MANAGEMENT OF GROUND AND SURFACE WATER QUALITY DURING MINING

The proposal involves ground disturbing activities within the catchments of a number of seasonal water courses, one of which will be temporarily diverted during the course of mining.  The discharge of surplus water in an uncontrolled manner may cause flooding to downstream environs. 

Occasionally, excavation into orebodies below the water table requires the water table to be sufficiently lowered to gain access to the ore, and this lowering not only competes for the groundwater resource with other users, but the extent of drawdown sometimes goes beyond the immediate area of excavation and can expose acidic soils or affect moisture availability to surrounding vegetation and wetlands. 

Abstraction of significant volumes of water from the deeper Yarragadee formation may affect Yarragadee ground water levels in the vicinity of the production bore, which could in turn result in draw-downs in the overlying formations. Poor rehabilitation of disturbance areas could reduce soil infiltration characteristics, resulting in changes to stream-flows over the long term and reduced groundwater recharge. 

Surface water

The significant water courses in the proposal area are shown in (Creek & Drain locations (map)), namely:

a seasonal stream that flows northwest from state forest through the HVN deposit, under Gavins Road (Figure 20), through a gully dam on Loc. 215 and into another gully dam on Lot 217, where it terminates.

two seasonal tributaries that flow northwest from state forest, through cleared blue gum plantations on which the HVS deposit is partially situated, and then combining to flow down through the current Gwindinup South mining area, after which it disappears into the sandy soils where it contributes to the recharge of the Leederville aquifer.

The relative length of the proposed disturbance compared to the length of the first stream is very small (less than 5%) and this stream is associated with agricultural and domestic uses.  The level of disturbance to the second two tributaries will be significantly higher, with one of the tributaries crossing the HVS orebody, while the second will be affected by dams and other infrastructure. 

The three watercourses only flowed two out of the last six seasons and although flow volumes have not yet been recorded, they are not expected to be significant to downstream users or ecosystems.  However, flows will be estimated through catchment modelling and the importance of flow to downstream users will be determined as part of the preparation of the ERMP. 

Groundwater

Hydrogeology of the mining area

The Happy Valley deposits are part of the Ridge Hill shoreline (Baxter 1977), which is approximately 76 - 82 mAHD and was eroded into the Leederville formation in the Pleistocene era (approx 2 million years BP).
The deposits form part of the Ridge Hill superficial formation that overlies the Leederville and are dominated by sandstones and gravelly and sandy soils (Baxter 1977, Wilde & Low 1980). The proposal does not intersect the Leederville formation, so studies have focussed on the presence or absence of shallow (perched) water tables or deeper but unconfined water tables in the Ridge Hill sandstones in which the mineral sands deposits are situated.  Test pits and hand-augering indicated that a shallow water table was only present in the Rosa subsystem, which is outside the pit area. Two pits were excavated to the depth of 8 m at HVN and a single pit to the depth of 10 m was excavated at HVS.  No watertable was intercepted at any of these pits.

In March 2006, 4 groundwater monitoring bores were installed along the Happy Valley deposits, to the basement of the orebody (21). The basement is at a fairly consistent depth of 86 mAHD.  All bores are screened at this depth.  The four bores have been monitored monthly for water levels and to date show no evidence of a water table within the deposit.  These bores will continue to be monitored. A program for the construction of five additional deeper monitoring bores has been approved. These additional bores will monitor heads in the Leederville formation.

Hydrogeology of the abstraction site

The hydrogeology of the abstraction bore, situated in the Yarragadee Formation and licensed by the Department of Water, is described in detail in URS 2000. The URS report is based on an abstraction rate of 2.9 GL/annum, which exceeds the current Gwindinup abstraction limit of 1.5 GL/annum (approximate).  A maximum of 2.9GL/annum would only be required if two wet separation plants were operational at the same time.  This will be dependent on project approvals, timing and market factors. Nevertheless the URS report is based on a maximum water requirement scenario and therefore potential worst case impacts have been considered. Once production at Gwindinup commences, the results from the groundwater monitoring program will be available to verify and/or adjust the impacts to the Yarragadee and Leederville aquifers of abstraction. This information should be available, in part, in time for the release of the ERMP. 

Potential Impacts on Water Resources

Surface water quality

The quality of surface waters may be affected by an increase in suspended solids (sediments) as a result of runoff from disturbed areas or overflows from the mine water circuit.  The areas for the assessment of impacts to surface waters are those streams and surrounding catchment (directly or indirectly associated with a beneficial use) that may be affected by the proposed area of disturbance. There are three key watercourses that are significant in the project area as previously outlined. Although the beneficial use (groundwater recharge) is perhaps not as sensitive to deteriorating quality as other uses, such as domestic, any deterioration in surface water quality in these two tributaries has the potential to be cumulative, as the water quality may also by under pressure from its passage through the Gwindinup South minesite. Consequently it is considered that the protection of water quality in all three watercourses is essential and will be an important consideration during the mine planning process. 

Groundwater

Groundwater quality may be susceptible on a small scale to contamination as a result of poor management of fuel and other hydrocarbons on the mine site, however, it may also be susceptible on a larger scale if potential acid sulphate soils (PASS) are present and are disturbed through the mining process or exposed if mining is responsible for a significant lowering of the water table. The assessment area, therefore, will be the various aquifers beneath and adjacent to the disturbance area to the areal extent of groundwater drawdown and any users of groundwater from the same formations down-gradient of the area.

The potential presence of ASS in the proposed excavation areas has assessed, with results suggesting that the risk was relatively low. However recent advice from DEC is that there may be a higher than anticipated risk of soil acidity problems in the area and that these can be related to high sulphur PASS materials that accumulate in association with perched water tables and soaks. Such conditions are not present in the proposed disturbance area. In order to allay DEC concerns regarding this issue, an additional investigation into ASS in the project area is underway. 

Management Strategies

Bemax proposes the following management measures in order to maintain the health and integrity of water resources associated with the project:

• Drainage Management Plan – to address:
• temporarily redirecting drainage around disturbance areas to maintain flows and reduce contamination
• collecting stormwater from site and use within process and for dust control
• integrating drainage into the rehabilitation plan and restoring flows only when rehabilitation is sufficiently stable
• consulting with and reporting to downstream users
• maintaining site rainfall records
• establishing monitoring stations downstream from disturbance areas and comparing results to pre-disturbance flows.
• Identifying water quality objectives, consistent with ANZECC Guidelines (NWQMS 2000)
• Implementation of stream buffers during clearing and construction, consistent with water quality protection guidelines.
• Constructing dams and residue areas in accordance with engineering and legal requirements and guidelines.
• Managing dam levels to accommodate seasonal, peak and perpetual storm events
• Monitoring surface water quality at established monitoring stations, to compare with background data
• A plan for the protection of wetlands, GDEs and the satisfaction of EWRs, if identified as a requirement by the study.
• An Operating Strategy as part of the DoW allocation license process for the groundwater production bore, addressing management items such as:
• water (abstraction) requirements (and limits)
• water auditing
• efficiency measures/targets
• monitoring and reporting
• contingency measures.
• A Local Water Users Plan, addressing: location, ownership, supply requirements and reliability/reliance of local water resources; consultation, monitoring and reporting measures; contingency measures and operating agreements.
• Procedures for management of spills, handling and storage of environmentally hazardous materials to minimise risk of contamination of groundwater.
• Integrating mining and rehabilitation programs, with an objective of recreating a landscape & soil profile with similar hydrological properties to pre-mining.
• Minimising rehabilitation delays by proper planning.

In addition, it expected that the studies conducted during the preparation of the ERMP will propose additional recommendations applicable to management.Where beneficial, these recommendations will be considered in the preparation of the management plans.