Where does drinking water come from?

Almost all drinking water starts as rain or snow. Where it goes next decides what is in it.

• Surface water is water in rivers, lakes and reservoirs. It is easy to collect and there is a lot of it, but it is open to the outside world. Sediment, leaves, animal droppings, farm runoff and spills can all reach it, so surface water almost always needs full treatment.
• Groundwater is water that has soaked down through soil and rock and fills the tiny gaps in a layer called an aquifer. People reach it with wells and boreholes. The soil acts as a slow natural filter, so groundwater is often clearer and has fewer germs than a river, but it can pick up things from the rock it flows through (naturally occurring arsenic or fluoride in some regions) and it can be polluted from above (for example by nitrate from fertiliser or leaking tanks).

Both kinds can be perfectly good sources, and both can be unsafe. The only way to know is to measure.

What can be wrong with water? Water scientists sort pollutants into four families:

• Particles: soil, silt and clay make water cloudy. Cloudiness is called turbidity. It is not always harmful in itself, but particles can hide germs from disinfectants.
• Microbes: bacteria, viruses and tiny parasites from human or animal waste. These cause the diseases people most often link to unsafe water, such as cholera and typhoid.
• Chemicals: dissolved substances such as nitrate, arsenic, lead picked up from old pipes, or industrial solvents. Most are invisible and tasteless.
• Nutrients: nitrogen and phosphorus compounds that feed algae. They matter mostly for the lake itself (World 2), but they also make treatment harder.

Safe drinking water is a matter of degree, not a yes or no. Health agencies such as the World Health Organization (WHO) publish guideline values: the highest level of each substance that is considered safe for lifelong drinking. Water suppliers test against these numbers again and again.

Why it matters. According to the WHO and UNICEF Joint Monitoring Programme, about 2.2 billion people (figure for 2022) still did not have a safely managed drinking-water service, meaning water on the premises, available when needed and free from contamination. Treatment plants, protected wells, testing and good pipes are the tools that close that gap, and the rest of this world follows water from the source to the tap.

In the Water Test Kit you will meet three sample jars, labelled A, B and C. They are teaching samples with made-up numbers, not real measurements. For now, just look at what the strips can read. Your first job as a Planet Fixer is simply to notice that the three jars all look like clear water.