Yandicoogina Baseline Hydrology Page 1 of 60 Contents Page
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Resource Development Surface Water Management Baseline hydrology assessment for Yandicoogina discharge 02 April 2012 (Update to Report – Baseline hydrology assessment for Marillana Creek discharge released 4 May 2010) Future expansion at RTIO Yandicoogina has the potential to increase the volume of surplus water generation and discharge into Marillana and Weeli Wolli Creeks. A baseline hydrological/hydraulic assessment was carried out to access the existing situation and movement of discharged water along the creek systems and to predict the likely behaviour of the released water for various discharge scenarios. Surplus water is discharged by BHPBIO and RTIO Yandicoogina mine operations at several locations along Marillana Creek (BHPBIO discharge outlet is located 2 km up gradient of the RTIO Oxbow deposit; RTIO discharge network is located adjacent to the Junction Central mine) and two outlets by RTIO Hope Downs 1 and Yandicoogina operations in Weeli Wolli Creek (Hope Downs 1 discharge outlet is located 500 m up gradient of the Weeli Wolli Spring; RTIO Discharge Outlet 6 is positioned adjacent to the Junction South East mine). In December 2009, the total average discharge rate to the creek systems from all three mining operations was 116 ML/day or 42 GL/year. The assessment demonstrated that historically discharged water would not have extended past the Marillana Creek catchment outlet at the confluence with Weeli Wolli Creek; hence anecdotally no flow contribution to Weeli Wolli would have been expected. This suggests the wetting front in Weeli Wolli Creek is likely to have resulted from surplus water discharged from the Hope Downs 1 mine operation. The average maximum steady state inundation or discharge footprint produced from the total average discharge, from 1998 to 2009, was expected to extend approximately 2.5 km down gradient from the confluence of Weeli Wolli and Marillana Creeks. On the other hand, the discharge footprint would extend 5.5 km downstream from the confluence if 60 GL/year (peak discharge from 1998 to 2009) of surplus water was released at a constant rate into the creek systems. A discharge footprint of 35.4 km from the BHPBIO discharge outlet and 36.1 km from the Hope Downs 1 discharge outlet or 16.3 km down gradient from the Weeli Wolli – Marillana Creek confluence was estimated for the maximum modelled 110 GL/year regional surplus discharge (based on existing infrastructure). An option to relocate the discharge location in Marillana Creek down gradient from the mine operations was also investigated. Modelling indicated that the footprint distance would extend from approximately 6.7 km to 17.3 km down gradient from the Weeli Wolli – Marillana Creek confluence for modelled volumes 55 GL/year to 110 GL/year. Due to the dynamic nature of the Weeli Wolli Creek system, the creek channel and in particular the section that drains the alluvial plain is capable of changing course during large flood events. This type of event would modify the existing drainage flow path of the discharged water and alter the course of the discharge footprints. Yandicoogina baseline hydrology Page 1 of 60 Contents page Introduction 3 1. Issue 3 2. Objectives 3 Catchment characteristics 4 3. Hydrology 4 4. Rainfall 5 5. Hydrogeology 7 6. Alluvium characteristics 8 7. Riparian vegetation 11 7.1 Mapped vegetation 11 7.2 Evapotranspiration 14 Discharge modelling 16 8. Modelling approach 16 8.1 Defining the discharge footprint 16 8.2 Surface water – groundwater connectivity 18 9. Discharge water balance calculations 20 9.1 Methodology 20 10. Modelling scenarios 28 10.1 Existing discharge regime 28 10.1.1 Scenario 1: Comparison of observed and predicted wetting fronts 28 10.1.2 Scenario 2: Average discharge between 2007 and 2009 29 10.1.3 Scenario 3: Peak discharge between 1998 and 2009 29 10.2 Proposed discharge scenarios 29 10.2.1 Scenario 4: Discharge rate options – 60 GL/year 29 10.2.2 Scenario 5: Discharge rate options – 90 GL/year 29 10.2.3 Scenario 6: Relocation of Marillana Creek discharge outlets 29 11. Modelling results 30 11.1 Existing discharge footprints 30 11.2 Proposed discharge footprints 33 11.2.1 Scenarios 4 and 5 33 11.2.2 Scenario 6 35 Conclusion 40 Appendix A – Base case output 42 Appendix B – Scenario output 47 Addendums– Scenario 7 to 9 including BHPBIO Jinidi 52 References 60 Yandicoogina baseline hydrology Page 2 of 60 Introduction 1. Issue Surplus water is generated as a result of mining below the groundwater table. The volume and rate of surplus water generation will be determined by the difference between the rate at which site and other parties can use the water and the rate of abstraction. The current total abstraction rate for Yandicoogina Junction Central (JC) and Junction South East (JSE) is 35 GL/year, with the potential to increase to a maximum 55 GL/year as a result of expansion and development of the Junction South West (JSW) and Oxbow deposits. Management of water on Rio Tinto sites follows strict environmental and water use standards (refer to Rio Tinto Environmental Standards). These standards align with the Western Australian State Government Department of Water hierarchy of disposal options informal regulatory guidelines (Bessan Consulting Services, 2007) that recommend (in order of preference): Use on site; Transfer to another site or industrial location; Re-injection; Controlled discharge to surface (irrigation); and Uncontrolled discharge to surface (creek discharge). Any surplus water not utilised in the first four strategies is currently being discharged into Marillana and Weeli Wolli Creeks. Discharge of surplus water from the BHPBIO Yandicoogina mine operation into Marillana Creek began in May 1992. In 1998 RTIO started releasing surplus water into Marillana Creek and by December 2006 the combined operations were discharging at an average rate of 34 ML/day. Release of surplus water into Weeli Wolli Creek began in 2007 with expansion of the JSE mine and development of the Hope Downs 1 operation up gradient of Yandicoogina. By December 2009, the combined average discharge rate to Marillana and Weeli Wolli Creeks from all three operations was 116 ML/day. 2. Objectives The purpose of this study was to predict the hydrological reaction of the creek systems to artificial discharge. The specific objectives of the assessment were: To characterise the stream morphology of sections of Marillana and Weeli Wolli Creeks downstream of the discharge outlets. This was accomplished by reviewing the stream pattern, river/floodplain geometry, soil profile/geological logs, vegetation patterns and community distribution, and other characteristics of the receiving creeks to establish representative “reaches”. To determine the hydraulic characteristics associated with different volumes of water released into the system. This work was accomplished in order to determine the capacity and reaction (water movement) of the creeks at different discharge volumes. To establish the area over which the released water could travel (or discharge footprint). Yandicoogina baseline hydrology Page 3 of 60 Catchment characteristics The dearth of hydrological data in the Pilbara, especially related to small and medium sized catchments, their stream flow and rainfall distribution patterns, makes it impossible to use standard hydrology assessment methodologies to determine surface water flow characteristics in local Pilbara catchments. To overcome this limitation and to provide a methodology for estimating the potential flow conditions of artificial discharge into ephemeral Pilbara creeks, hydroecology techniques for evaluating water movement have been adopted. Hydroecology is the inter-disciplinary study of the interactions between ecological processes and water movement. For the purpose of this study, information that could be extracted from the Pilbara landscape to help describe baseline hydrological characteristics, in the absence of historical stream flow or climate series data, included: vegetation patterns and communities in the riparian zone, pool location/absence and water quality, geomorphology of the creek bed, banks and floodplain, and soil/regolith geology properties and patterns (particularly as recorded in satellite remote sensing imagery). These catchment characteristics are described below and are subsequently used to define the inputs to the discharge modelling. 3. Hydrology The Weeli Wolli Creek regional catchment covers an area of approximately 4,000 km2. The upper Weeli Wolli Creek catchment, up gradient of the Hope Downs 1 mine, is characterised by relatively wide, flat plains of gentle gradient, surrounded by rugged hills of outcropping Marra Mamba Iron and Brockman Iron Formation. The Creek and its tributaries drain in a north easterly direction past the Hope Downs 1 mine site, converging at a narrow valley system. Weeli Wolli Spring is located at the mouth of the valley, approximately 10 km down gradient from the Hope Downs 1 mine site. The Creek continues its path downstream into the lower Weeli Wolli Creek catchment, joins with Marillana Creek to the west and drains onto a wide alluvial floodplain 20 to 30 km across in the Fortescue Valley, producing a meandering channel that can alter its course following large flood events. The flows terminate at the Fortescue Marsh, an internally draining basin of the Upper Fortescue River catchment. Marillana Creek, a major tributary of the Weeli Wolli Creek system, has a total catchment area of 2,230 km2. The headwaters rise from the high relief areas of Hamersley Range where the Creek drains in an east and north easterly direction into the Munjina Claypan. The Claypan, an internally draining basin, has a total area of approximately 274 km2. It is subject to periodic inundation following rainfall events and has the potential to retain surface water flows for lower flood events, ≤ 1 in 10 year annual recurrence interval (ARI). Surface water exceeding the internal holding capacity of the basin spills south east into the lower Marillana Creek catchment. The lower Marillana Creek drains in an easterly direction through the existing BHPBIO and RTIO Yandicoogina operations before merging with Weeli Wolli Creek.