Groundwater Geology
by Owen Borville
January 30, 2021
Learning, Geology, Science
Groundwater is water found beneath the Earth's surface where part or all of the pore space in the rock is filled with water. Groundwater is one of the major sources of water for homes and industry. Porosity is the percentage of open spaces or pores in rock or soil. When these spaces are interconnected, water, air, or other fluids can migrate from space to space. Interconnected spaces make the soil or bedrock permeable. Permeability is the ability of a rock or other material to allow water to flow through its interconnected spaces.
The water table is the boundary between the unsaturated zone and the saturated zone below the Earth’s surface. The water table is flat where the land surface is flat. However, where the land contains hills or ridges the water table will also have an irregular surface with high or low parts roughly corresponding to the overlying topography.
The unsaturated zone (also called the aeration zone or vadose zone) is the zone between the land surface and the regional water table. The aeration zone contains pore space partially filled with air and partially filled with water. The aeration zone includes the capillary fringe and may also include localized perched ground water. The saturated zone (phreatic zone) is the underground zone completely saturated or filled with water and is located directly below the water table.
The capillary fringe is a belt layer of variable thickness just above the water table where water is drawn up by capillary action. This layer is saturated with water. The capillary fringe contains porous material just above the water table that holds water by capillarity, which draws water upward into the smaller void spaces.
An aquiclude is a material that tends to exclude the flow of groundwater and has a low permeability, such as clay. An aquifuge is a rock layer that completely blocks the flow of water and has no interconnected pores. An aquitard is a rock layer which slows the flow of water.
A spring is a water resource formed when the side of a hill, a valley bottom, or other excavation intersects a flowing body of ground water at or below the local water table, below which the subsurface material or aquifer is saturated with water and overflows onto the land surface.
An aquifer is a section of the subsurface that contains sufficient saturated permeable rock or material to yield significant quantities of water to wells and springs. An unconfined aquifer is an aquifer that can receive water from the surface and whose water table surface is free to fluctuate up and down, depending on the recharge or discharge rate. There are no overlying confining beds of low permeability to physically isolate the aquifer.
A perched aquifer is a relatively small aquifer located above the water table in the unsaturated zone and rests on top of an impermeable rock layer which prevents the water from reaching the water table.
A confined aquifer is a subsurface layer of rock containing groundwater located below a confining layer, or a layer of rock which blocks or slows down the passage of water. The water inside the confined aquifer can be reached by drilling a well through the confining layer. The rock or strata containing the confined aquifer must connect to the surface at some point or the aquifer cannot be recharged with water. The confined aquifer, also known as an artesian well or aquifer, is overlain by a layer of impermeable rock which causes hydrostatic pressure to build up and pull the water to the surface against gravity. If a hole or well is drilled through the confining layer, water will rise through the well. Water flows from higher pressure to lower pressure and a well or hole drilled through the confining layer will create a lower pressure exit for the water to rise through. The potentiometric surface (also called head) is the level in which groundwater in a confined aquifer rises when a well or hole is constructed to reach the aquifer.
The cone of depression is the region affected by drawdown or the decline of water level observed in wells in an aquifer being pumped. Excessive groundwater pumping can have several important effects including lowering the water table and the loss of hydrostatic pressure, which makes access more difficult. Specific yield is the amount of water drained by gravity in a saturated aquifer in relation to the total volume of the aquifer. Some water sticks to soil particles due to surface tension, so specific yield is always less than porosity.
Saltwater intrusion is the contamination of groundwater (freshwater) with salty water in coastal areas, commonly caused by excessive pumping of groundwater but can also occur from other human related activities in coastal areas. Subsidence occurs when land sinks, commonly resulting from excessive withdrawals of groundwater.
Groundwater contamination and quality are very important issues to society. The most common sources of groundwater contamination are sewage, landfills, toxic waste disposal sites, and contamination from chemicals used in agricultural activities. If these contaminants enter the groundwater system, the water can become polluted and unsafe to drink.
Hydrothermal fluid is hot water inside the Earth usually heated by magma and the activities associated with the hot water. Hydrothermal alteration is a change in the mineralogy as a result of the interaction of rock with hot water containing solutions of minerals. A hot spring is naturally hot water inside the Earth which is heated by magma and rises to the surface. Geysers are hot springs that episodically erupt fountains of boiling hot water and steam from fractures and conduits in the ground. Geyserite is a mineral that forms at the surface of a geyser. The hot silica-dissolved waters form geyserite, or sinter, creating the massive geyser cones, the scalloped edges of hot springs, and the expansive, light-colored, barren landscape characteristic of geyser basins. Travertine is a type of calcium carbonate formed from geothermal hot springs and is commonly found in caves. Natural steam derived from the Earth's internal heat produces an energy resource called geothermal energy.
Karst Topography
Groundwater erosion is caused by dissolution of limestone rock by acidic water. Dissolution is the process of chemical weathering of bedrock in which the combination of water and acid slowly removes mineral compounds from solid bedrock and carries them away in liquid solution. Sinkholes are depressions in the surface commonly found in karst or limestone environments. Sinkholes often form where limestone or some other soluble rock is partially dissolved by groundwater, causing the ground surface to collapse and form a depression or bowl shape, which can be small or hundreds of meters in diameter (also called dolines).
Karst topography is a distinctive landscape that can develop where the underlying bedrock, often limestone or marble, is partially dissolved by acidic water. A cave is a natural opening in the ground extending beyond the zone of light and large enough to permit the entry of man. Caves can form by the dissolving of limestone rock by acidic groundwater. Solution valleys (karst valleys) are the remains of former surface stream valleys whose streams were diverted underground as karst developed and may develop a series of sinkholes in the valley floor. Sinkholes formed in karst environments are called dolines. Stalactites are mineral deposits (a type of speleothem) which hang downward from the roof or wall of a cave. Stalagmites are mineral deposits (a type of speleothem) which project upward from a cave floor.
by Owen Borville
January 30, 2021
Learning, Geology, Science
Groundwater is water found beneath the Earth's surface where part or all of the pore space in the rock is filled with water. Groundwater is one of the major sources of water for homes and industry. Porosity is the percentage of open spaces or pores in rock or soil. When these spaces are interconnected, water, air, or other fluids can migrate from space to space. Interconnected spaces make the soil or bedrock permeable. Permeability is the ability of a rock or other material to allow water to flow through its interconnected spaces.
The water table is the boundary between the unsaturated zone and the saturated zone below the Earth’s surface. The water table is flat where the land surface is flat. However, where the land contains hills or ridges the water table will also have an irregular surface with high or low parts roughly corresponding to the overlying topography.
The unsaturated zone (also called the aeration zone or vadose zone) is the zone between the land surface and the regional water table. The aeration zone contains pore space partially filled with air and partially filled with water. The aeration zone includes the capillary fringe and may also include localized perched ground water. The saturated zone (phreatic zone) is the underground zone completely saturated or filled with water and is located directly below the water table.
The capillary fringe is a belt layer of variable thickness just above the water table where water is drawn up by capillary action. This layer is saturated with water. The capillary fringe contains porous material just above the water table that holds water by capillarity, which draws water upward into the smaller void spaces.
An aquiclude is a material that tends to exclude the flow of groundwater and has a low permeability, such as clay. An aquifuge is a rock layer that completely blocks the flow of water and has no interconnected pores. An aquitard is a rock layer which slows the flow of water.
A spring is a water resource formed when the side of a hill, a valley bottom, or other excavation intersects a flowing body of ground water at or below the local water table, below which the subsurface material or aquifer is saturated with water and overflows onto the land surface.
An aquifer is a section of the subsurface that contains sufficient saturated permeable rock or material to yield significant quantities of water to wells and springs. An unconfined aquifer is an aquifer that can receive water from the surface and whose water table surface is free to fluctuate up and down, depending on the recharge or discharge rate. There are no overlying confining beds of low permeability to physically isolate the aquifer.
A perched aquifer is a relatively small aquifer located above the water table in the unsaturated zone and rests on top of an impermeable rock layer which prevents the water from reaching the water table.
A confined aquifer is a subsurface layer of rock containing groundwater located below a confining layer, or a layer of rock which blocks or slows down the passage of water. The water inside the confined aquifer can be reached by drilling a well through the confining layer. The rock or strata containing the confined aquifer must connect to the surface at some point or the aquifer cannot be recharged with water. The confined aquifer, also known as an artesian well or aquifer, is overlain by a layer of impermeable rock which causes hydrostatic pressure to build up and pull the water to the surface against gravity. If a hole or well is drilled through the confining layer, water will rise through the well. Water flows from higher pressure to lower pressure and a well or hole drilled through the confining layer will create a lower pressure exit for the water to rise through. The potentiometric surface (also called head) is the level in which groundwater in a confined aquifer rises when a well or hole is constructed to reach the aquifer.
The cone of depression is the region affected by drawdown or the decline of water level observed in wells in an aquifer being pumped. Excessive groundwater pumping can have several important effects including lowering the water table and the loss of hydrostatic pressure, which makes access more difficult. Specific yield is the amount of water drained by gravity in a saturated aquifer in relation to the total volume of the aquifer. Some water sticks to soil particles due to surface tension, so specific yield is always less than porosity.
Saltwater intrusion is the contamination of groundwater (freshwater) with salty water in coastal areas, commonly caused by excessive pumping of groundwater but can also occur from other human related activities in coastal areas. Subsidence occurs when land sinks, commonly resulting from excessive withdrawals of groundwater.
Groundwater contamination and quality are very important issues to society. The most common sources of groundwater contamination are sewage, landfills, toxic waste disposal sites, and contamination from chemicals used in agricultural activities. If these contaminants enter the groundwater system, the water can become polluted and unsafe to drink.
Hydrothermal fluid is hot water inside the Earth usually heated by magma and the activities associated with the hot water. Hydrothermal alteration is a change in the mineralogy as a result of the interaction of rock with hot water containing solutions of minerals. A hot spring is naturally hot water inside the Earth which is heated by magma and rises to the surface. Geysers are hot springs that episodically erupt fountains of boiling hot water and steam from fractures and conduits in the ground. Geyserite is a mineral that forms at the surface of a geyser. The hot silica-dissolved waters form geyserite, or sinter, creating the massive geyser cones, the scalloped edges of hot springs, and the expansive, light-colored, barren landscape characteristic of geyser basins. Travertine is a type of calcium carbonate formed from geothermal hot springs and is commonly found in caves. Natural steam derived from the Earth's internal heat produces an energy resource called geothermal energy.
Karst Topography
Groundwater erosion is caused by dissolution of limestone rock by acidic water. Dissolution is the process of chemical weathering of bedrock in which the combination of water and acid slowly removes mineral compounds from solid bedrock and carries them away in liquid solution. Sinkholes are depressions in the surface commonly found in karst or limestone environments. Sinkholes often form where limestone or some other soluble rock is partially dissolved by groundwater, causing the ground surface to collapse and form a depression or bowl shape, which can be small or hundreds of meters in diameter (also called dolines).
Karst topography is a distinctive landscape that can develop where the underlying bedrock, often limestone or marble, is partially dissolved by acidic water. A cave is a natural opening in the ground extending beyond the zone of light and large enough to permit the entry of man. Caves can form by the dissolving of limestone rock by acidic groundwater. Solution valleys (karst valleys) are the remains of former surface stream valleys whose streams were diverted underground as karst developed and may develop a series of sinkholes in the valley floor. Sinkholes formed in karst environments are called dolines. Stalactites are mineral deposits (a type of speleothem) which hang downward from the roof or wall of a cave. Stalagmites are mineral deposits (a type of speleothem) which project upward from a cave floor.
