Below A general understanding of water's journey through the ground can and should inform public debate about the policies and management programs to prevent pollution from pesticides and other contaminants. Consider again the sponge analogy. It has many layers, laid out in a complex pattern.
Water is held within a given layer or geologic formation.
While the standard diagram of groundwater flow shows layer-cake formations of several homogenous rock formations, things are rarely so simple in nature. Diagram from Baldwin, Helene L. and C. L. McGuinness, U.S. Geological Survey, A Primer on Ground Water , 1963. Water moves over areas in nine states through the Dakota Sandstone, for example. Aquifers can be found a few feet or many hundreds of feet below the land surface. Vertically they can extend for a few feet or hundreds of feet.