The DSITI Guidelines for draft environmental values and Water quality guidelines for Fitzroy Basin fresh, estuarine and marine waters, including Keppel Bay (Newham et al. 2017) identify the Dee River as a highly disturbed ecosystem with modified environmental values and water quality objectives. In January 2013, ex-cyclone Oswald caused unusually intense rainfall over the Mount Morgan Mine that resulted in the first-ever overtopping of the spillway of the OCP. The impacts of this runoff extended down the Dee River for approximately 50 km. Despite this, monitoring on the Don River at Rannes (approximately 70 km downstream of the mine) showed only very minor changes to water quality (AARC, 2017a). Sampling of the Dee, Don and Dawson rivers after the January 2013 event showed that water quality returned to pre-flood conditions shortly after the event. Contaminated water seepage and runoff from the Mount Morgan Mine is currently managed by the State via the seepage interception system. While this has reduced contamination loads, the system cannot collect all contaminated water and water quality values in the Dee River remain adversely affected. The effects of this can be observed in monitoring data downstream from the Mount Morgan Mine in the Dee River. Due to impacts from the Mount Morgan Mine, water in the Dee River not suitable for drinking or recreation, such as swimming or fishing. The DNRMMRRD have placed signage at all crossings along the Dee River advising no drinking, no swimming and no recreational activities can occur.
7.3.2
Impact Assessment
7.3.2.1 TSF The overall Heritage Minerals Project will ultimately result in improved environmental outcomes at the Mount Morgan Mine through the excavation and placement of historical ARD generating mine waste and redeposition into the Sandstone Gully TSF, the OCP and (if approved) the Upper Mundic Gully TSF. This will result in the removal of the source of ARD generating material from across the site and consolidation where modern capping and closure solutions can be employed. This will reduce impacts from the Mount Morgan Mine to the Dee River in accordance with the State’s Rehabilitation Plan (Unger et al, 2003) and Phase 3 Agreement. As referenced in Section 4.3 overtopping/ environmental spill and seepage consequence categories were considered to be “low’ for both ANCOLD and DETSI. Given the total volume capacity of both assessed stages of the TSF, a flood included overtopping failure was deemed as non-credible. The results of the sunny day piping failure of both the Stage 1 and Stage 2 embankment layouts indicated that the OCP is the primary receptor of breach outflows in all scenarios with these being heavily attenuated and controlled by its spillway discharge. The PMP storage allowance will provide more than adequate capacity to meet the Queensland DSA and ESS requirements. Given the total volume capacity of both assessed stages of the TSF, a flood included overtopping failure was deemed as non-credible. These findings are important to EA conditioning as the Upper Mundic Gully TSF does not require an operational spillway, DSA or ESS requirements to be included in Table H3 of the EA. The TSF design considers mitigation of impacts to groundwater through the proposed installation of a liner on the upstream batter of the embankment to mitigate seepage through the embankment. A geomembrane liner will be required as no suitable low permeability clayey material is found on the mine site. The liner bedding will be selectively borrowed from finer mine waste, however as it is difficult to remove coarse particles, a more robust bituminous geomembrane liner is proposed. Piezometers are proposed to be installed within the embankment to monitor the phreatic surface and detect potential seepage/ discharge of contaminants. The locations of the piezometers are to be finalised during the next design phase and are typically installed following construction of the Stage 1 embankment.
Project number: 25B061
Page 148
Powered by FlippingBook