5 Potential Impacts and Mitigation – Water Quality
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5 POTENTIAL IMPACTS AND MITIGATION – WATER QUALITY The approach adopted for this study to evaluate potential water quality impacts associated with the proposed discharge relied on the application and interpretation of calibrated far-field, two-dimensional hydrodynamic (HD) and advection dispersion (AD) modelling tool built using MIKE21. This modelling was supported, guided and informed by a range of data and other relevant information. These data, the work conducted, and key findings are presented below. 5.1 Numerical Modelling Numerical modelling was used to simulate the transport, dilution and dispersion of the proposed discharge at Gunn Point and surrounding waters. The proposed discharge was modelled (see Appendix A) as a conservative tracer. This allows the dilution and dispersion of the effluent to be understood. The numerical modelling, and therefore the modelled tracer concentrations, can be considered conservative for the following reasons: Biological and physical processes such as the deposition of particulate material or the take up of bioavailable nutrients or absorption by sediments and algal mats (microphytobenthos) growing on the sediments of the significant intertidal areas in and around Shoal Bay are not included in the modelling. Three-dimensional turbulent dispersion associated with wave action has not been included in the modelling. Because the model was very computationally demanding, all scenarios were undertaken in 2D. However, during the model calibration and sensitivity testing, 3D simulations were carried out to confirm the mixing processes were resolved appropriately. The strong tidal currents and shallow water mean the site is well mixed, and the 3D modelling did not provide significantly different results. 5.1.1 Simulation Scenarios The model was run for two separate years: June 2005 – May 2006 inclusive May 2016 to April 2016 inclusive Whilst tropical conditions are highly variable, 2005-2006 was considered a ‘typical’ wet season, and 2016- 2017 a season with higher than average rainfall. The modelling indicated less dilution of the tracer occurred in the vicinity of the intake site during high flood events. Thus the 2016-2017 simulation scenario allowed an assessment of worse than average discharge mixing conditions. 5.2 Dispersion & Dilution of Discharge 5.2.1 Discharge Characteristics The modelled intake and discharge locations are displayed in Figure 5-1. The intake is located at -4 m AHD; the discharge -7 m AHD. The proposed discharge will have the following characteristics: Average daily flow rate of 954 kL/day. Salinity +/- 5PSU of the ambient intake water 26_R01v04_GunnPt_NOI - Temperature +/- 5°C if the ambient intake water 3894 Seafarms Group Limited | October 2017 Stage 1 Hatchery Coastal Environment and Impact Assessment Page 66 The proposed facility will have operating conditions and discharge characteristics similar to the existing Seafarms Hatchery operations at Flying-Fish Point (FFP), Innisfail in North Queensland. As such, Seafarms propose to adopt the water quality licence conditions for that facility set by the Queensland Department of Environment and Heritage Protection (QLD DEHP). The median concentrations for the key nutrients assessed in the modelling exercise are equal to the median licence conditions and are displayed in Eight samples were collected over May and June 2017 at the existing Seafarms hatchery at Flying-Fish Point near Innisfail in North Queensland (this facility is only required to monitor twice per year and only if the ponds are in operation. The sampling was undertaken once a week for eight weeks to investigate the water quality exiting the hatchery and then leaving the settlement ponds). These results indicate that the effluent quality is typically significantly better than the median licence conditions. Table 5-1. Eight samples were collected over May and June 2017 at the existing Seafarms hatchery at Flying-Fish Point near Innisfail in North Queensland (this facility is only required to monitor twice per year and only if the ponds are in operation. The sampling was undertaken once a week for eight weeks to investigate the water quality exiting the hatchery and then leaving the settlement ponds). These results indicate that the effluent quality is typically significantly better than the median licence conditions. TABLE 5-1 LICENCE DISCHARGE CONDITIONS Median (g/L) Total Nitrogen 2000 Total Nitrogen FFP performance 1250 Total Phosphorous 400 Total Phosphorus FFP performance 110 Chlorophyll a 20 Chlorophyll a FFP performance 1 g/L 26_R01v04_GunnPt_NOI - 3894 Seafarms Group Limited | October 2017 Stage 1 Hatchery Coastal Environment and Impact Assessment Page 67 FIGURE 5-1 MODELLED DISCHARGE AND INTAKE POINTS; FACILITY FOOTPRINT 5.2.2 Feasibility Investigation Water Technology undertook a feasibility assessment of the intake / discharge locations. In total, this involved assessing over 20 different intake and discharge configurations. The location of the source point within the model was reviewed to determine any improvement in position of the proposed discharge. As expected, there was greater initial dilution when the discharge was placed in deeper water. Placement of the discharge in deeper waters also resulted in less “bank attached” flow where the discharge material was higher along the inshore zone. The feasibility assessment covered four stages. The aim was to minimise the potential environmental impact, as well as the risk of recirculation through the intake pipe: Initially, modelling of the discharge was completed over a period of four weeks for the wet season, dry season and astronomical tidal conditions. The discharge location was tested at 6 locations to review the impact at different locations within the proposed footprint. Two locations were selected to assess in 3-month simulation scenarios. The preferred scenario from the 3-month simulations was run for the 2016-2017 year. The full year results provided more clarity around the behaviour of the plume. As a result, a further 13 intake / discharge configurations were assessed using the full year simulation. The final three intake / discharge configuration options were then modelled for both the 2005-2006 and 2016-2017 years. Modelling both years ensured the prevailing conditions from a single year did not 26_R01v04_GunnPt_NOI - influence the locations. 3894 Seafarms Group Limited | October 2017 Stage 1 Hatchery Coastal Environment and Impact Assessment Page 68 All scenarios tested resulted in minimal increases in nutrient concentrations. The final assessment allowed the optimal configuration to be selected in terms of environmental impact, economics of construction of the intake and discharge pipes, operation of the pumps, and intake recirculation. 5.2.3 Performance Objectives As discussed in Section 3.2, the water quality objectives applied in this study are presented in Table 5-2. This indicates the background mid-estuary chlorophyll a concentration is above the corresponding water quality objective. When defining performance objectives, in accordance with National Guidelines (ANZECC 2000), there is an accepted hierarchy of documentation in this regard. This hierarchy requires that where there are no locally specific guidelines (which would require comprehensive local water quality data collection typically spanning at least a 1 to 2-year period) that management decisions should default to relevant State based guidelines, and in their absence to National guidelines. In this instance, local monitoring has been undertaken to obtain the background concentration. Given the Darwin Harbour WQO is assigned to the full harbour system, it is proposed that the WQO objective be adjusted at this location for the purposes of this assessment. The upper-estuary Darwin Harbour WQO is 4 g/L, and the mid-estuary WQO 2g/L. Examination of the available water quality data indicates an 80th percentile value of 3.4g/L. Using the 80th percentile as an objective is in line with the recommendations in the National Guidelines. This value sits between the mid and upper WQOs, which appears to be a reasonable interim trigger value to adopt on this basis. A less geographically relevant set of tropical water quality objectives are presented in DERM 2010 for Trinity Inlet, Cairns. These set the chlorophyll a water quality guideline concentration for mid-estuary to be 3 g/L, similar to the 80th percentile value provided above. It is proposed to adopt an alternative WQO of 3.4 g/L for the purposes of this assessment. This value is included in Table 5-2 in brackets. Further investigation will be conducted going forward to refine this value. In the analysis presented in Section 5.2.5, the background concentration is added to the discharge concentration, and compared to the water quality objective for mid and outer estuary respectively. TABLE 5-2 MEDIAN BACKGROUND NUTRIENT CONCENTRATIONS Parameter Background Concentration (g/L) Darwin Harbour WQO (g/L) Mid Estuary TN 230 270 TP 15 20 Chlorophyll a 2.1 2 (3.4) Outer Estuary TN 160 440 TP 8 20 Chlorophyll a 0.5 1 5.2.4 Initial Mixing / Near-field Mixing Upon discharge into the ocean, rapid initial dilution of the discharge water will occur at the discharge location. 26_R01v04_GunnPt_NOI The actual initial mixing will be determined by the ultimate design of the outfall structure. The structure should - be designed to achieve an initial dilution which ensures the discharge water plus the existing background water 3894 Seafarms Group Limited | October 2017 Stage 1 Hatchery