Finding fresh water on the coast

Salt water is denser and therefore heavier than fresh water. When rain soaks into sandy ground behind the dunes, it can collect as a thin layer above the salt water underground. Experts call this a freshwater lens. Whether one exists at a particular spot and how thick it is depends on rainfall, subsoil and distance from the sea.

Streams carry fresh water from the land to the sea. At their mouth, though, the tide pushes salt water in, so the two mix. This mixed water is called brackish water. Only above the highest tide line is stream water free of sea salt – that still does not make it free of pathogens.

Why seawater is so dangerous: the kidneys have to excrete the salt you take in with urine, and they need water to do so. Seawater contains more salt than the body can get rid of using the same amount of water. Overall it loses water. This also applies to small amounts and to seawater “stretched” with fresh water.

On many coasts, rain is the best source. Rig tarps or sheets at every chance of rain and channel the water into clean containers. The first minutes of rain wash salt, dust and bird droppings off the tarp; you can let this first water run away.

If there are streams, follow them inland beyond the marks of the highest tide. You can recognise these by a line of driftwood and seaweed. Only collect there. Right at the mouth the water can be brackish, even if it looks like stream water.

If neither exists, you can dig in a hollow behind the first row of dunes until water seeps in. Skim off only the top few centimetres, because salt water may lie below. If you dig too deep, you reach salt water. Check the taste for salt with a small sip: if it tastes salty, discard it.

Never drink seawater, not even in small amounts or mixed. Filters, chlorine, UV and boiling do not remove salt. Only distillation or a purpose-built desalination device does. Improvised distillation usually yields little and costs a lot of fuel or time.

Keep an eye on the tides while searching and digging on the beach. Rising water can cut you off on rocks, in coves or on sandbanks, and waves can knock you over. Find out about the local tides if possible and always keep a way back open.

Any fresh water you find is raw water and must be disinfected. If oil, sewage or harbour activity is suspected, find another source, because coastal waters can also be chemically polluted. Seeking shade and moving exertion to cool times of day helps you make the water you have last longer.

Coastal hikes and kayak trips: here the best strategy is to carry enough drinking water and plan refill points in advance. Streams on the map are a clue, but small streams can dry up in summer or reach the coast only as brackish water.

Stranded, or on an island without supply: catching rain has top priority. Prepare tarps and containers before the next shower comes. Explore the land behind the beach for streams or damp hollows during the cool hours, and save energy and sweat in the heat.

Survival manuals describe digging behind the first dune as a way to tap a freshwater lens. Whether fresh water is actually there, however, is never certain. On dry coasts or after a long drought the lens may be missing or very thin.

A common myth is that you can “tolerate” seawater in small sips or use it to stretch fresh water. That only makes the water shortage worse. The basic rule remains simple: seawater is not a water source.