What is Urban Salinity?

Indicators of Urban Salinity Home

Urban salinity occurs as a result of a combination of excess water and salt in the environment. Some of this excess occurs naturally, but the way we use and manage our land and water resources has a large impact on salinity.

As with dryland salinity, urban salinity occurs when deep-rooted native vegetation is cleared and replaced with shallow-rooted garden plants and lawns, which use much less water. This creates an imbalance in the water cycle, allowing large amounts of water to escape past the root zone and down to the underlying groundwater (referred to as 'groundwater recharge'). The installation of roads, buildings and other infrastructure can also alter natural drainage patterns, while other sources of excess water may result from leaking sewerage, stormwater and water pipes.

Over time, the groundwater system fills, bringing with it dissolved salts that had been stored in weathered rocks deep below the surface and from other sources. Salt is naturally present in soil, groundwater, waterways and rain. Additional (very minor) sources of salt in urban environments include effluent (household and industrial), swimming pools, fertilisers, soap and detergents, and building materials.

Where salty groundwater rises to within about 1.5 metres of the soil surface, it is drawn up by capillary action to form 'saline groundwater discharge' sites. These sites most often occur in low-lying areas.

Whilst the causes are somewhat similar in most areas experiencing dryland salinity, the processes vary from region to region according to differences in geology, geomorphology (landforms) and climate. This is because groundwater flow is also important. Groundwater generally flows from positions high in the landscape to positions lower in the landscape. Water escaping the root zone in one area of the city may, for example, contribute to salinity occurring in another area of the city much further down the catchment.

The capacity for groundwater to flow underground depends on the nature of the geological material that the water must flow through. Where this is very porous and permeable, groundwater may flow freely down the catchment over large distances, ultimately discharging in the lowermost part of the landscape (along creeks and valley floors). On the other hand, where geological materials restrict water movement, flow may occur over relatively short distances, with salinity discharge occurring in close proximity to salinity recharge.

Beneath the city of Bendigo groundwater moves through open fractures occurring within the upper 50 to 100 metres of the ancient marine sandstones and shales that comprise the region. In some instances, flow also occurs via numerous underground tunnels created by extensive historical underground gold mining activities. The network of fractured rock and tunnels forms a large groundwater system that operates throughout most of the urban area.

Urban Salinity in Bendigo
This brochure provides a general overview of urban salinity in Bendigo

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References

Collings, A (2002) Indicators of Urban Salinity. NSW Department of Land and Water Conservation, Sydney, New South Wales. ISBN: 07347 5276 9.

Ryan, M. (2003) Introduction to Urban Salinity. NSW Department of Infrastructure, Planning and Natural Resources, Sydney, New South Wales. ISBN: 0 7347 5376 4.