So one of our readers asked us this question the other day: How is water ‘purified’ naturally (where it is safe to drink)?
The different substances that make up the layers of the ground have different shapes, and are differently malleable, so they pack together in different ways. One possible arrangement is described below:
Humus: Very loose compaction, made of leaves and other organic material. Water flows through quickly, removing only the largest of impurities.
Topsoil: Still fairly loose, broken up by fewer deep reaching roots, etc. water does not flow quite as quickly through, and slightly larger impurities are left behind.
Eluviation Layer: Mostly sand and silt. This layer packs together better, causing water to flow through more slowly and catching smaller impurities as it passes through smaller gaps.
Subsoil: Clay and minerals here which pack together very tightly. Water flows through the tiny cracks very slowly, catching insanely small impurities. This is where your water also picks up most of the minerals that are found in water (iron, calcium, etc.)
Regolith: Sits directly above the bedrock and is mostly pieces of broken bedrock mixed with whatever is composing the layer above. At this point, the water has mostly reached its maximum filtering. Most wells draw from water in this layer.
Bedrock: Solid rock which is (mostly) impermeable to water (excepting cracks, fissures, etc.), allowing pockets of water to sit and wait to be tapped for wells.
This is ONE simple example as there are tons of possibilities for how the strata are arranged, of what they are composed, etc. however, you should be able to see how the water percolates down through different layers, just like in a water filter, to remove different size/types of impurities in the different layers.