Water Journal : Water Journal April 2011
water APRIL 2011 117 artificial aquifer recharge At the Old Bar exfiltration site the groundwater flow is to the east and south- east towards the Pacific Ocean. Hydraulic gradients and, hence, groundwater flow directions are influenced by the volume of water in each exfiltration basin at any given time. Groundwater mounding periodically develops beneath the basins following prolonged discharge; however, the relatively high aquifer hydraulic conductivity allows effluent to laterally discharge rapidly. At Hallidays Point the exfiltration basins are situated on a local topographical high. The water table beneath the WWTP and exfiltration basins is part of a groundwater mound centred beneath the topographic high at an elevation of approximately 3.0m--3.5m AHD. Groundwater flow is in two directions, north-west towards Frogella Swamp and south-east towards the Pacific Ocean. Groundwater modelling of the local groundwater regime indicates the predominant groundwater flow direction is towards Nine Mile Beach and the Pacific Ocean. Water quality monitoring Groundwater quality monitoring has been undertaken quarterly at Old Bar and Hallidays Point since 1996. The monitoring programs ensure impacts to groundwater quality are detected, enabling management practices to be altered to minimise impacts to local groundwater quality. Effluent loadings discharged to the basins have the potential to increase nutrient concentrations (specifically nitrogen and phosphorus) in the groundwater if not managed appropriately. Prior to the construction of additional exfiltration basins at Old Bar, periodic overflowing of the exfiltration basins impacted groundwater quality by increasing groundwater nutrient concentrations. Since the upgrade, groundwater nutrient concentrations have decreased significantly. This decrease is attributed to the exfiltration basins no longer periodically overflowing and the nutrients within the aquifer system being flushed by continual effluent discharge. Following the construction of the northern exfiltration basins at Old Bar, impacts to groundwater quality in the former quarry have been minimal with recorded nutrient concentrations similar to background levels. Analysis of the trends within the hydrochemical data set indicates that the upgraded exfiltration basins at Old Bar are operating efficiently and not impacting significantly on the natural environment. The hydrochemical trends indicate groundwater quality around the southern basins has also improved significantly since the upgrade. Since the upgrade at Hallidays Point, mean nutrient concentrations have typically decreased and the range of occasional anomalous elevated concentrations has also decreased. The improvement in groundwater quality post-upgrade is attributed to better management of effluent discharge due to the increased surface area of exfiltration basins, allowing effluent to be discharged more evenly. Slightly elevated nitrogen concentrations were periodically recorded in some monitoring wells close to the exfiltration basins. These events occurred after December 2007 within the first six months that flow was transferred from Tuncurry. Since the increased flows to Hallidays Point, on-site management has been improved by increasing basin cycling to reduce groundwater nutrient levels. Prior to development of the Hallidays Point WWTP background groundwater was degraded, possibly due to former mineral sand-mining operations or the application of fertilisers for agriculture. Planting of native vegetation around the exfiltration basins has assisted in improving groundwater quality by lowering the water table and reducing nutrient levels. Since the WWTP upgrade, discharge volume has increased significantly; however, groundwater quality and groundwater levels down-gradient of the exfiltration basins have not been significantly impacted. The relationship between nutrient concentrations in groundwater and distance from the exfiltration basins has been investigated. Adjacent to the Hallidays Point western exfiltration basin nitrogen and phosphorus concentrations are moderately elevated but rapidly attenuate down-gradient as the contaminants degrade and undergo significant dilution. Concentrations of nutrients and phosphorus monitored in the outlying monitoring wells approach background levels, before discharging into Frogella Swamp. Conclusions The groundwater conditions at Old Bar and Hallidays Point WWTPs have been simulated by building groundwater models. These models have been used to predict impacts on the local groundwater regime due to increased effluent loadings. Prior to expansion at Hallidays Point, deep aquifer injection was considered as a flexible integrated re-use scheme sufficient for long-term requirements, but investigations were unsuccessful in locating the target. The remaining option was to expand the existing exfiltration systems at Old Bar and Hallidays Point. Even though the natural storage in the unconfined coastal aquifers is relatively small, the exfiltration areas should be considered as prospective ASR areas in the longer term. Since the Old Bar and Hallidays Point WWTP upgrades, discharge to the expanded exfiltration systems has increased; however, the impact on groundwater quality and groundwater levels down-gradient of the exfiltration basins has not been significantly impacted, nor the beneficial uses of the groundwater resource degraded. The high quality of treated effluent is compatible with the quality of natural groundwater, allowing extensive tree planting in buffer zones around the sewerage treatment works and rehabilitating degraded areas previously subject to sand mining and quarrying. Acknowledgements We thank MidCoast Water for permission to publish the results of these investigations and groundwater modelling studies. These groundwater investigations were conducted by Parsons Brinckerhoff (formerly PPK Environmental and Infrastructure) between 2000 and 2010. The experience and professionalism of our subcontractors also contributed to the success of this investigation, namely Macquarie Drilling and Engineering Exploration (drilling contractors), Groundwater Data Collection Services (geophysical contractor) and David O'Neill (Consulting Geophysicist). The Authors Graham Hawkes is a Principal Hydrogeologist with Parsons Brinckerhoff, based in their Sydney office (email: firstname.lastname@example.org). John Ross is the Manager of Hydrogeology with AGL Energy Limited. Graeme Watkins is a Strategic Operations Manager with MidCoast Water, based in Taree. References Turner IL, Coates BP and Acworth I, 1996: The Effects of Tides and Waves on Water Table Elevations in Coastal Zones. Hydrology Journal V4, No 2.
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