Unstable Flow in Soils

Percolation

• The rate of percolation is a measure of how quickly water moves through soil. The rate is highest in soils with high concentrations of sand and lowest in soils made primarily of clay. Not only does water move downward in soil, it moves in a lateral fashion as well, passing between particles as it seeks out a lower point. The water absorbed by porous soil such as loam also travels as it exits the particles, but while it was once thought that water moved through the soil in a generally uniform pattern, it now appears that this is not always the case.

Conveyance Properties

• As water moves vertically and horizontally through the soil, it picks up other substances along the way. These hitchhikers could be nutrients that are beneficial to plants, but they could also be pollutants that can potentially contaminate the water supply. Soil particles generally act as a filter, removing most impurities from rainwater. But rainwater, as well as water from a garden hose or spring runoff, sometimes penetrates the soil more aggressively in some areas than others, creating larger pathways, called fingers, that allow the flow to move more rapidly into the aquifer, or groundwater supply. In some cases, the buildup of water pressure within these pathways is high enough to open other exit points for it to pass through.

Flow Paths

• Pathways that lead through the soil to the groundwater table include larger spaces between the particles in soils with a coarse texture, such as sand; worm tunnels, openings created by plant roots, and the markedly larger spaces between chunks of compacted soil or rocks. Water passes quickly through these openings, saturating only some of the soil particles in its path while other nearby soil particles stay dry during the process.

Effects

• Water that moves quickly through isolated pathways from the force of gravity not only has the potential of introducing contaminants into the groundwater supply, it also deprives plants of the nutrients it may be transporting as well as the moisture they need to grow. According to the Journal of Hydrology, unstable flow occurs most often in very dry soil, in areas where hydraulic conductivity, or the speed of water, is increased at lower depths, and in situations where the air becomes compressed ahead of the moving water. For any of these factors to be viable, the soil's known water retention capabilities must be considered for any changes to become apparent.