What can affect our water resources?

Table 14: Key pressures and impacts

The Bureau of Meteorology and the CSIRO predict that SA's rainfall will decrease in the future with a greater number of severe droughts. Rainfall is expected to decline in winter and spring by up to 15% by 2030, resulting in reduced streamflow and in turn, reduced water in storages.

The May to July rainfall has already reduced by around 19% since 1970 in southwest Australia, with streamflow falling by 50%. The predicted increase in extreme weather events will also affect water quality through greater runoff.

In the southern agricultural regions of SA, there has been a prolonged period of drying from the 1990s, particularly in early winter and autumn. There is good understanding of the underlying physical mechanisms driving this change (southward shift of winter and spring storm systems).

The amount of runoff from catchments, and the annual recharge to groundwater systems has a non-linear relationship to changes in seasonal rainfall. In SA, reductions in runoff and groundwater recharge can commonly be between 2 and 4 times the reduction in annual rainfall.

During dry times, Adelaide can rely on desalination as a reliable, climate-independent water source for its domestic and industrial water needs. However, this will not meet all our water needs, and the Mount Lofty Ranges storages and the River Murray will continue to be important water sources into the future.

The Bureau of Meteorology estimates the climate resilient component of SA’s water supply capacity is almost 290 GL, with seawater and wastewater each contributing just over 100 GL. This comes from 13 desalination plants and 27 recycled water plants across the state.

South Australia’s largest climate-independent source of water is the Adelaide Desalination Plant that, in full operation, is capable of delivering 100 GL of water per annum, currently around half of metropolitan Adelaide’s annual water needs.

The influence of a future hotter and drier climate on groundwater will vary markedly between different aquifer types, with studies suggesting significant reductions in sustainable capacities for some groundwater resources.

Activities on the land largely determine what happens to our water. Sedimentation, and nitrogen and phosphorus pollution work together to decrease water quality and damage freshwater ecosystems. Sediment builds up in layers on riverbeds and lakebeds in which much of the phosphorus is stored. Nitrogen, mostly as highly soluble nitrate, eventually washes out to sea, but the harm done before this happens is not easily reversible.

The more contained and slowly flowing a waterway is, the higher the accumulation of sediment. The Coorong and Lower Lakes, at the end of the very long River Murray system, are particularly vulnerable. Sedimentation increases as rivers flow through developed catchments, with the worst build-up of sediment occurring in estuaries.

Wetlands, considered as the kidneys of catchments, have largely been cleared and now only occupy a small fraction of their former extent. Growth in intensive farming, including dairy farms, has increased the nitrogen in freshwater. This, together with phosphorus, fertilises unwanted plant growth.

Changes in farming practices are helping to reduce pathogens, sediment and phosphorus pollution. However, it is much harder to stop nitrogen getting into water and impossible to stop the nitrate in groundwater, accumulated over decades, slowly making its way to lakes and streams.

Agricultural activities have changed much of SA’s landscape. Previously rich and abundant native vegetation has been cleared to make way for crops and grazing land to produce food products and textiles. An unfortunate consequence of agricultural development is to damage many of our river systems.

The typical effects of urbanisation on the water cycle are described as:

• increased runoff volumes and peak flows, altered timing of flows and reduced infiltration due to impervious surfaces such as buildings and roads
• water pollution due to the influence of urban land-use activities and landscapes designed to rapidly divert water to receiving waterways without adequate measures to capture pollutants
• catchment drying resulting from reduced infiltration and storage of rainfall runoff
• increased dependence on imported water due to a reduced area of natural catchment available for harvesting good quality water.

Conventional wisdom has it that in urban areas, impermeable hard surfaces created by roofs, roads, pavements and concrete do not allow rain to filter through, and consequently greatly reduce groundwater recharge.

However, there is evidence that the opposite may be partly true because of the presence of sources of water other than rainfall in urban areas, such as leaking water mains, sewers, septic tanks and soakaways. Often, the net effect is to increase recharge to pre-urbanisation rates. Other studies have found that the complex interaction between groundwater levels, climate, geology, stormwater practices and leaky pipes could lead to either rising or falling groundwater levels.

The clearance of native vegetation has raised the water table in many parts of SA. As a consequence, the soils and watercourses in many areas have become increasingly saline.

Historically, water extraction and river regulation has adversely affected the health and productive use, that is water quality, of rivers and streams because of reduced environmental flows and changed flow patterns. This has been particularly significant for the River Murray and its wetlands and floodplains.

During the last 5 years, the provision of environmental water has contributed, on average, an additional 800 GL of water flowing over the SA border each year. Full implementation of the Murray–Darling Basin Plan will result in further environmental benefits to SA and the wider Basin.

Many crops rely on the addition of fertilisers to promote rapid growth. Fertilisers usually contain readily soluble nitrogen and phosphorous compounds. Some of these fertilisers are washed off the land or through the soil to surface water areas where they can create excessive algal growth. 

Livestock access to rivers and streams can also introduce nutrients as well as cause excessive bank erosion and increase water cloudiness.

Wastewater storage and reuse also contribute to the nutrient load.

Pesticides used in agriculture can be washed into rivers and streams after rain. Some pesticides are persistent and can be detected in water long after their use.

In SA, pesticides have been found in waterways and groundwater, and historical contamination of soil is common around cattle and sheep-dip sites. Pesticides have killed fish and aquatic invertebrates in inland and estuarine waters. Bird deaths have also been attributed to pesticides, and spraying to kill locusts has been shown to affect other organisms. 

In some areas, natural recharge of groundwater has increased due to land clearance and irrigation, leading to increased leaching of nutrients and pesticides into aquifers. Groundwater provides much of SA’s water for human use and is the source of some of the water in many of the state’s creeks, rivers, wetlands and estuaries.

One risk to groundwater quality is increasing salinity as a result of extraction, either due to the downward movement of naturally occurring high salinity water in the overlying unsaturated zone or the lateral movement of more saline groundwater. Both these risks are managed through water allocation plans. Water table aquifers are particularly susceptible, although leaky wells are also potential pathways for pollution to deeper confined aquifers.  

As recently as the 1980s, chemicals used by industry were simply tipped down drains and poured onto soil and left to evaporate. In some cases, this has contaminated upper aquifers. Chemicals found in groundwater across metropolitan Adelaide can include volatile organic compounds (petroleum hydrocarbons, chlorinated hydrocarbons and other organic compounds), pesticides, polycyclic aromatic hydrocarbons and nitrates.

As stormwater travels over surfaces it picks up pollutants, including litter, nutrients such as phosphorous and nitrogen, metals such as zinc, copper and lead, oil and grease from roads, garden waste, bacteria and sediments. 

With the higher and changed flow regimes of urban waterways, greater erosion in streams further adds to the pollutant load and affects aquatic species living in waterways. In the regional centre of Mount Gambier, urban stormwater discharged into groundwater drainage bores filters its way through the limestone karst geology to the Blue Lake.

In addition to climate variability, land use and the risk of over-consumption, other threats to water quality include bushfire and introduction of invasive species.