School of Earth & Environmental Science
Palma, Jack, Water quality in Lake Illawarra, New South Wales: Groundwater contamination and migration from septic and nightsoil waste deposits in the coastal Windang unconfined sandy aquifer, BEnvSci Hons, School of Earth & Environmental Science, University of Wollongong, 2016.
Approximately 13% of the world’s coastline is occupied by coastal lagoon systems which are often very important centres for recreational and commercial industries. Nutrient loading from nearby waste deposits into coastal bodies such as estuaries and lakes is not uncommon, and the leaching and migration of waste into such water bodies can lead to eutrophication. Hence, the understanding of contaminant plumes and nutrient loading mechanisms such as submarine groundwater discharge is of particular importance to coastal managers.
The Windang peninsula is a sandy coastal barrier 80 km south of Sydney, Australia, and forms the north-eastern edge of Lake Illawarra. The dumping of nightsoil and septic effluent into unlined trenches in the dune system adjacent Port Kembla Golf Course on the Windang peninsula has occurred from the early 1940s until the 1990s, and is thought to have led to eutrophic events including algal blooms in Lake Illawarra during times of intermittent tidal flushing.
The unconfined aquifer in Windang around the south-western portion of Port Kembla Golf Course was assessed in this study with the goal of identifying the characteristics of a known ammonia plume including migration, discharge and attenuation. Eight boreholes were assessed over 4 ‘dry’ sampling events and 3 post-rainfall ‘wet’ sampling events to determine chemical concentrations and groundwater properties. Very little NOx was found in the study area, and ORP results suggest that groundwater is in a reduced to slightly oxidising state. Ammonia concentrations exceeded ANZECC trigger values for estuarine bodies by a factor of 24 times on average, with a mean plume concentration of 36 mg/L and a range of 1.8 to 103 mg/L recorded in groundwater during the study period. Ammonia concentrations were found to be higher following rainfall periods, which is most likely due to the leaching of nitrate and organic nitrogen from the surface and subsequent conversion to ammonia via ammonification and dissimilatory nitrate reduction to ammonium. The plume is moving through the aquifer in a westerly direction via advection, moving upwards over a saltwater wedge that intrudes from Lake Illawarra in the western edge of the peninsula and upwelling close to the surface near the shoreline. The plume is also discharging into Lake Illawarra via submarine groundwater discharge as contaminated groundwater from the peninsula seeps into lake sediment. Hydraulic characteristics of the Windang sandy unconfined aquifer were established using manual and automatic standing water level measurements, as well as slug testing. Groundwater flow and plume migration takes place in a west-north-westerly direction at a rate of 5.4 metres per year. Tidal pumping was observed between the western edge of the aquifer and Lake Illawarra based on statistical correlations with automatic data loggers and a Lake Illawarra tidal gauge. Ammonia concentrations in the plume are likely primarily derived from the leaching of septic effluent and nightsoil waste deposits rather than fertiliser leaching from Port Kembla Golf Course. Between 12 and 17 tonnes of ammonia are thought to be within the study site groundwater, with 0.42 tonnes discharging into Lake Illawarra from the study site per year from steady-state groundwater flow. An approximate plume shape was determined, showing dispersive or diffusive processes as the probable mechanisms for migration of the ammonia plume perpendicular to groundwater flow. Historical concentration data exhibits attenuation of the plume over time via discharge into adjacent Lake Illawarra, and based on two independent methodologies, this study presents time-till-depletion values of between 7 and 40 years for the ammonia plume.