Seep well beside a lake or river (bank filtration)

If you dig a hole beside a body of water that goes deeper than its water level, the water pushes in through the ground from the sides and below. On this short path, larger particles such as mud, algae and part of the microorganisms get caught between the grains of sand and gravel. The result is water that is much clearer than the open water.

Large waterworks also use this principle, under the name bank filtration. There, however, the water often flows through the ground over long distances, and proper treatment still follows. A seepage path of a few metres achieves correspondingly much less.

Bacteria, viruses and dissolved substances such as chemicals or salt can pass through the ground over a short distance. That is why the seep well is pre-clarification – comparable to letting water settle and filtering through cloth – and not disinfection.

Choose a level, sandy or gravelly spot a few paces from the bank. Sandy ground lets water through well; in dense loam or clay hardly anything seeps in. Keep away from animal trails, watering places and above all latrines, so that no faeces get into your hole.

Dig until water seeps in from below or the sides. The bottom of the pit should be lower than the water level of the pond or river. The first water is usually cloudy because you stirred up soil while digging. Scoop it out and let the pit refill. Repeat until the water runs in clearer.

Then scoop carefully without stirring up the bottom and pour the water through a cloth into a raw-water container. If you use the pit for longer, you can line the edge with stones or gravel. That stops soil sliding in and clouding the water again.

Clear seep water is not ready to drink. Boil it, or use a suitable filter followed by chemical disinfection. The seep well is still very useful here: clearer water clogs filters less and makes chemical disinfection more effective.

With an oil film, foam, chemical smell, dead fish, or industry, intensive farming or sewage nearby, bank filtration does not help either. Dissolved chemicals travel through the ground with the water. Find another source in that case. Seawater or brackish water also stays salty, however far it has seeped.

Do not dig at undercut or steep bank edges. Such edges can break off, and you can fall into the water. Also avoid flood zones, because the water there is almost always heavily contaminated.

A seep well is especially worthwhile with muddy or algae-rich ponds and pools and with cloudy rivers in which a filter would clog quickly. It spares filters and chemicals and needs hardly any equipment – in an emergency a stick, a flat stone or a pot lid is enough for digging.

It makes less sense when clear water is directly available or the ground is too hard to dig. Then it only costs time and energy. In such cases it is enough to let water settle and pre-filter it through a cloth before you disinfect it.

Try a seep well somewhere digging is allowed, for example at a campsite by a pond with a sandy bank. For comparison, fill one glass with pond water and one with water from your pit. You will see the difference in clarity – and remember that you still cannot see germs.

Notice how long it takes for the pit to fill and run clear. That gives you a feel for how much time to plan in a real situation. Afterwards fill the pit in again and leave the site as you found it.