Geology Lab: Groundwater

Aquifer

A permeable underground rock layer saturated with slowly moving groundwater is an aquifer. The most effective aquifers are deposits of well-sorted and well-rounded sand and gravel. Limestones in which fractures and bedding planes have been enlarged by solution are also good aquifers. Shales, and many igneous and metamorphic rocks, make poor aquifers because they are typically impermeable, unless fractured. Rocks and other materials that prevent the movement of groundwater are aquicludes. A confined aquifer is an aquifer with an aquiclude above.

Interactions between the water table and surface water

Although surface water in lakes and streams are considered a separate reservoir from groundwater resources, there can be significant natural interaction between surface and subsurface water. Gravity and pressure drive the movement of groundwater, additions to the water table are referred to as recharge, subtractions from the water table are referred to as discharge.

The Water Table

As gravity moves water down the surface, it collects and fills all of the available pore spaces. Two zones are defined by whether their pore spaces contain mostly air, the zone of aeration, or mostly water, the underlying zone of saturation; the boundary of the two zones is the water table. The base of the zone of saturation varies, but usually extends to a depth where an impermeable layer is encountered or to a depth where confining pressure closes all open space.

Formation of Caves

As groundwater moves from the surface through the zone of aeration, it slowly dissolves the carbonate rock and enlarges its fractures and bedding planes. Caves that form in the unsaturated zone tend to form vertical pits and incised canyons as water movement is driven by gravity. Upon reaching the water table, the groundwater migrates laterally toward the region's surface streams. As the groundwater moves through the zone of saturation, it continues to dissolve the rock and gradually forms a system of elliptical horizontal passageways driven by fluid pressure. Caves at the water table tend to be horizontally widened as fluid velocity switches from gravitational to pressure driven.

Caves

As groundwater percolates through carbonate rocks, it dissolves and enlarges fractures and openings to form a complex, interconnecting system of crevices, caves, caverns, and underground streams. A cave is usually defined as a naturally formed, subsurface opening that is generally connected to the surface and is large enough for a person to enter. A cavern is a very large cave or system of interconnected caves.

More formation of caves

As surface streams erode by downcutting into the valleys, the water table drops in response to the lower elevation of the stream. The water that flowed through the system of horizontal passageways now flows to the lower water table where a new system of lateral passageways begins to form. The abandoned channel forms an interconnecting system of caves and caverns. Slightly acidic surface water infiltrates the rock units and dissolves along flow paths. The paths, now filled with water, will one day be a network of cave passages as the water table drops. The water table has lowered in response to stream erosion. Surface erosion has breached the areas formerly filled with water, which are now a network of cave rooms and passages.

Groundwater Movement

As water enters the subsurface from rain, snowmelt, or streams, it continues to percolate downward through soil, sediment, and porous rock until it reaches the zone where water fills all spaces or pores in rock and sediment, the zone of saturation. The top of the zone of saturation is known as the water table, and once water reaches this zone, it continues to flow slowly from areas of higher pressure to areas of lower pressure. Gravity and pressure drive the movement of groundwater; additions to the water table are referred to as recharge, subtractions from the water table are referred to as discharge. Groundwater movement, and its eventual recovery at wells, depends on two critical aspects of the materials that it moves through: porosity and permeability.

Figures on perched water tables and faults

As water infiltrates from the surface, if it meets an impermeable layer, it will travel laterally until it intersects the landscape. This water comes to the surface between bedding planes as springs or seeps. These types of springs are common in east Texas where alternating layers of sandstone and shale are found in the subsurface. Faults are fractures in rock that cut across units. They can act as conduits for groundwater to move between layers and to the surface as a spring. These types of springs are common in Central Texas; the Edwards Aquifer has been extensively faulted ad these faults bring water to the surface in the area.

Groundwater

Groundwater is the third largest reservoir of water on Earth and the largest reservoir of fresh liquid water; therefore it is an important resource for agriculture, industrial, and domestic use. More than 65% of the groundwater used in the United States each year is used for irrigation, with industrial use second, followed by domestic needs. Groundwater supplies 80% of the water used for rural livestock and domestic use, as well as providing 40% of public water supplies. In fact, of the 100 largest cities in the United States, 34 depend solely on local groundwater supplies.

Sinkholes

In karst regions, the ground surface may be pitted with numerous depressions that vary in size and shape, called sinkholes. Solution valleys form when sinkholes intercept and capture overland flow.

More Groundwater

In many cases, groundwater takes a long time to accumulate underground and once these resources are depleted, they may not recover quickly or easily. As the world's population and industrial development expand, the demand for water, particularly groundwater, will increase. Not only must new groundwater sources be located but, once found, these sources must be protected from pollution and managed properly to ensure that users do not withdraw more water than can be replenished.

Drought water table figure

In times of drought without adequate inputs to the groundwater system, the elevation of the water table will be the inverse of the landscape. In this scenario, water will migrate from higher pressure recharge areas in the stream valley to a lower pressure environment below. Stream channels that lose water to the groundwater system are called losing streams.

Introduction to Groundwater

Most of Earth's 1.33 billion km^3 of water is in the oceans and nearly all the rest is frozen in glaciers, leaving less than 1% of Earth's water to exist in the atmosphere, lakes, swamps, rivers, and underground. Earth's water moves through these different environments, referred to as reservoirs, as a result of the hydrologic cycle. This cycle is powered by solar radiation as water evaporates from the oceans, condenses into clouds in the atmosphere, and eventually falls as precipitation back to the Earth's surface. Most precipitation falls directly back into the ocean, but approximately 20% will fall on land and can be channeled into streams and rivers, temporarily stored in lakes and swamps, in snowfields and glaciers, or percolate into the subsurface and exist as groundwater.

Two ways sinkholes form

Most sinkholes form in one of two ways: Soluble rock below the soil is dissolved by seeping water, and openings in the rock are enlarged and filled in by the overlying soil. As groundwater continues to dissolve the rock, the soil is eventually removed, leaving shallow depressions with gently sloping sides. Sinkholes also form when a cave's roof collapses, usually producing a steep-sided crater.

Permeability

Porosity determines the amount of groundwater that geologic materials can hold, but it does not guarantee that the water can be extracted. So, in addition to being porous, geologic materials must have the capacity to transmit fluids, known as permeability. Generally speaking, more porous materials are also permeable, but the pores must be connected and large enough for the water to flow through rather than cling to the pore surface. Pressure also plays a role; higher pressure creates faster water flow through permeable material. In layered sedimentary rock, water will tend to move more quickly along bedding planes, and more slowly across the layers. Both porosity and permeability are important in groundwater movement and recovery.

Porosity

Porosity is the percentage of a material's total volume that is open space. Porosity most often consists of the spaces between particles in soil, sediment, and sedimentary rocks, but other types of porosity include cracks, fractures, faults, and vesicles in volcanic rocks. Porosity varies among different rock types and is dependent on the size, shape, and arrangement of the geologic material. Igneous and metamorphic rocks, as well as some sedimentary rocks, have very low porosity because they consist of interlocking crystals. Their porosity can be increased, however, if they have been fractured or weathered by groundwater. This is particularly true for carbonate rocks such as limestone and dolomites as fractures can be enlarged by groundwater moving through these mediums. Detrital sedimentary rocks composed of well-sorted and well-rounded grains can have high porosity because any two grains touch at only a single point, leaving relatively large open spaces between the grains. Poorly sorted sedimentary rocks typically have lower porosity, because smaller grains fill in the spaces between the larger grains, further reducing porosity.

Losing Streams

Precipitation and water from snow melt are the main vehicles for groundwater recharge. Water can also infiltrate into the ground from stream channels if the water table level is below the bottom of the stream channel. This commonly occurs in arid environments, where meteoric inputs occur seasonally. Streams that lose part of their flow to the subsurface are referred to as losing streams.

Danger of Sinkholes

Sinkholes are a serious hazard, particularly in populated areas. In regions prone to sinkhole formation, extensive geologic and hydrogeologic investigation must be performed to determine the depth and extent of the underlying cave systems prior to any site development. Sinkholes are shown on topographic maps as depressions with hachure marks pointing towards the center of the feature. The blue area in the center of the sinkhole represents the water table. The number 51 is the elevation of the water table.

Sinkholes, Solution valleys, and Caves

Solutional erosion below the surface creates distinct features at the surface, referred to as karst topography. Karst develops largely by groundwater erosion in areas underlain by soluble rocks and is characterized by solution features such as sinkholes, solution valleys, and caves.

Perched Water Table

Springs can also develop wherever a perched water table intersects the surface. An impermeable lens of rock within a larger unit, such as a shale lens within a larger sandstone unit, can hold water above the main water table; hence this small zone of saturation is "perched" above the main water table.

Faults

Springs form when water infiltrating from the surface meets an impermeable layer and travels laterally along bedding planes. If this bedding plane intersects the landscape, a spring will develop. Geologic structures such as faults can also act as conduits to deliver water to the surface. The Balcones Fault system in Central Texas and the springs associated with it were influential in the early settlement of San Antonio, New Braunfels, San Marcos, and Austin.

Water on Earth

The approximate percentage of Earth's water existing in various reservoirs. The average residence time of a water molecule existing in these locations ranges from days to thousands of years, although special circumstances can cause water to move more rapidly or slowly through these reservoirs. Oceans have 1,338,000,000 volume, 96.50% of total, 4000 years is the average residence time. Ice caps and glaciers contain 24,364,000 volume, 1.76% of total, and an average residence time of 1,000 to 10,000 years. Groundwater contains 23,400,000 volume, .76% of total, and average residence time from 2 weeks to 10,000 years. Freshwater and saline lakes and inland seas have 230,325 volume, .0170% of total, and an average residence time of 10 years. Atmosphere at sea level has 12,982 volume, .001% of total, and average residence time of 1-2 weeks. Stream Channels have 2,120 volume, .0002% of total, and an average residence time of 2 weeks.

Configuration of water table

The configuration of the water table is usually a subdued replica of the overlying land surface; it rises beneath hills and has its lowest elevations beneath valleys. Several factors contribute to the fluctuation of a region's water table, including regional differences in amount of rainfall, permeability, and rate of groundwater movement. During periods of high rainfall, groundwater tends to rise beneath hills because it cannot flow fast enough into adjacent valleys to maintain a level surface. During droughts, the water table falls and tends to flatten out because it is not being replenished. In arid and semiarid regions, the water table is usually quite flat regardless of the overlying land surface. When there have been adequate inputs to the groundwater system, the elevation of the water table will mimic the landscape. In this scenario, water will migrate from higher pressure recharge areas toward the stream valley. Stream channels that take in water from the groundwater system are called gaining streams.

Porosity values for geologic materials

Unconsolidated Sediment: Percentage Porosity. Soil: 55%, Gravel: 20-40%, Sand: 25-50%, Silt: 35-50%, Clay: 50-70%. Rocks: Percentage Porosity. Sandstone: 5-30%, Shale: 0-10%, Solution activity on carbonate rock: 10-30%, Fractured basalt: 5-40%, Fractured granite: 10%. Porosity values for unconsolidated sediment and consolidated rock. Porosity in solid rock depends on the size, shape, and arrangement of the material. Well-sorted sedimentary rock has higher porosity, but a poorly sorted one has lower porosity. In soluble rock such as limestone, porosity can be increased by dissolution of bedrock by groundwater, and crystalline igneous or metamorphic rocks can exhibit increased porosity values as a result of fracturing.

Karst Topography

When rainwater begins to seep into the ground, it immediately starts to react with the minerals it contacts, weathering them chemically. In an area underlain by soluble rock such as limestone and gypsum, groundwater is the principle agent of erosion and is responsible for the formation of many major features of the landscape. Although limestone is mostly insoluble in pure water, it readily dissolves if a small amount of acid is present. When carbon dioxide in the atmosphere and soil combines with water, it acidifies rainwater and soil water, which eventually makes its way into the groundwater system. When acidic groundwater travels through various fractures and openings in limestone, the water reacts with calcite to dissolve rock by forming soluble calcium bicarbonate, which is carried away in solution.

Springs

Where the water table intersects the landscape, springs deliver groundwater to the surface; in the shallow subsurface, groundwater flows laterally into streams and lakes where it discharges at the surface, contributing to stream flow and surface water. Streams that gain water from groundwater discharge are referred to as gaining streams; as the volume of water in the stream channel increases with inputs from the water table. This commonly happens when the water table rises in response to adequate precipitation, and when surface soil and sediments are saturated.