 2005
ISBN 0 642 24395 6
ISSN 0 158-4030
Discussions between Hanson Pty Ltd and the Northern Land Council (on behalf of the Traditional Owners) on issues related to the rehabilitation expectations for Nabarlek continued during 2004–05. This includes arrangements to complete site clean-up, particularly the former camp area and laboratories.
During its September 2003 meeting, the Alligator Rivers Region Technical Committee was advised that the rehabilitation bond for Nabarlek had been reduced from $10 000 000 to $400 000. The Supervising Scientist subsequently raised concerns regarding the process for changing the rehabilitation bond.
At the December 2003 meeting of the Nabarlek MTC, DBIRD advised that the bond was revised subsequent to the submission of the last Mining Management Plan (MMP). It was agreed that the bond would be reassessed on the submission of the next MMP with the consultation of the MTC members.
Hanson submitted a draft MMP in May 2004. The MTC discussed the MMP and the bond during its October 2004 meeting. At that meeting scientists from eriss and Charles Darwin University presented the findings of recent revegetation studies and provided a preliminary cost estimate of work they considered necessary to achieve sustainable revegetation. Stakeholders accepted that the revegetation plan in the MMP required amendment and that costs for successful closure could not be assessed until acceptable criteria for closure, encompassing detailed revegetation objectives are set. Subsequently, the acceptance of the draft MMP was deferred and revision of the security is consequently pending.
The NLC and Hanson continue to work on identifying suitable closure criteria.
The Nabarlek Minesite Technical Committee met twice during the year. Table 2.12 provides information on the meeting and the major points of discussion.
|Date||Significant agenda items|
|21 October 2004||Revegetation studies, radiologically anomalous area, rehabilitation and management activities on site, environmental monitoring, Mining Management Plan and security|
|3 December 2004||No new items|
There were no changes to the Authorisation during the reporting period.
There were no reportable incidents at Nabarlek during the year.
Staff from eriss continue to undertake research programs at Nabarlek and the site is subject to at least two formal visits from oss staff during the year. In addition, oss often carries out opportunistic site inspections if in the area on other business (eg exploration inspections). Environmental monitoring of the site is carried out by DBIRD and is reported in its six-monthly report.
Two formal site inspections are carried out each year – the annual audit of the Mining Management Plan and the post wet season inspection. The intent of this inspection is to check site stability and erosion following the wet season and to plan works for the coming dry season.
Highlights of the post wet-season inspection were:
Erosion noted previously at a culvert under the bitumen road near the Nabarlek main gate, on the road to the camp, is severe and poses a safety hazard. In previous years Hanson has addressed this issue by filling the erosion after the wet season. Hanson proposed that this year a more permanent repair be undertaken by removing the culvert pipe work and restoring the natural drainage with battered sloping sides so as not to pose a hazard for vehicles.
The small areas of tube-stock planting adjacent to the former mill location appear to be thriving.
Extensive areas of para grass were noted extending to the northwest over the area of the previous evaporation ponds and in the immediate vicinity of the plant runoff pond.
Inspection of the area formerly occupied by the grizzly revealed no open voids and it is concluded that fill of the subsidence in this area that was undertaken last season has successfully survived the 2004–05 wet season.
The 2004 annual environmental audit was undertaken on 30 November 2004. The aim was to assess the effectiveness of management systems and to provide feedback to the audited company on establishment and status of these systems. The audit outcomes were:
No Category 1 or Category 2 non-conformances were identified by the audit team.
One Conditional issue that related to the radiologically anomalous area was identified. Further investigation into the radiologically anomalous area was undertaken by eriss.
Due to Gunbalanya being temporarily inaccessible at the time of the audit, the audit team could not verify the establishment and/or success of the nursery in Gunbalanya.
Acceptable – the remaining 13 of the 15 issues audited were considered acceptable.
The Radiological Anomalous Area (RAA) continues to be investigated and was a standing agenda item for MTC meetings during the year. The RAA is approximately 0.4 ha and is located southwest of the former pit area.High levels of salts and an excess of Ra226 over Ra228 in soils from the RAA indicate that the material is, or has been, in contact with, processed material. The area has elevated levels of radioactivity and has been identified to contribute about one-quarter of the total radon flux from the rehabilitated mine site (Figure 2.20) and three-quarters of the radionuclide flux from the site via the erosion pathway. Gamma dose rates across the Nabarlek site have been determined from an airborne gamma survey in 1997.
Figure 2.20 Radon flux densities from surfaces of the rehabilitated Nabarlek mine
However, dose rates from the RAA could not be resolved due to its relatively small extent and the resolution of the airborne gamma survey.
In June 2005 eriss conducted a detailed ground based gamma survey of the RAA. Figure 2.21 shows external gamma dose rates in micro Grays per hour ( µGy/hr) determined at the area.
Figure 2.21 Gamma dose rates [ µGy-hr-1 ] measured at radiological anomalous area
Maximum dose rates at the RAA are higher than the averages across site. The maximum measured was 14 µGy/hr at the RAA compared to a site average of 0.31 µGy/hr. From the field observations and results of the gamma survey it appears that some material is eroding from the RAA towards the southeast. Lead isotope ratio studies of surface soils taken from the RAA, adjacent drainage lines and a sediment settling pond just outside the fenced area will ascertain the fate of the eroding material.
This information will aid in determining remediation strategies for the RAA.
Statutory monitoring of the site continues to be undertaken by DBIRD and the lease holder, Hanson Pty Ltd. DBIRD carries out all surface and groundwater monitoring on and off-site, including surface water monitoring downstream of the mine in Kadjirrikamarnda and Cooper Creeks. DBIRD reports the results of this monitoring in the six-monthly Northern Territory Supervising Authorities Environmental Surveillance Monitoring in the Alligator Rivers Region reports. These creeks are reported to have low electrical conductivities (<24 µS/cm) and low concentrations of the key mining indicators, sulfate (< 1 mg/L) and uranium (< 0.1 µg/L).
eriss continues to undertake research programmes at Nabarlek including radiation assessments, revegetation success and monitoring techniques, and erosion and contaminant transport. The research is aimed at enabling an overall assessment of rehabilitation success at Nabarlek. Progress on these programs are reported in Supervising Scientist Annual Reports and in the Internal Report series.