Potable water: choosing an appropriate source of fresh water

The idea

Fresh water is water with low levels of dissolved salts, and it is obtained from sources such as rivers, lakes, reservoirs and groundwater held in aquifers. Choosing a source depends on local availability, the volume of water needed, the distance to the treatment works and the level of contamination. In the UK, rainfall collected in reservoirs and abstracted river water are common choices because they need relatively little treatment. In dry coastal regions, seawater is not fresh water, so desalination by distillation or reverse osmosis may be needed, which uses large amounts of energy. Groundwater from aquifers is often cleaner because soil and rock filter it, but it can still contain dissolved minerals and microbes. A sensible choice balances water quality, reliability of supply, cost and environmental impact, and the chosen source must be treated to make it potable.

What earns the marks

• Identifies fresh water as water containing low concentrations of dissolved salts, distinguishing it from seawater which has a high salt concentration.
• Names suitable sources such as rivers, lakes, reservoirs, groundwater or aquifers, and links the choice to local availability and reliable supply.
• Compares sources using criteria such as volume of water available, distance to the treatment works, level of contamination and energy or cost needed for treatment.
• Explains that groundwater is often less contaminated because it has filtered through soil and rock, but may still contain dissolved minerals or microbes.
• Recognises that seawater is not a fresh water source and that desalination is an energy-intensive alternative used where fresh water is scarce.
• Justifies a chosen source for a given scenario by balancing water quality, reliability, cost and environmental impact.

Common mistakes and how to put them right

• Treating seawater as a fresh water source.

  Put it right by stating that seawater has a high concentration of dissolved salts and requires desalination, which is expensive in energy.

• Assuming all fresh water is automatically safe to drink.

  Put it right by explaining that even clean-looking river or groundwater must be treated to remove microbes and dissolved substances before it is potable.

• Choosing a source only because it is nearby.

  Put it right: Also considering volume, contamination and treatment cost, since a close but polluted source may be unsuitable.

Exam tips

• Read the scenario carefully and name a specific source, such as a reservoir or an aquifer, rather than writing only 'water from the ground'.
• When comparing sources, use comparative language such as 'less contaminated' or 'requires more energy to treat' to show reasoning.
• Link the choice of source to the later treatment steps, for example stating that clearer groundwater may need less filtration than muddy river water.