What Kind of Soil Filters Water Better?

Particle Size

• Emergency water filters constructed from natural materials use coarse particles such as gravel for the first rough screen. A layer of clean sand beneath the gravel filters out finer contaminants. Smaller particles in the sandy soil give better filtration but many pollutants still pass through soils such as sandy loam. Sandy soils form the coarsest soil class, followed by silt and clay. Sand particles range from 1.999 mm to .050 mm across. Silt measures between .049 mm and .002 mm; and particles less than .002 mm form clay, according to the Portage County, Wisconsin, government website.

Porosity

• To filter water efficiently, soils must allow water to move steadily through spaces between the soil grains. Porosity depends on how particles fit together as well as on the particular blend of particles in soil. Clean sand measures high in porosity, but clay mixed with sand fills the spaces between grains and lowers porosity. Even pure clay soil could have high porosity if the soil structure remains open. Particle shapes affect porosity by allowing soil to pack together tightly. If clay contains mainly rounded particles, clay's porosity increases, but if clay particles resemble shards the amount of porous space drops and water flow decreases.

Trapped Water

• Small particle size both enhances the filtration ability of soil and decreases the permeability of soil. Water forms a film on the surface of the individual particles in soil. Because the total surface area of the particles in clay soil exceeds that of sandy soil, clay soil holds more water in place, preventing large amounts of water from recharging deep aquifer layers. Clay with irregular particle shapes also compacts tightly, forming a water-saturated layer that blocks all water flow. Clay composed of rounded particles filters water well, remaining porous and bonding with many contaminants that flow through coarser soil layers.

Soil Layers

• The most efficient soil type for water filtration includes several layers with different filtration qualities. Sandy loam, silt or sandy clay loam at the surface allows water penetration and filters out the coarse pollution. Subsoil with higher amounts of clay and less organic matter filters out finer particles and traps chemical pollutants. Deep layers of clay could hold contaminants long enough for natural decomposition to reduce toxic effects, according to the University of Missouri Extension. Deep clay subsoils block many pollutants from reaching groundwater, but large volumes of chemicals could overwhelm natural filtration and penetrate the water table.