Groundwater monitoring and water quality in Lake Illawarra: contaminiation and migration from night soil deposits in the Port Kembla sand dunes

Nutrient inputs to marine ecosystems is an area of growing concern in coastal areas experiencing rapid urbanisation. Of most concern are increased nitrogen inputs to coastal estuaries from fertiliser use, stormwater runoff and subsurface septic deposits. Thus, there is a need to identify, monitor and, where possible, remediate both point and non-point sources of nutrient pollution. Lake Illawarra, located on the South Coast of New South Wales is a barrier lagoon which has been experiencing macro algal blooms for many years associated with eutrophication. This study is a continuation of a number of groundwater quality assessments that have been conducted in the Windang Peninsula over the past two decades. The main objective of this preliminary study was to identify the north-south, east-west extent of ammonia in the unconfined aquifer of the Windang Peninsula. Three boreholes (4 m, 8 m and 12 m) were sunk at each of the four monitoring locations in the Windang Sandy Aquifer to identify ammonia trends with depth. Nitrate, nitrite, ammonia and TKN were sampled to show the species of nitrogen present in the aquifer. A dry period and rain event sample rounds have identified the presence of ammonia concentrations well above ANZECC guidelines for the 12 m bores. The dry period sample round and the rain event sample round identified maximum ammonia concentrations of 71.0 mg/L and 73.3 mg/L respectively. Nitrate and nitrite concentrations were quite low with maximum concentrations for both rounds of 0.59 mg/L and 0.01 mg/L respectively. Analysis for heavy metals showed that soluble iron is present at a maximum of 9.1 mg/L. Cadmium, lead and copper were either close to or below the detectable limit whilst zinc was detected below ANZECC guidelines. The 12 m holes were cored for the reconstruction of the Windang Barrier Holocene evolution. Amino acid racemisation was used to date A. trapezia found in the core samples. The evolution of the Windang Barrier is consistent with similar studies of Holocene evolution of Lake Illawarra. The monitoring location along the lake’s edge suggests that tidally driven, circulating estuarine water is responsible for a subterranean mixing zone. It has been hypothesized that this zone is responsible for significant attenuation of the plume. Concentrations of ammonia in the salt water wedge are above natural levels in the lake and this suggests that advection of ammonia across the subterranean estuary is likely. Regular monitoring on a monthly basis and directly after a rainfall event is recommended to validate temporal and seasonal trends of the ammonia plume.