WATER UNDER THE GROUND

Terminology



Porosity - the percentage of open spaces within 
the rock




Permeability - a measure of the 
interconnectedness of the pores.  A measure 
of the capacity of the rock to transmit fluids



The Water Table



The WATER TABLE separates the
ZONE OF AERATION (water plus air in the 
pore spaces) from the
ZONE OF SATURATION (water in the pore 
spaces)





The water table is not a flat surface -- follows 
the topography


AQUIFER - a permeable unit which can 
transport ground water.



If the GWT intersects the surface a spring 
can form 




If the rate of withdrawl exceeds the rate of 
recharge a cone of depression can form 
around the well.




In general, the rate of recharge tends to be 
quite small


Caves



CaCO3 : calcite or aragonite - soluble in weak 
acids (like rain water) and precipitate in weak 
bases (like sea water)



Carbonates precipitate in warm waters 
(thermal springs) and dissolve in cold water 
(deep marine ~4,000 meters)




Any process that contributes CO2 tends to 
favor the precipitation of carbonate - decay 
of vegetation, warming solutions



Caves tend to form in carbonate-rich rocks 
(sedimentary)


Active caves contain ³formations² formed by 
the active precipitation of calcium carbonate 
in various forms and shapes




If the GWT is raised, cave growth may cease.  
If the GWT falls, cave growth may be 
initiated




KARST Topography - abundant sink holes or 
collapse features