What is downward seepage (wegzijging)?
Downward seepage, in Dutch “wegzijging”, is the slow downward movement of water from the topsoil, ditches and surface water into deeper soil layers. It is the opposite of upward seepage (kwel): there groundwater rises, here it flows down.
When does it occur?
Water always flows from high to low hydraulic head. Downward seepage occurs where shallow water stands higher than the pressure head in the deeper aquifer, for example:
- in higher-lying areas and on sandy soils;
- in polders that lie higher than their surroundings;
- in dry summers, when rivers are low and the water table drops with them;
- near groundwater abstraction or dewatering.
Effects
- Ditches run dry, because water drains away faster than it is replenished. Water authorities may need to let water in to maintain levels.
- Drying out of nature areas and farmland.
- Timber pile foundations: if the water table drops for long periods, pile heads can dry out and decay.
What it means for house and garden
- Rainwater infiltration works well: a soakaway or rain garden drains water quickly into the ground.
- Be careful with private abstraction such as a garden borehole or dewatering: it can increase seepage locally, and abstraction is often regulated.
- Older home on timber piles: keep an eye on the groundwater level via local monitoring wells, especially after dry summers.
Related terms
- Groundwater seepage
- Groundwater level
- Surface water
- Groundwater monitoring well
- Infiltration
- Stormwater retention
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