Arsenic in groundwater: chemistry, health risks and remediation

Arsenic in groundwater is a chemical and public-health problem shaped by geology, water chemistry and human exposure. It can enter aquifers naturally as minerals weather, or through mining, metal processing, pesticides and industrial waste. Because arsenic is colourless, odourless and usually tasteless at environmentally relevant concentrations, contamination cannot be identified reliably without laboratory testing.

Its toxicity depends strongly on chemical form. In groundwater, inorganic arsenite, As(III), and arsenate, As(V), are the most important species. Organic arsenic compounds may occur in some environments, but inorganic forms generally create the greatest long-term concern for drinking-water safety.

The issue has direct relevance in Australia, where households, farms and small communities may rely on private bores. The Great Artesian Basin supports communities and agriculture across Queensland, New South Wales, South Australia and the Northern Territory, while groundwater is also important in Western Australia. Water quality can vary substantially between neighbouring bores, so a broad regional assumption is not a substitute for site-specific analysis.

Where arsenic comes from

Arsenic is a naturally occurring element found in minerals containing iron, sulphur and other metals. Groundwater may dissolve arsenic when it moves through sediments or rocks under particular pH and redox conditions. Reductive dissolution of iron oxides can release arsenic that was previously attached to mineral surfaces, while oxidation of sulphide minerals can also mobilise it.

Human activities can intensify this process. Historical mining districts, including parts of Victoria and New South Wales, may contain arsenic-bearing mine wastes or tailings. Agricultural chemicals containing arsenic are largely legacy concerns in Australia, but old contaminated sites can still affect soil and groundwater. A private bore near a former mining area should therefore be assessed more carefully than its clear appearance might suggest.

Chemical speciation controls toxicity

Arsenite, As(III), is often more mobile and more toxic than arsenate, As(V). It commonly occurs in reducing groundwater, where oxygen is scarce. Arsenate is more prevalent in oxygen-rich conditions and behaves chemically like phosphate. Both species can be absorbed by the body, but As(III) binds strongly to sulphydryl groups in proteins and can disrupt enzymes involved in cellular energy production.

Speciation also controls treatment performance. Iron and aluminium oxides generally adsorb arsenate more effectively than arsenite. Oxidising As(III) to As(V) with chlorine, permanganate, ozone or other suitable oxidants can therefore improve removal. However, treatment design should account for pH, competing ions, iron concentration, phosphate, silicate and organic matter rather than relying on oxidation alone.

How exposure affects health

Long-term ingestion of inorganic arsenic is associated with skin changes, peripheral neuropathy, cardiovascular effects and increased risks of cancers affecting the skin, bladder and lungs. Repeated exposure may also affect child development and pregnancy outcomes. Health risk depends on concentration, duration, body weight, nutrition and the route of exposure.

Drinking contaminated water is usually the main concern, although cooking with that water can increase dietary exposure when water is absorbed into food. In Australia, the Australian Drinking Water Guidelines set a health-based value of 0.01 milligrams per litre, equivalent to 10 micrograms per litre, for arsenic. Private bore owners do not always receive routine monitoring from a water utility, so responsibility for testing can sit with the household, farm or facility.

Testing groundwater properly

A reliable investigation begins with a representative sample collected in a laboratory-approved container and handled according to the laboratory’s instructions. Samples should be preserved promptly because changes in oxidation state, suspended solids and microbial activity can alter measured concentrations. Total arsenic testing is useful for screening, while speciation analysis is valuable when treatment selection depends on the ratio of As(III) to As(V).

Testing should include pH, oxidation-reduction potential, dissolved oxygen, iron, manganese, phosphate, silicate, sulphate, alkalinity and total dissolved solids. These measurements help explain why arsenic is present and whether a treatment system is likely to work. In Australia, selecting a NATA-accredited laboratory is a practical quality-control step, particularly for rural properties in Queensland or Western Australia that depend on a private bore.

Remediation and treatment options

Oxidation followed by adsorption is widely used for drinking-water treatment. Granular ferric hydroxide, activated alumina and iron-coated media can capture arsenate, while coagulation with ferric salts can remove arsenic with the resulting floc. These systems require correct pH control, regular media replacement or regeneration, and safe management of arsenic-containing waste.

Reverse osmosis can remove both major inorganic species, although it produces a concentrated reject stream and uses significant pressure and energy. Ion exchange may be effective under suitable water conditions but can be affected by competing anions. Point-of-use systems can protect drinking and cooking water in a single dwelling, whereas whole-house systems are more appropriate when bathing, food preparation or multiple users create wider exposure concerns.

Managing treatment in Australian settings

Treatment should be verified through before-and-after laboratory results, not inferred from equipment labels. A system that reduces arsenic in one bore may perform poorly in another because groundwater chemistry changes with depth, season or pumping rate. Filters also require maintenance; exhausted media can allow breakthrough, and poorly managed waste can transfer arsenic back into soil or surface water.

For small Australian communities, schools, Aboriginal communities and remote stations, a durable design must include operator training, spare parts and a realistic servicing plan. Local water authorities may provide guidance, but private bore owners generally need to organise their own testing and treatment. In areas affected by mining legacies, remediation may require a hydrogeological assessment, monitoring wells and source control rather than a domestic filter alone.

The safest approach combines prevention, monitoring and treatment. Avoid using an untested bore for drinking, cooking or making infant formula when contamination is plausible. Begin with a total arsenic test and basic groundwater chemistry from a NATA-accredited laboratory, then use the results to select an oxidation, adsorption, membrane or combined treatment process.

For a bore in or near a mining district, irrigated farming area or Great Artesian Basin community, collect the first laboratory sample before installing treatment and record the result as the baseline.