5.5 Groundwater quality

More information is needed on groundwater contamination and salinity to properly assess status and trends.

The natural and human-induced salinity of NSW groundwater is highly variable and there are many areas where groundwater is not suitable for consumption or irrigation. Activities such as over-pumping can result in the salinisation of good quality groundwater resources.

Contamination of groundwater by nitrates, pesticides, pathogens, hydrocarbons and other substances is known to occur in isolated instances, but only limited data is available to define the full extent of the problem.

In general, more knowledge is needed to ensure that the resource is managed appropriately.

NSW indicators

Introduction

Many ecosystems require good quality groundwater for their ongoing health and maintenance. Ecosystems depending on groundwater include surface water bodies, such as wetlands, rivers and lakes that are connected to groundwater, and also some terrestrial ecosystems. Changes in groundwater quality have the potential to degrade these ecosystems causing a loss of terrestrial and aquatic species.

Groundwater is a vital resource, supplying approximately 11% of the total water used in NSW. Its uses include drinking water for human consumption and other domestic uses; irrigation, stock watering and other agricultural uses; and industrial purposes. The availability of groundwater for these uses depends not only on the quantity of the resource, but also on its quality.

The main impacts on groundwater quality in NSW are increased salinity, and contamination by pollutants. Poor-quality groundwater significantly limits its environmental value and increases the cost of water treatment. It may also prevent some types of water use altogether. Once an aquifer is polluted, it is extremely difficult and expensive to restore.

Current status and trends

Groundwater salinity

Natural salinity levels in groundwater are predominantly determined by a combination of geology and the distance to the recharge source. Many areas of south-western NSW are underlain by sediments of marine origin which yield regionally saline groundwater. Although groundwater quality may be influenced by the type of aquifer, good- and poor-quality water can be found in most aquifers, and the quality can be highly variable within that aquifer. Natural salinity may be exacerbated by human activities such as irrigation, inappropriate disposal of wastewaters, and the clearing of land. Excessive extraction may cause saline groundwater to enter a freshwater aquifer where these are hydraulically linked, such as where freshwater coastal sand aquifers are surrounded by saline groundwater linked to the sea – extraction depletes the freshwater resource, which is then replaced by inflowing saline water.

The potential to use groundwater resources increases at higher yields and with decreasing salinity. Higher salinities tend to limit the use of groundwater. However, improvements in desalination technology may allow greater future use of saline water, if this is not contaminated with other pollutants.

It is difficult to assess how increases in the salinity of groundwater might affect groundwater-dependent ecosystems. This is because the impact of saline groundwater on water quality in rivers and wetlands is generally related to the level of dilution by surface flows. Naturally saline groundwater may significantly increase the salinity of aquatic ecosystems, while high surface flows from rainfall or flooding may lower it. These ecosystems may also receive saline discharges from sources other than groundwater (see Water 5.3), so any observed salinity increases may not be the result of groundwater alone. Also, little is known about the tolerances of many groundwater-dependent flora and fauna species, especially those living within saline aquifers, to changes in salinity.

Groundwater contamination

The NSW Government regulates contaminated sites that represent a significant risk of harm under the Contaminated Land Management Act 1997 and other sites with notices issued under sections 35 and 36 of the Environmentally Hazardous Chemicals Act 1985 (now repealed). The Contaminated Land Management Public Register lists 237 sites, of which 117 are currently being actively remediated, 77 have been remediated, and a further 43 are being assessed (see Land 4.3). Groundwater contamination is largely associated with long-standing existing and former industrial areas, and occurs at about 90 of the currently regulated sites. These tend to be in urbanised areas and concentrated in Sydney, Newcastle and Wollongong. The sources of contamination are distributed between purpose-built hydrocarbon storage sites such as service stations and depots (30%), industrial sites (38%), and landfills, gasworks and other land uses (32%).

The main contaminants at 46% of sites are hydrocarbons – including total petroleum hydrocarbons; benzene, toluene, ethyl benzene and xylenes; and polycyclic aromatic hydrocarbons. Twenty-five per cent are affected by heavy metals, 9% by chlorinated solvents and 7% by nutrients. Several sites are affected by more than one contaminant.

Response to the issue

Most of the responses to groundwater management address both quantity and quality issues (see Water 5.4). For example, the statutory plans for sharing groundwater also provide for management actions such as cessation of pumping if water quality problems arise.

For salinity, a key part of the response has been to improve understanding and acceptance that groundwater in many areas is naturally very saline and that bringing it to the surface can cause problems. The NSW Government provides financial assistance towards research into the use of saline groundwater in aquaculture. The Inland Centre for Saline Aquaculture Research and Development at Wakool is a joint venture between the NSW Department of Primary Industries (Fisheries) and Murray Irrigation Ltd. It has successfully raised salt-tolerant fish species in saline groundwater. There is commercial interest in the most successful of the trials – raising trout during the winter and prawns over summer.

The National Action Plan for Salinity and Water Quality and the MDBMC's Basin Salinity Management Strategy 2001–2015 (MDBMC 2001) are important responses that will help to manage groundwater quality (see Land 4.3). A brief summary of the achievements of the NSW Salinity Strategy (DIPNR 2004c) can be found in Water 5.3.

The NSW Government is developing groundwater guidelines for contaminated sites, and is requiring more monitoring of groundwater during operations of high-risk activities such as oil refineries and landfills in licences issued under the Protection of the Environment Operations Act 1997. Exclusion zones prohibiting groundwater extraction can be imposed under the Water Management Act 2000, as they have been at Botany, to protect users from polluted groundwater originating from the Orica and other contaminated sites.

Future directions

Government policies are in place to better manage both the quantity and quality of groundwater resources in NSW. The limited information available on groundwater quality makes it difficult to determine current status and trends. However, in many cases the changes in water quality result from extraction; therefore, managing groundwater extraction will also manage groundwater quality issues to some extent. Groundwater-sharing plans include performance indicators for assessing their operation over the ten-year life of the plans.

Groundwater contamination is generally local in occurrence, and it is often easy to predict where problems will exist. However, greater coordination of groundwater quality monitoring would help in evaluating the effectiveness of management measures over the long term.

There are no guidelines for water quality needed to sustain groundwater-dependent ecosystems. Developing such guidelines, within the framework of the National Water Quality Management Strategy, is a priority.