#capillary

This graphic from the U.S. Geological Survey labels the various parts of a shallow groundwater system. First we have the land surface where rainwater falls; this area is typically dry unless it is raining or you’re standing in a river. Below the surface there is a layer of soil that is called the vadose zone or the unsaturated zone. Soil minerals hold onto water and stay damp unless the surrounding area is completely dry, but in the vadose zone there isn’t enough water to fill all the space between soil grains. Instead, this zone stays damp by holding onto water that moved through it the last time it rained and by wicking a bit of water upwards from what lies below. The capillary zone is the source of water wicked upwards. In the capillary zone, water actually moves upwards due to electrostatic interactions between the molecules of water and the molecules in the soil, with an assist from surface tension that holds water beads together. Finally, if you dig deep enough, you reach the water table; a level where there is enough water stored that all of the space between mineral grains is filled by water. While it's shown fully blue here, it's still mostly dirt - maybe 80-90%. You might even think of it like a saturated sponge - filled with water in the gaps, but the substrate is held together by the solid part. When it rains, water moves down through the vadose zone until it reaches the water table. If it rains too much, the tiny spaces in the vadose zone can’t carry the water downward quick enough and it starts flowing over land and drains to rivers. If something is spilled on the surface or in the soil, a pollutant or fertilizer, rainwater will eventually carry that pollutant downwards to the water table below. -JBB Image credit: USGS https://wikivisually.com/wiki/Infiltration_(hydrology) https://www.usgs.gov/ecosystems/toxic-substances-hydrology-program