The Chemistry of Groundwater: Hardness, TDS, and Alkalinity Explained

Groundwater can look perfectly clear while carrying a complex mixture of dissolved minerals. Calcium, magnesium, bicarbonate, chloride, sulphate, sodium and many other ions enter water as it moves through soil, fractured rock and aquifer sediments. Understanding these ingredients helps explain why one bore supplies pleasant drinking water while another leaves scale on a kettle or tastes noticeably salty.

For Australian households, farmers, water utilities and environmental managers, three measurements are especially useful: hardness, total dissolved solids (TDS) and alkalinity. They describe different chemical properties, yet they influence each other and often appear together in groundwater reports. Interpreting them correctly supports safer bore use, better treatment decisions and more sustainable aquifer management.

What Hardness Reveals About Bore Water

Hardness mainly comes from dissolved calcium and magnesium. Water becomes harder when it passes through limestone, dolomite, calcareous soils or other mineral-rich formations. The result is familiar to anyone using bore water around Perth, regional Queensland or the Murray-Darling Basin: soap produces less lather, white scale forms on taps and hot-water systems need more frequent cleaning.

Laboratories usually report hardness as milligrams per litre (mg/L) of calcium carbonate equivalent, written as mg/L as CaCO₃. A broad guide classifies water below about 60 mg/L as soft, 60–120 mg/L as moderately hard, 120–180 mg/L as hard and above 180 mg/L as very hard. These are practical categories rather than health limits. Hard water is generally not considered a health risk and supplies useful calcium and magnesium, although it can affect plumbing, appliances and taste.

Temporary hardness is associated with bicarbonate and can be reduced when water is heated, because calcium carbonate may precipitate as scale. Permanent hardness is linked more closely with sulphate, chloride or nitrate and remains after boiling. Water softeners, reverse osmosis and some central treatment systems can reduce hardness, but the best option depends on the complete chemical profile and the intended use.

TDS Measures The Overall Mineral Load

Total dissolved solids represent the combined concentration of dissolved substances that pass through a very fine filter. The measurement may include calcium, magnesium, sodium, potassium, bicarbonate, carbonate, chloride, sulphate, silica and small amounts of other compounds. TDS is commonly reported in mg/L or parts per million, which are approximately equivalent for ordinary water.

A low TDS result does not automatically mean that water is safe, and a high result does not identify the contaminant responsible. TDS is a broad indicator of mineralisation and salinity, not a substitute for testing bacteria, nitrate, arsenic, fluoride, pesticides or other health-related substances. Conductivity, often measured in microsiemens per centimetre, is frequently used to estimate TDS, but the conversion factor varies with the water’s ionic composition.

Australian conditions make this distinction important. Groundwater in the Gnangara system near Perth may experience increasing salinity where groundwater extraction, reduced rainfall and land-use pressures alter recharge. In the Great Artesian Basin, water can have a long residence time and a distinctive mineral signature. Near coastal towns, excessive pumping may draw seawater into an aquifer. A “salty bore” therefore requires investigation rather than a decision based on TDS alone.

Rainfall, geology and recharge also shape dissolved-solids levels. Short, intense rain may run off rather than replenish groundwater, while managed recharge or infiltration through sandy soils can introduce relatively dilute water. Guidance on rainwater harvesting techniques is relevant when designing supplementary supplies, because the interaction between collected rainwater and local aquifer conditions affects both water quality and long-term recharge.

Alkalinity Controls Resistance To Acidity

Alkalinity is the capacity of water to neutralise acids. In most groundwater, bicarbonate is the main contributor, with carbonate and hydroxide playing smaller roles depending on pH. Some waters also contain alkalinity from borate, phosphate, silicate or organic compounds. The result is usually reported as mg/L as CaCO₃, the same reference unit often used for hardness.

Alkalinity is different from pH. pH describes how acidic or alkaline water is at a particular moment, whereas alkalinity describes how strongly the water resists a fall in pH. A sample can have a near-neutral pH but low alkalinity, meaning a small acid input changes its pH quickly. Another sample can have a similar pH and high alkalinity, giving it greater buffering capacity.

Carbon dioxide in soil and groundwater reacts with minerals to form bicarbonate. In limestone terrain, this process can create groundwater with substantial alkalinity and hardness because calcium or magnesium dissolves alongside bicarbonate. That relationship explains why hard groundwater often has moderate to high alkalinity, though the two values need not match. Sodium-rich water, for example, may have high alkalinity without equivalent hardness.

How The Three Measurements Interact

Hardness and alkalinity often travel together because calcium carbonate dissolves into groundwater as calcium and bicarbonate. When water is heated or its carbon dioxide balance changes, calcium carbonate can fall out of solution and form scale. This is why a bore-water sample with high hardness, high alkalinity and a high pH may cause problems in hot-water cylinders, irrigation equipment and reverse-osmosis pre-treatment.

TDS includes the ions responsible for hardness and alkalinity, but it also includes many others. A water sample can therefore have high TDS and low hardness if sodium and chloride dominate. Conversely, groundwater can be hard while its total mineral content remains moderate. These distinctions matter in Australia, where irrigation managers may be more concerned with sodium adsorption, chloride toxicity or salinity than with bathroom scale.

Treatment should follow a full laboratory report rather than a single number. Softening can reduce calcium and magnesium but may add sodium. Reverse osmosis can lower TDS substantially, yet it produces a concentrated reject stream and may need pre-treatment to control scaling. Aeration can remove some dissolved gases and improve taste, while acid dosing changes alkalinity and pH. A qualified water professional can match treatment to drinking, household, stock, irrigation or industrial requirements.

Groundwater chemistry can also reveal aquifer processes. Trends in alkalinity, hardness and TDS may indicate changing recharge, mixing between aquifers, mineral dissolution, evaporation or seawater intrusion. The Central Ground Water Board’s aquifer mapping work demonstrates how chemistry becomes more useful when combined with geology, water levels, land use and pumping information. Studies of alluvial aquifers near Delhi show why the wider setting matters when interpreting a groundwater resource.

Using Chemistry In Sustainable Water Planning

A single bore sample is only a snapshot. Results can vary between wet and dry seasons, across different pumping rates and between shallow and deep screened intervals. Samples should be collected in clean containers, labelled with location and date, and tested by a laboratory using appropriate methods. For household supplies, testing should cover microbiological quality as well as chemical indicators; clear water is not proof of safety.

Australian users should compare results with the Australian Drinking Water Guidelines for drinking supplies and with advice from the relevant state or territory regulator. Irrigation assessment may require electrical conductivity, sodium, chloride, boron and sodium adsorption ratio, while livestock supplies may need attention to salinity, sulphate, nitrate and particular trace elements. A local council, water authority or accredited laboratory can help interpret results for a specific property.

At aquifer scale, chemistry supports decisions about allocation, recharge protection and monitoring. Protecting recharge areas from contamination, limiting excessive pumping and tracking water-level and quality trends can prevent a minor change becoming a costly salinity or contamination problem. Rainwater tanks, recycled water and managed aquifer recharge may supplement supplies, but each option needs careful assessment of geology, pathogen risks and chemical compatibility.

For Australian landholders, the practical message is straightforward: hardness helps explain scale, TDS indicates the total mineral load, and alkalinity shows buffering capacity. None of the three provides a complete health assessment on its own. Read them together, alongside pH, conductivity, major ions and targeted contaminant tests, to build a reliable picture of groundwater quality.

Use the NAQUIM portal to explore aquifer maps, groundwater studies, water-quality resources and management information from the Central Ground Water Board. Comparing its mapped approach with local Australian monitoring can strengthen bore-water decisions, guide conversations with laboratories and support long-term planning for reliable groundwater supplies.