Surface water quality

The Sydney Basin bioregion varies from relatively pristine wilderness areas within national parks in the northern and western parts of the Hawkesbury-Nepean river basin to highly modified urban environments around Sydney and Wollongong (see Figure 1 5 in Section 1. 1.2), where the surface drainage network is highly engineered and the sources of contaminants highly variable. Treated wastewater, stormwater, runoff from disused mines and other industrial sites are sources of contaminants from the urbanised areas. In south-western Sydney, up onto the Southern Highlands and in the coastal strip south of Wollongong, runoff from agricultural lands is also a source of surface water contaminants.

Nutrients, particularly nitrogen and phosphorus, are of concern because elevated loads can lead to excessive algae and aquatic weed growth, which affects aquatic habitat and can limit commercial and recreational uses of the river. Diffuse sources, including urban, grazing, cropping, intensive horticulture, intensive animal production areas, contribute the majority (70 to 80%) of the total nutrient load to the lower Hawkesbury-Nepean river basin (Davis et al., 1998), with the highest rates around the Penrith-Windsor area and elevated rates southward towards Picton (NSW Department of Environment and Climate Change and Water, 2010). Point sources (predominantly sewage treatment plants (STPs)) contribute a much lower proportion of total nutrient loads in the Hawkesbury-Nepean river system. However, during dry weather they contribute the majority of nutrients because discharges are not driven by runoff events, but remain relatively constant year round. Over the last 20 years, upgrades to STPs, decommissioning of poorly performing plants and increasing wastewater recycling have resulted in significant reductions in nutrient exports from STPs (45% and 75% for TN and TP, respectively, between 1996 and 2008) (NSW Department of Environment and Climate Change and Water, 2010). These reductions in nutrient loads from point sources suggest that the relative contribution from diffuse loads will increase to more than 80%. Trend analysis of nutrient levels in the Hawkesbury-Nepean river basin indicates that phosphorus have been generally declining throughout the river system, whereas nitrogen levels have decreased at many sites and increased at some others. Nitrogen levels often remain above ANZECC/ARMCANZ (2000) guidelines throughout the system.

WaterNSW is mandated to ensure a safe and reliable supply of water suitable for treatment to drinking water standards. To this end, it manages the water supply catchments and infrastructure to protect water quality. Protected and special areas are declared catchment areas where public access and activities are restricted to protect water quality (Figure 15 in Section 1.1 .2). Three classes of protection zone have been defined: (i) special areas – no entry, (ii) special areas – restricted access and (iii) controlled areas – no entry. The no entry special areas include the reservoirs and surrounding land, except for Fitzroy Falls Reservoir and part of Lake Yarrunga, which allow for restricted access. Lake Burragorang has both a no entry zone around the reservoir enclosed by a more extensive second protection zone of restricted entry. Restricted entry prohibits vehicles, horses, pets and firearms within the zone. The controlled areas prohibit public access to water supply infrastructure, including land along the Warragamba pipelines and Upper Canal. In all, 365,000 ha of the water supply catchments are within protected and special areas. Underground coal mining predates the declaration of special areas and 8% of the special areas are currently undermined (NSW Chief Scientist and Engineer, 2014).

Water quality monitoring is undertaken by WaterNSW and an annual report has been published since 2000–01 by the Sydney Catchment Authority. The focus of monitoring is on ensuring the Australian Drinking Water Quality guidelines are met for a range of water quality parameters, including pathogens such as Cryptosporidium and Giardia, algae (e.g. cyanobacteria), nitrogen, phosphorus, cholorophyll-a, turbidity, toxins, heavy metals, pesticides and synthetic organic compounds. The monitoring program includes routine sampling and special sampling during wet weather and for research, using manual and automated sampling technologies. The safety limits for most substances dissolved in drinking water are set at about 1% of the amount that could potentially harm humans. Should levels exceed these limits, prompt response procedures are in place to ensure ongoing supply of safe water (Sydney Catchment Authority, 2014).

Analysis of water quality data between 2000 and 2009, reported in the State of the Science – Catchment Impacts Summary Report (Sydney Catchment Authority, 2011), indicated that 12 of the 19 catchments which flow into Greater Sydney water supply storages posed no risk to water quality except during very high flows. Five catchments were identified as high risk across a range of pollutant groups: Wollondilly, Kedumba (lower Coxs River), Kangaroo, Shoalhaven and the Upper Nepean rivers. All had excessive levels of nitrogen, phosphorus (except Shoalhaven), pathogens (except Shoalhaven) and metals (except Kedumba and Upper Nepean). Two catchments, Waratah Rivulet and the Cataract River, had some water quality issues relating to metals associated with mining activities. Turbidity and suspended sediments were not found to be a significant problem in any catchment, except during very high flows. Aluminium and iron, which are naturally occurring in groundwater discharges, were found to exceed ANZECC/ARMCANZ (2000) guidelines.

All development within Greater Sydney’s drinking water catchments is required by the State Environmental Planning Policy (Sydney Drinking Water Catchment) 2011 (SEPP) to have a sustainable neutral or beneficial effect on water quality. In May 2014, the NSW Chief Scientist and Engineer delivered a report to the NSW Government on the state’s capacity to assess the cumulative impacts of activities which impact groundwater and surface water in the Sydney water supply catchments. The report found that there was insufficient data in a coherent form to enable a deep and reliable assessment of cumulative impact. WaterNSW has declared a set of principles for managing mining and coal seam gas impacts in declared catchment areas. Consistent with the SEPP, mining and coal seam gas activities must not result in a reduction in the quality of surface water and groundwater inflows (WaterNSW, 2015).

The salinity of Hawkesbury-Nepean water, as indicated by its electrical conductivity (EC), shows low salt concentrations (range 30 to 990 μS/cm, in Markich and Brown, 1998) throughout most of the river system until about 70 km or so from the coast (between where Colo River and Macdonald River meet the Hawkesbury) where the tidal influence begins. Water upstream of the Colo River is used for irrigation, but salt concentrations increase to levels that are detrimental to most irrigable crops downstream of this point. Some tributaries of the Nepean River have relatively high ECs at low flows, reflecting greater contribution of more saline groundwater at low flows, but the water is still generally suitable for most fruit and vegetable crops (NSW Environment Protection Authority, 1992). Analysis of EC data from the Hawkesbury-Nepean Environmental Monitoring Program suggest small increases in EC are occurring at the majority of monitoring sites, but levels remain well within ANZECC/ARMCANZ guidelines for lowland rivers (NSW Department of Environment and Climate Change, 2009).

Last updated:
21 January 2019
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