Groundwater and Karst

In the hydrologic cycle

precipitation = runoff + evapotranspiration + infiltration

which means that the water that falls to the ground in the form of rain, snow, etc will either soak into the groundwater, runoff into surface streams, or be evaporated from the surface or transpired through plant leaves. In the U.S. the average precipitation is on the order of 30 inches per year; of which 8.9" runs off into streams, 0.1" infiltrates the ground, and 21" is returned to the atmosphere via evapotranspiration.

The water that infiltrates the ground will percolate (seep) downward through porous and permeable soil, sediment, and rock until it reaches an impermeable unit. Porous means having void spaces between grains, permeable means the voids are connected so water can pass through. Porous and permeable materials include soil (if not too clay rich), sand, sandstone, limestone, fractured igneous and metamorphic rock, vesicular basalt and scoria. Impermeable and/or non-porous materials include clay, shale, non-fractured igneous and metamorphic rocks

Porous/permeable layers are called aquifers. Impermeable layers called aquicludes. In an unconfined aquifer the zone of saturation (all voids filled with water) lies above an aquiclude. The top of the zone of saturation is called the water table. Above this is the zone of aeration (voids filled with air, though grains may be wet - coated with water).

Water table elevations generally follow the topography (lay of the land). Water tables are high where the land is high and low where the land is low. High areas are recharge zones where water is added but not removed from the groundwater. Water is removed from the ground in low-lying discharge areas, such as lakes, streams, and springs. Groundwater flows from areas with a higher water table surface to areas with a lower water table.


Moisture condenses in the atmosphere to form water droplets that may eventually fall to the ground as rain. CO2 from the air dissolves in the water. Some of the dissolved CO2 reacts with the water to form the weak acid carbonic acid (H2CO3). Carbonic acid is a principal agent of chemical weathering. [See Weathering]

This mixture of carbonic acid in water makes most natural surface waters slightly acidic. As slightly acidic water infiltrates the ground to become part of the groundwater system it causes weathering of the rocks. The abundant silicate minerals are chemically weathered into clay minerals and soluble byproducts. A few common minerals dissolve into soluble weathering products that are removed with the slowly flowing groundwater, leaving little solid residue.

Evaporite minerals such as halite (NaCl) and gypsum (CaSO4·2H2O) are quite soluble. But the most common soluble minerals are calcite (CaCO3), the principal mineral in the rocks limestone and marble, and the related mineral dolomite (Ca Mg(CO3)2), the principal mineral in the rock dolostone (also simply called dolomite). These carbonate rocks are abundant in the upper crust in many areas. They are susceptible to dissolution.

Where slightly acidic surface waters seep down to limestone/dolomite rock strata the rock slowly dissolves away. The water is best aerated and acidified near the top of the aquifer near the water table. Most dissolution of carbonate rocks occurs in this interval. But since water tables rise and fall over time due to seasonal changes in rainfall as well as long term climate change, large thicknesses of carbonate rock may be exposed to the corrosive effect of this slightly acidic water.

Dissolution is most effective where there are conduits for the transmission of water. Joint systems make for easy passage of downward-seeping groundwater. Cavern systems typically form along preexisting joint patterns. Eventually caverns may become so large that the roof collapses, filling in the cavern. This may be gradual or sudden. The ground overhead subsides forming a sinkhole or even a karst valley. In areas with well developed karst most streams are short. They become sinking (disappearing) streams. Streams may flow through underground caverns for many miles and rise back to the surface at a lower elevation.

The degree of karstification depends not only on the underlying geology but also on the climate (temperature and rainfall). Limestone regions in hot, wet climates may show extreme cases such as the tower karst of south China and southeast Asia and cockpit karst in Puerto Rico. Limestone regions in cold, dry climates may show little development of karst.

Long Island Groundwater Geology