Microbiological, Hydrochemical, and Isotopic Characterisation of the Aquifers at Alstonville, NSW, Australia

摘要

The Alstonville Reclaimed Water Scheme (the Scheme) will provide treated effluent from the Alstonville sewage treatment plant to irrigate eight properties, including commercial nurseries, agricultural and sporting enterprises. The Scheme will commence in early 2008. Due to the importance of the groundwater resource of the Alstonville Plateau, Ballina Shire Council commissioned a series of groundwater investigations to: determine the possible impacts on groundwater resources associated with irrigation using treated effluent; and to ensure that the shallow and deep basalt aquifers, and associated groundwater dependant ecosystems, would not be compromised by the Scheme. Of particular importance is the proximity of the Scheme to the town water supply bore, which utilises water from the productive deep basalt aquifer. A number of groundwater investigations have been conducted since 2003, including baseline investigations that detected elevated faecal microbiological and nutrient results in some of the Scheme's network of piezometers and private bores. There was also uncertainty regarding the relationship/connectivity between the underlying aquifers, and thus the potential risk of impact and the potential pathways for contaminant transport to the water supply aquifer. Additional microbiological, hydrochemical, and isotopic investigations were undertaken prior to the Scheme's commencement: to establish 'background' contaminant levels, including faecal and nutrient contaminants, and the likely sources; and to provide a better understanding of the hydrochemistry and isotopic characterisation of the sand and basalt aquifers, and thus the connection between the aquifers. Faecal coliform to enterococci ratios were calculated to determine the faecal contamination source(s), with one bore obtaining a ratio indicating a human source, which was supported by historically elevated microbiological results. Nitrogen isotopes proved useful in determining sources of elevated nutrients in various locations. Results indicated various sources, including human waste, human or animal waste, and organic fertiliser. A Combination of hydrochemistry and stable isotopes (oxygen-18 and deuterium) proved useful in determining the connection between the aquifers. The techniques applied suggested a good connection between the deep basalt and deep sand aquifers, and a limited connection between the shallow basalt aquifer and the deep basalt and deep sand aquifers. It is likely that the shallow basalt aquifers are 'leaking', providing some recharge to the deep basalt aquifer, probably via vertical fracturing and/or columnar jointing.