Water Journal : Water Journal April 2011
technical features 116 APRIL 2011 water artificial aquifer recharge Additional modelling for forecasted effluent loads for 2021 (4ML/day ADWF and 15.5ML/day PWWF) were modelled but shown not to be sustainable without increasing the number of exfiltration basins (PB, 2007). Potential Deep Aquifer Storage and Recovery Prior to the expansion of the Hallidays Point wastewater treatment plant, groundwater modelling indicated that the local groundwater regime was incapable of accepting additional wastewater without expansion of the exfiltration system. Thus alternative disposal options were considered. Deep aquifer storage and recovery was an option to provide MidCoast Water with a flexible integrated re-use scheme sufficient for long-term requirements. The target for deep aquifer disposal was a suspected palaeochannel associated with the Wallamba River, north of Tuncurry, which could be suitable for long-term aquifer storage and recovery (ASR) of the treated effluent. No previous geological exploration had been undertaken in the area to identify the possible location of palaeochannels. Palaeochannels associated with the major drainage systems have been located elsewhere in South Eastern Australia and are dated as Miocene to Pliocene age. During these periods the palaeoclimatic data indicates the climate was significantly wetter than present. Consequently these palaeorivers were much larger than those of today, with a higher energy and carrying a larger sediment load. A resistivity imaging survey, electromagnetic (EM34-3) survey and drilling investigation was conducted to locate suitable deep aquifers. The location of geophysical surveys and the Wallamba River is shown in Figure 3. Seven anomalies were detected and, of these, two anomalies (one high resistivity and one low resistivity) were assessed as having moderate prospects for locating the palaeochannel. Two deep stratigraphic holes (T7 and T8) were drilled at the identified geophysical anomalies. In each case the boreholes intersected a sequence of dune sand underlain by sand and clay to a depth of 32.5m terminating in weathered laminated siltstone. No palaeochannel was intersected. The geophysical anomalies have since been reinterpreted as being related to heterogeneity within the bedrock and silty clay interface rather than a palaeochannel. Given the much higher rainfall conditions during the late Pleistocene, lower sea levels, faster-flowing rivers from the Eastern Australian Highlands, and the presence of palaeochannels along the eastern Australian coastline, it is considered likely an undiscovered palaeochannel associated with the Wallamba River palaeovalley is present between Hallidays Point and Bennetts Head (Foster). Further exploration work is required to further evaluate this deep ASR option. Since locating a suitable aquifer for deep storage and recovery was unsuccessful in the area north of Tuncurry, the primary effluent disposal option was to expand the existing exfiltration system at Hallidays Point. Exfiltration System Performance Following WWTP upgrades at Old Bar and Hallidays Point, and associated expansion of the exfiltration systems, increased effluent loadings have been successfully discharged at each site. Groundwater monitoring networks have been expanded at each WWTP within the unconfined sand aquifers to monitor potential impacts of the expanded exfiltration systems on groundwater levels and quality (PB, 2010). Monitoring wells are located next to the exfiltration basins and down-gradient along groundwater flow lines to monitor groundwater mounding and any contaminant migration and attenuation. At selected sites, dual monitoring wells have been installed to monitor shallow and deeper groundwater to detect any lateral aquifer variation. As part of the plans of management, a strategy has been developed to manage any impacts to the local hydrogeological regime. In the event that elevated groundwater levels are measured, or seepage outbreaks are noticed, it must first be established that the impacts are attributable to the exfiltration operation. Following confirmation of impacts caused by operations related to the exfiltration process, there are a number of management options including: • Cycling effluent to other basins; • Reducing the rate of infiltration to the basins that are significantly impacted; • Increasing maintenance on the floor of the basins to remove sludge and any biomass accumulations; • Planting more trees down-gradient of the basins. Water table movements Analysis of hydrographs from Old Bar and Hallidays Point indicate that the water table fluctuates in response to rainfall with the influence of effluent discharge superimposed on the hydrograph. Since the expansion of the exfiltration basins at each site, the water table has not been recorded as reaching ground level. No seepage from the dune aquifers has been observed down-gradient of the exfiltration basins or other low-lying areas, indicating the increased effluent loading has not exceeded the aquifer capacity. Figure 3: Location of geophysical surveys.
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