Water Journal : Water Journal April 2011
water APRIL 2011 115 artificial aquifer recharge to a depth of 50m. The sediments are mostly Pleistocene to recent alluvial and Aeolian sands associated with the Foster-Tuncurry embayment. Alluvial sand and gravel deposits are thought to occur at depth within the palaeovalley of the Wallamba River located between Tuncurry and Hallidays Point. At Old Bar the original southern exfiltration basins are underlain by shallow Quaternary sands to a depth of approximately 8m--10 m that thicken towards the beach. To the west the former quarry is underlain by dune sand and iron-indurated sand known as ferricrete or "coffee rock". Locally the Hallidays Point WWTP is underlain by a thick sequence of Pleistocene sands that are typically leached of calcareous material and appear generally clean and bleached. Drilling indicates the beach deposits are relatively uniform, consisting of medium grained sands and coarsening with depth to at least 18m. The coarser sediments are thought to be relic beach deposits and extend from Darawank Wetland to the west to Nine Mile Beach to the east. Recent sands form low dunes with the elevation ranging between 6m--7m AHD along the dunes and 3m--4m AHD along swales. Mineral sand deposits (rutile, zircon, ilmenite and monazite) have previously been mined within the dune systems and coastal barrier beaches. Groundwater Local groundwater use is minimal within the Old Bar to Hallidays Point region, because the area receives a relatively high rainfall and MidCoast Water provides a reticulated water supply to residential, industrial and commercial properties. However, due to predicted population growth along the NSW coast, groundwater is becoming increasingly important for irrigation, domestic and garden use. MidCoast Water is also expanding its potable water supply borefields to cater for increased population growth. The beneficial use of groundwater in most of the dune sediments is for stock/domestic, irrigation, and recreation purposes. Groundwater also contributes to the sustainability of wetlands within the dune areas and adjacent creeks and estuaries. Recharge to the shallow unconfined alluvial aquifers is by rainfall runoff and infiltration. Groundwater monitoring networks at Old Bar and Hallidays Point WWTPs are located down-gradient of the exfiltration basins and consist of 20 and 25 monitoring wells respectively. Groundwater levels and quality are monitored in the unconfined shallow aquifers on a quarterly basis for a variety of analytes, based on license conditions issued by DECCW. The water table at Old Bar is typically a subdued reflection of the topography, with groundwater discharging eastward into the Pacific Ocean. Within the alluvial sands the hydraulic conductivity is relatively constant; however, within the former quarry the ferricrete has variable water-transmitting qualities. Groundwater quality at Old Bar is sodium-chloride dominated and slightly saline (1500 to 2500mg/L) which may be due to windblown salt. At Hallidays Point the WWTP and exfiltration basins are located on a groundwater divide with groundwater flowing westerly towards the Darawank Wetland and easterly towards the Pacific Ocean. The water table is shallow, varying between 2m and 4m below ground surface and reaching the surface at Darawank Wetland, 600m to the west. Groundwater is of low salinity, sodium- calcium-chloride dominated, which is typical of groundwater recharged by rainfall in a coastal environment. Groundwater Modelling The objectives of the groundwater modelling were to simulate the dynamics of the shallow groundwater flow system at each site and evaluate the suitability of expanding the existing exfiltration systems. In addition, the calibrated groundwater models were used to assess impacts on the local hydrogeological regime based on forecasted effluent loadings for up to 20 years. Prior to developing groundwater models for each site program, a series of field investigations was undertaken at each site to assess model parameters and boundary conditions. The field investigations included drilling, monitoring well installation, permeability testing, groundwater characterisation, geological mapping and collation of groundwater levels. The conceptual groundwater flow model at each site is similar but with differing boundary conditions. The model consists of one layer representing the unconfined sand aquifer underlain by impermeable bedrock. Under natural conditions the only water inputs are rainfall recharge and discharge to the exfiltration basins. Discharge is via evapotranspiration and discharge to the Pacific Ocean and swamps. Boundary conditions have been selected to coincide with known or assumed natural boundaries such as the coastline and bedrock outcrop. Other boundaries either align perpendicular to the dominant groundwater flow direction or are located at sufficient distances from areas of special interest to ensure simulation errors are minimised. The coastline was set at a constant head of 0.4m AHD to allow for tidal, wave and wind fluctuations (Turner et al., 1996). Water inputs to the models were simulated by groundwater recharge (rainfall) and exfiltration (enhanced recharge). Water outputs were simulated by evapotranspiration and discharge to the Pacific Ocean and swamps (Hallidays Point). At Hallidays Point, Frogella Swamp was simulated by the river module. Hydraulic parameters of hydraulic conductivity and rainfall recharge were refined during the calibration process. The modelling approach was steady state calibration followed by transient calibration using historical rainfall data and effluent discharge rates. Transient calibration was conducted by running each model over the period that discharge rates and groundwater level monitoring was available (typically three to four years) and simulating the groundwater hydrographs by replicating groundwater recharge (rainfall) via a sophisticated but conservative routine. To evaluate the most practical, efficient and environmentally acceptable effluent exfiltration system, a series of predictive modelling runs was conducted based on predicted future effluent loading scenarios. The effluent loading algorithm is based on historical peak wet weather flows (PWWF), and average dry weather flow (ADWF) with 50% loading added to the ADWF to simulate peak loading during holiday seasons. At Old Bar, groundwater modelling predicted the exfiltration system could be redesigned and expanded to the north into the former sand quarry to increase effluent discharge capacity. By redesigning the southern basins and constructing three additional basins, the exfiltration capacity was expanded to a capacity of 22L/sec ADWF and 200L/sec PWWF. Similarly, groundwater modelling predicted that the exfiltration system capacity at Hallidays Point could be expanded approximately 10-fold from two basins to eight basins and increasing effluent loading to 47L/sec ADWF and 361L/sec PWWF. This expanded scheme accounts for the predicted expansion of Hallidays Point and Wallamba and effluent transfer from Tuncurry.
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