Aquifers We know that clean, fresh water is essential for life. Although 75% of the Earth’s surface is covered by water, only 3% of that is fresh water. It just so happens that a large percentage of the world’s fresh water supply is stored below ground in what we call aquifers. An aquifer is an underground, saturated layer of porous rock (usually limestone) or unconsolidated material like gravel or sand. Think of it as a big underground sponge. An aquifer temporarily stores ground water that has percolated down through the soil and rock. You can also think of an aquifer as a big water filter. As groundwater moves through the rock or sand, it is strained of impurities just like your water filter at home.
Floridan Aquifer In Florida, most of the water we consume comes from one of two aquifers. The Floridan Aquifer is one of the world’s most productive, and the largest and oldest aquifer in the southeastern United States. It runs from the southern part of South Carolina to the tip of Florida. It consists mostly of limestone rock. Limestone is a sedimentary rock primarily made up of the skeletal remains of ancient marine organisms such as coral. In some parts of the Floridan Aquifer, the limestone is 3000 feet thick. Think about the vast amount of geologic time it would take for this layer to develop, and what Florida might have looked like before!
Biscayne Aquifer In South Florida, we get most of our drinking water from the Biscayne Aquifer. The Biscayne Aquifer actually sits on top of the Floridan Aquifer in the southern part of the state. It is a shallow aquifer made of highly permeable limestone, sand and shells. Because it is shallow, it is very vulnerable to human activity. Pollutants and agricultural run-off percolate into the Biscayne Aquifer very quickly. Also, there is great concern that humans are drawing water out of the aquifer faster than it can be replaced.
Biscayne Aquifer Because the Biscayne Aquifer actually merges with the floor of Biscayne Bay and the Atlantic Ocean, saltwater can actually percolate into the aquifer to replace the diminishing freshwater. This is called saltwater intrusion, and it is bad news for the freshwater supply. Luckily water is a renewable resource. But problems occur when our demand for water out- paces the ability of the water cycle to replenish our aquifers.
Karst Topography Karst Topography is a term used to describe geological formations caused by the dissolving of soluble rock. Karst Topography is usually characterized by sinkholes at the Earth’s surface and underground caves carved out by groundwater. Typically, the bedrock in a Karst region is limestone. This is because limestone tends to be really soluble (dissolves easily). In central and north Florida, an example of Karst topography is seen in the vast network of sink holes, springs and underground caves and rivers. In Mexico’s Yucatan peninsula, there is a similar landscape which is pock-marked with sinkholes, also called Cenotes. Karst topography occurs over time when lightly acidic groundwater dissolves the surrounding bedrock. As rain, water picks up CO2 in the atmosphere. As groundwater, it picks up even more CO2 from the soil or submerged leaves and other organic material. This dissolved CO2 creates a weak carbonic acid solution which dissolves calcium carbonate - the main ingredient of limestone.
Artesian Springs Florida has over 700 springs. Combined, they release more than 8 billion gallons of freshwater each day, which makes Florida’s spring system the most productive in the world. An artesian spring is constantly discharging water that is under pressure. Sometimes the force of the rushing water is so great the surface of the spring appears to be boiling. This is why the mouth of a spring is often referred to as a spring “boil.” An artesian spring exists because of high subsurface water pressure, also called hydrostatic pressure. There are two factors that contribute to high hydrostatic pressure: First, the aquifer must be confined by an upper layer of low permeability such as clay. Second, the opening of the spring must be at a lower elevation than the water table.