Posted 12 January 2009
Coastal aquifers are important strategic resources that may be at risk of salinisation from rising sea level and climate change.
The impacts of climate change and sea level rise have the potential to affect both the yield and quality of important strategic water resources provided by coastal aquifer systems.
Coastal freshwater aquifers are strategic resources that provide water for many important uses including town water supply, domestic water supply, irrigation of crops and pastures and industrial processes. Coastal aquifers also provide base flow to creeks and rivers during dry periods, thus supporting diverse ecosystems.
It has been estimated that 46,060 hectares or 1.4% of Australia's irrigation area is coastal land lying less than 5m above sea level (i.e. between 0 and 5 m AHD) and is therefore potentially at threat from seawater salinisation (Werner et al., 2008). The degree to which productivity of these coastal irrigation is reliant on groundwater supplies is yet to be fully quantified.
Sea level rise contributing to saline intrusion or inundation of coastal freshwater resources is probably the most direct impact of climate change, particularly for shallow sandy aquifers along low-lying coasts. The natural groundwater equilibrium is also susceptible to changes in recharge and discharge associated with climate change.
Fresh water contaminated by seawater at the level of only 5% renders it unsuitable for many important uses including drinking water supplies; irrigation of crops, parks and gardens and sustaining groundwater dependent ecosystems.
Sea-level rise and climate change can potentially impact groundwater resources in the following ways:
The occurrence of these impacts can vary substantially between different localities due to site-specific factors. Some impacts, such as changes to recharge and discharge patterns may also be naturally influenced by climatic variability. In some areas, groundwater extraction from water bores and subsidence of the land surface can exacerbate the potential impacts of climate change and sea-level rise.
On average, sea-levels rise naturally by about 2 mm/year. Globally there has been a total rise of about 0.1 to 0.2 m in the last 100 years. According to measurements from tide gauges the average rise in sea-level around the Australian coast is 1.2 mm/year (BOM). For more information see the OzCoasts Sea level rise web page.
Long-term global sea-level measurements have indicated sea-levels are rising at increasing rates. Currently, the best estimates provided by the IPCC (2007) indicate sea-level increase of 0.2 to 0.5 m by 2050 and 0.5 to 0.9 m by 2100. Current coastal management and construction guidelines around Australia generally allow for up to 1.0 m of sea-level rise by 2100 (NCCOE, 2004). A planning period to 2100 is considered to be appropriate for the design life of most coastal structures.
In addition to these natural rises in sea-level, melting of the Arctic, Antarctic and Greenland icesheets that may occur due to climate change could cause further increases in sea-level. According to IPCC (2007) total melting of the Greenland ice sheet over a period spanning thousands of years would eventually elevate global sea levels by an estimated 7m.
Saline intrusion and/or inundation caused by rising sea-level would probably be the most significant impact on coastal groundwater resources due to climate change, particularly for shallow sandy aquifers along low-lying coasts. Seawater movement into a coastal aquifer can be initiated by the breakdown of the natural barrier system and salinisation of fresh water wetlands. See the OzCoasts saline intrusion page for more information.
It is commonly - although wrongly - assumed that groundwater level is at mean sea-level, with continuous groundwater discharge below this level (0 m AHD, Turner et al., 1996, 1997; Turner 1998). However, the action of waves causes 'mounding' of groundwater at the coast, resulting in the water table occurring at about mean high tide level (~1.0 m AHD). Storm events, and local wind & wave climate can push the coastal groundwater level to ~2.0 m AHD. Increased mean sea-levels could further increase this groundwater 'mounding' and therefore change groundwater discharge dynamics as shown in Figure 3.
In order to better understand how these processes are affected by groundwater extraction and climate change, more detailed knowledge is required of hydrogeological processes, how groundwater interacts with surface waters and aquatic ecosystems. This is particularly important for aquifers that are yet to reach equilibrium with current groundwater extraction rates.
Powerful storms can generate storm surges that inundate thousands of miles of coast lines along the world's oceans every year with seawater. This saline water can flow down submerged storm-damaged water supply wells and contaminate boreholes (well casings and filter packs) in inundated low-lying areas. This can then lead to contamination of surrounding coastal aquifers (Carlson et al. 2007). The risk of bore contamination is increased as sea-levels rise.
For example, extensive low-lying areas in coastal Bangladesh of approximately 22,000 km2 are vulnerable to inundation by a surge of only 1.5 m. This has the potential to affect thousands of water supply wells in these areas that supply fresh water to approximately 17 million people (United Nations Environment Programme 2006).
In the United States, a study of a contaminated bore in southeastern Louisiana flooded by storm surges caused by Hurricane Katrina in 2005 found that the extent of contamination required purging of far greater volumes of water than conventionally used to re-establish water quality in wells of this kind (approximately 200 casing volumes, Carlson et al., 2007).
Saline intrusion has affected many coastal aquifers around the world, such as aquifers in Los Angeles (USA) and localities along the Mediterranean coast including 60% of Spanish coastal aquifers (FAO, 1997, Herrera, 2007). Over-extraction of groundwater from these aquifers has contributed to saline intrusion.
Aquifer salinisation in north-eastern Denmark was caused by or seawater flooding low-lying coastal plain. High density seawater plumes migrated down into the aquifer after the initial seawater inundation event (Andersen et al. 2006, 2007).
While there are reports of seawater intrusion in many coastal areas of Australia, comprehensive investigations confirming the occurrence of seawater intrusion have only recently been completed for coastal systems in Queensland (Pioneer Valley and Burnett Basins)and to a lesser degree in Western Australia and South Australia (Narayan et al, 2003, Werner et al. 2008).
Saline intrusion is known to have occurred in the Botany Aquifer, near Sydney airport in the 1960s (Cornell, 1964). Groundwater bores supplying industrial process water were subsequently shut down, and usage from the aquifer moved inland from Botany Bay.
Climate change is expected to exacerbate groundwater salinisation processes in many of areas where it has already occurred.
The UNSW Water Research Laboratory has conducted many studies of coastal aquifers, with special focus on unconfined sandy aquifers.
Research currently undertaken Ian Turner with European and US ACE collaborators is measuring the response of coarse sediment beaches to changing tide, wave and groundwater conditions. This project employs a wave machine the size of a two-storey building that uses a million litres of water and 500 tonnes of gravel simulate a full-scale 'beach' within the laboratory.
Understanding processes inside coastal aquifers, particularly how water moves within and between these aquifers and adjacent saline groundwater is essential for management of salinisation. Professor Ian Acworth, the Gary Johnston Chair of Water Management at UNSW, has led the development of techniques for monitoring water movement in costal sandy aquifers. New sensor equipment has been designed to measure the underground water pressure that drives flow in coastal sandy aquifers, and tracking of a radioisotope tracer (Br82) injected into a coastal sandy aquifer has measured the changes in underground flow direction in response to tidal movement (Acworth 2007, Acworth et al. 2007).
A feasibility assessment for desalination using beach well systems on the Central Coast of New South Wales in eastern Australia has been conducted by WRL to assist addressing water shortages there. This involved site investigations, monitoring, analytical assessments, aquifer pump testing and 3D numerical flow modelling for the purpose of detailed design (eg. Timms et al. 2004, 2006, Anderson et al. 2005a, b; Anderson et al. 2009). Beach well systems (both vertical wells and radial lateral wells) have been successfully used with minimum pre-treatment for large desalination plants in Israel, Malta, Saudi Arabia and numerous islands.
These subsurface techniques make use of natural sand filteration and offer a number of advantages over surface-based means for extracting saline water in coastal environments including: better water quality, reduced need for chemical pre-treatment of source water, reduced risk of mechanical failures, minimisation of impact on marine biota, improved visual amenity and smaller construction footprint (Cunningham et al., 2007). For more information download the Optimising desalination feed water quality using subsurface intakes poster (download PDF 283KB).
Sustainable yield estimates for coastal aquifer systems should account for possible seawater intrusion, along with employing appropriate monitoring systems and application of adaptive management procedures to groundwater resources.
Assessment should commence with detailed background study using available information (desktop evaluation ) in order to identify high, medium and low priority risks (Carley et al., 2008). Information required for such an assessment would include:
Case study:
Clarence City, Hobart, Tasmania
An assessment of the impact of seal level rise on groundwater resources in Clarence City, Hobart, Tasmania found the magnitude of potential risks to groundwater was variable, and that the possibility of high water tables causing damage to infrastructure was a major concern. This information can assist planning of future work to ensure focus on this impact. (Carley et al., 2008).
If high value water resources or infrastructure threats are identified, field investigations, monitoring and computer groundwater modelling can assist decision-making and management. Geophysical surveys can identify targets for test bores and monitoring. The most reliable information on groundwater flow direction and salinity concentrations is obtained from nested monitoring bores (or mini-piezometers) with short intake screens that are positioned at different depths.
Installation of automated loggers to record groundwater level and salinity changes in monitoring bores is a cost-effective monitoring strategy. Sampling of groundwater chemistry on at least an annual basis is also important because geochemical changes can provide early warning of saline intrusion.
Adaptive management of coastal groundwater supplies provides a number of options for handling the risk of salinisation.
'Retreat' options include restricting groundwater use or optimising pumping locations and schedules to reduce the impact of extraction on the fresh-saline equilibrium.
'Accommodation' options include building raised bore heads to reduce the risk of bore flooding and desalination of saline borewater that has already contaminated coastal aquifers. Where other water sources are limited, this can be more efficient than desalinating raw seawater obtained directly from the ocean.
'Protection' options include engineered flow barriers (eg. Southern Los Angeles), managed aquifer recharge and active management of catchment water balances - particularly by the use of vegetation cover that helps to maintain the equilibrium by transpiring water.
Further study and research is required to develop adaptive and management responses for local aquifers that are valuable and strategic water supplies, where these systems are at risk due sea-level rise and climate change.
Timms, W, Andersen, M.S. and Carley, J (2008). Fresh-saline groundwater boundaries below coastlines - potential impacts of climate change. Coast To Coast Crossing Boundaries Conference, 18-22 August, 2008, Darwin.
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