Groundwater Formation

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Groundwater Formation

Groundwater is fresh water in the rock and soil layers beneath Earth's land surface. Some of the precipitation (rain, snow, sleet, and hail) that falls on the land soaks into Earth's surface and becomes groundwater. Water-bearing rock layers called aquifers are saturated (soaked) with groundwater that moves, often very slowly, through small openings and spaces. This groundwater then returns to lakes, streams, and marshes (wet, low-lying land with grassy plants) on the land surface via springs and seeps (small springs or pools where groundwater slowly oozes to the surface).

Groundwater makes up more than one-fifth (22%) of Earth's total fresh water supply, and it plays a number of critical hydrological (water-related), geological and biological roles on the continents. Soil and rock layers in groundwater recharge zones (a entry point where water enters an aquifer) reduce flooding by absorbing excess runoff after heavy rains and spring snowmelts. Aquifers store water through dry seasons and dry weather, and groundwater flow carries water beneath arid (dry) deserts and semi-arid grasslands. Groundwater discharge replenishes streams, lakes, and wetlands on the land surface and is especially important in arid regions that receive limited rainfall. Flowing groundwater interacts with rocks and minerals in aquifers, and carries dissolved rock-building chemicals and biological nutrients. Vibrant communities of plants and animals (ecosystems) live in and around groundwater springs and seeps.

Almost all of the fresh liquid water that is readily available for human use comes from underground. (The bulk of Earth's fresh water is frozen in ice in the North and South Pole regions. Water in streams, rivers, lakes, wetlands, the atmosphere, and within living organisms makes up only a tiny portion of Earth's fresh water.) For thousands of years, humans have used groundwater from springs and shallow wells to fill drinking water reservoirs, and water livestock and crops. Today, human water needs far exceed surface water supplies in many regions, and Earth's rapidly-growing human population relies heavily upon groundwater to meet its ever larger demand for clean, fresh water.

Aquifers: fresh water underground

An aquifer is a body of rock or soil that yields water for human use. Most aquifers are water-saturated layers of rock or loose sediment. With the exception of a few aquifers that have water-filled caves within them, aquifers are not underground lakes or holding tanks, but rather rock "sponges" that hold groundwater in tiny cracks, cavities, and pores (tiny openings in which a liquid can pass) between mineral grains (rocks are made of minerals). The total amount of empty pore space in the rock material, called its porosity, determines the amount of groundwater the aquifer can hold. Materials like sand and gravel have high porosity, meaning that they can absorb a high amount of water. Rocks like granite, marble, and limestone have low porosity, and make poor groundwater reservoirs.

Karst and the Edwards Aquifer

The Edwards Aquifer is a groundwater reservoir made of limestone rock that today provides water to nearly 2 million people in 10 central Texas counties. Clear, cool, clean water flows from natural springs and shallow wells along the Balcones Fault zone (a fracture in the crust of the Earth along which rocks on one side have moved relative to those on the other side) that runs through the cities of San Antonio, Austin, and Waco. Diverse communities of plants and animals, including humans, have thrived in the Edwards aquifer discharge zone for tens of thousands of years. Native American tribes including Comanche, Apache, and Tonkawa had been living beside the spring-fed pools of the Edwards, drinking cool water and hunting plentiful game, for more than 12,000 years before the arrival of Spanish explorers and European settlers.

The rock layers that make up the Edwards aquifer are filled with a honeycomb of caves, cavities, and conduits that were created by the chemical reaction between water and limestone. Rainwater dissolves limestone. Each slightly acidic raindrop that falls in the recharge zone of the Edwards aquifer dissolves a little bit of limestone. Over geologic time, the limestone has dissolved has and carved the "plumbing" of the Edwards aquifer. The landscape and geologic features created by the dissolving of the limestone—sinkholes, disappearing and reappearing streams, caves, and caverns—are called karst.

Today, millions of humans share groundwater from the Edwards aquifer with its native biological users. Overuse and pollution are threatening the quantity and quality of groundwater flowing from the Edwards. Although karst aquifers like the Edwards are relatively fast flowing, an average water molecule still spends about 200 years traveling through the aquifer. (Some of the water flowing from Barton Springs in downtown Austin probably entered the aquifer around the time of the American Revolution!) Human activities that prevent water from entering the aquifer, like installing pavement in the recharge zone, or that remove water faster than it enters, like pumping large quantities of water for crops, can lower the water level in the aquifer, and cause springs and wells to go dry. When pollutants like agricultural runoff or industrial chemicals make their way into the groundwater system, they emerge, almost unfiltered, in springs and wells in the discharge zone. Changes in the quality and quantity of the groundwater in discharge pools are threatening a number of species of salamanders, fish, and insects. Environmentalists, developers, and government officials in central Texas are working to find solutions that both protect the Edwards, its ecosystems, and the plentiful high-quality water it supplies the rapidly-growing human population of central Texas.

Aquifers must have high permeability in addition to high porosity. Permeability is the ability of the rock or other material to allow water to pass through it. The pore space in permeable materials is interconnected throughout the rock or sediment, allowing groundwater to move freely through it. Some high-porosity materials, like mud and clay, have very low permeability. They soak up and hold water, but don't release it easily to wells or other groundwater discharge points, so they are not good aquifer materials. Sandstone, limestone, fractured granite, glacial sediment, loose sand, and gravel are examples of materials that make good aquifers.

Water enters aquifers by seeping into the land surface at entry points called recharge zones and leaves at exit points called discharge zones. (Some aquifers discharge into the ocean.) Influent or " water-losing" streams, ponds, or lakes are bodies of surface water in recharge zones that contribute groundwater from their water supply. Groundwater flows into effluent or "water-gaining" streams and ponds in discharge zones.

For the water level in an aquifer to remain constant, the amount of water entering at recharge zones must equal the amount leaving at discharge zones. (Imagine a bucket punched with holes under a dripping faucet. If water drips in at the same rate that it drips out, the water level stays the same.) If water discharges or is pumped from an aquifer more quickly than it recharges, the groundwater level (water table) will fall. The time an average water molecule spends within an aquifer is called its residence time. Water in some fast-flowing aquifers spends only a few days underground, while other rock layers can hold water for ten thousand years. Average aquifers have residence times of about two hundred years.

The water table and unconfined aquifers Water enters aquifers by moving slowly down through a layer of surface rocks and soil whose pore spaces are partially filled with air (zone of infiltration). The water continues moving downwards until it reaches a level where all the pore spaces are completely filled with water (zone of saturation). The top of the zone of saturation is called the water table. In some wet, lowland regions, southern Florida for example, the water may be only a few feet (meters) below the surface. In others, like the American Southwest, water-saturated rocks may be hundreds of feet below the land surface.

Groundwater reservoirs that have uniform rock or soil properties (porosity and permeability) throughout are called unconfined aquifers. The water table forms the upper surface of an unconfined aquifer. The shape of the water table in an unconfined aquifer mirrors the shape of the land surface, but its slopes are gentler. In temperate (moderate) climates that receive moderate amounts of groundwater-replenishing rainfall, water infiltrates into unconfined aquifers in hilltop recharge zones and discharges into effluent streams and ponds in low areas where the water table intersects the land surface. Water will only rise to the level of the water table in a well, so a pump or bucket is required to extract water from an unconfined aquifer.

Confined aquifers and artesian flow Confined aquifers are pressurized groundwater reservoirs that lie beneath layers of non-permeable rock (granite, shale) or sediment (clay). Groundwater enters a confined aquifer in recharge zones beyond the uphill edges of the confining layer and discharges beyond the downhill edges. Groundwater trapped beneath an impermeable barrier cannot rise to the height of the water table, so pressure builds up in confined aquifers. Artesian wells are wells drilled in confined aquifers where the pressure is great enough to make water flow at the surface.

Laurie Duncan, Ph.D., andMarcy Davis, M.S.