What Is a Limestone Aquifer?

About Aquifers

• Aquifers are an important part of the water cycle. Rainwater that falls on the ground is used in several ways. It may evaporate, be taken up by plants, run off into streams or be absorbed into the soil and into groundwater supplies. The water that moves underground is called recharge. Rocks or sediments that store and move the water underground in amounts that are sufficient enough to be useful are called aquifers. Recharge water within an aquifer is stored in pores, or spaces within the rock and sediment. These pores range in size from tiny microscopic spaces between mineral grains to caves. Most of the pores within rock that sits below the water table are filled with water. Aquifers range in permeability, or the property that lets water move through the rock. Limestone aquifers vary in permeability, depending on their geologic history.

Limestone

• Limestone is any rock formed primarily of calcium carbonate and is considered to be one of several types of carbonate rocks composed of more than 50 percent carbonate minerals, such as calcite or dolomite or both. Most limestone is formed from seawater deposits and is constantly forming along coral reefs and shallow bodies of water. Seawater contains calcium and bicarbonate at high levels, both necessary for limestone formation.

Water Flow in a Limestone Aquifer

• Water in a limestone aquifer is typically moving under pressure. It enters the aquifer at one area, called the recharge zone, and leaves through another called the discharge zone. The discharge may be in the form of springs or wells, or it may be in areas along a river or coast. The path between these two areas might be very short or can stretch thousands of feet before the water comes to the surface. The flow and amount of water within a limestone aquifer can vary greatly,depending on its porosity. Some aquifers, called confining units, produce almost no usable water, while others produce large quantities. When wells are drilled in confined limestone aquifers, the resulting pressure causes the water to rise. This water is then pumped from the wells to use for irrigation, city and rural water supplies and for industrial purposes such as mining. The groundwater within the aquifer reacts with minerals in the limestone. The calcite within the limestone is what gives the taste and properties of hard water.

Potential Problems

• If water removed from an aquifer is greater than the water flowing into the recharge zone, the fresh water spring can dry up. As a result, salt can move into a limestone aquifer once used for freshwater. When there are large cavities within a limestone aquifer not filled with water, it may also result in the ground sinking, which can lead to surface water that often contains contaminants seeping into groundwater supplies. Other potential problems include spilling or mishandling of materials, such as chemicals near an aquifer, that contaminate the water within the aquifer, making the water unusable.