Groundwater is an important and vital source of drinking water in Australia. Geosciences Australia (link) report that Australia's use of groundwater has increased significantly over recent decades. For example, in the 13 years from 1983 to 1996, our national reliance on groundwater increased by nearly 90 per cent. Future use of groundwater in Australia is projected to rise, especially as surface water resources may become less available due to climate change and prolonged droughts. Groundwater is also an important water source in Australia for a wide range of other purposes, such as irrigation, agriculture and industrial use. In fact, irrigation practices account for just over half of all groundwater used by Australians. Many local councils and private households use bore water to maintain parks and gardens.
When we visit water utilities that supply drinking water from a groundwater supply, there is a common perception that the risk is low, the ground water supply is protected from contamination, and often disinfection is the only treatment carried out prior to supply for drinking water. This can, at times be a false sense of security, as we have seen with the recent widespread outbreak of campylobacter in North Havelock New Zealand (although this was an undisinfected supply).
Groundwater quality can be compromised in a number of ways that must be considered in the risk assessment. One of the primary pathways for groundwater contamination is infiltration of contaminants from the land surface (Australian Government 2013).
Pathways for contamination can affect both unconfined and confined aquifers, such as leakage of poor-quality groundwater into an aquifer; waste disposal via wells; or leakage from poorly constructed, maintained or decommissioned wells. In other cases, leakage can occur through aquitards that have been damaged by activities such as poorly managed mine subsidence or hydraulic fracturing.
Falling groundwater levels can result in the drying of some wetlands. This can oxidise acid sulfate soils, which creates acidic conditions that mobilise metals and sometimes release arsenic. Falling groundwater levels due to pumping can also result in seawater intrusion into a fresh aquifer or leakage of higher- salinity groundwater into a fresher aquifer. On the other hand, rising groundwater levels or changes in groundwater flow directions can cause flow of contaminated or poor quality groundwater into streams and wetlands.
Drinking water risk assessment must consider the range of relevant hazards and hazardous events that could affect groundwater supplies such as:
pathogens from stormwater intrusion from the extraction bore, unused bores and privately owned bores and wells, particularly in area where there are livestock and wildlife that could pose a protozoan threat
viral and bacterial hazards from septic tanks in the bore field
chemical hazards from nearby underground storage tanks, landfills
local land applications of pesticides and herbicides
changes in water quality due to oxidation of acid sulphate soils, and the mobilisation of metals
mobilisation of salts, metals, acidity, alkalinity or radionuclides from mining activities
nitrates, phosphates, salts, metals and pathogens from over-application of soil amendments
changing proportions of natural chemical constituents present in groundwater or introduction of additional constituents from CSG activities.
Options for managing risks to ground water supplies should be identified through a risk assessment process and may include:
replacing, refurbishing and repairing bore heads to ensure stormwater cannot enter the bore
proper decommissioning of old and disused bores
closing up wells
inspection and regulation of septic tanks within bore fields
development and enforcement of planning restrictions around bore fields
increasing monitoring to understand how the ground water is impacted by environmental or human interactions
developing operating strategies that respond to the relevant impacts, for example, tighter operational limits for turbidity during times when bore head are inundated by stormwater
assessing adequacy of drinking water treatment processes.
Whilst many utilities are implementing risk management actions to manage risk, there is a need to take a proactive approach to identifying and managing the risks to groundwater, we cannot wait for another outbreak before we take action.