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