Groundwater

The most common confining beds are

--Clays --Shales --Dense crystalline rocks

Porosity depends on

--Size and shape of grains --How grains are packed together

Examples of stalactites and stalagmites

Mammoth Cave, Kentucky, and Carlsbad Caverns, New Mexico

harder to see

Massive amounts of water stored in the atmosphere and underground, and the flows into and out of these storage places

Groundwater table (shortened to "water table")

The upper surface of the zone of saturation.

Porosity and permeability of course- to medium-grained sand

high porosity, high permeability

Porosity and permeability of fractured shale or metamorphic rocks

low porosity, very low permeability

fractured shale

small amounts of pore space along cracks in impermeable rock.

disappearing streams

streams that terminate abruptly by seeping into the ground

Stalactite

suspended from the ceiling formed by dissolution of limestone.

Effluent stream

when a stream channel intersects the water table, water discharges from the groundwater to the stream

Influent streams

when a stream channel lies at an elevation above that of the water table

The Ogallala Aquifer, part of the High Plains Aquifer System

--A vast yet shallow underground water table aquifer located beneath the Great Plains in the United States --One of the world's largest aquifers, it covers an area of approximately 174,000 mi^2 (450,000 km^2) in portions of the eight states of South Dakota, Nebraska, Wyoming, Colorado, Kansas, Oklahoma, New Mexico and Texas

A number of geologic conditions can lead to confined aquifers

--Alternating beds of dipping sandstones and shales --Fault zones in crystalline igneous rocks --Folded beds with alternating layers, recharged along joint fractures --Horizontal sedimentary rocks involving unconformable relationships --Glacial deposits consisting of till and outwash --Stabilizing sand dunes displaying impervious deposits in the interdune area

Perched water tables

--Discontinuous confining lenses may exist in the subsurface --These arrest downward infiltration to the water table --These lenses form perched water tables ----Overlie unsaturated material ----Represent a "false" water table ----More easily dewatered

Karst topography is most strongly advanced in regions with

--High-rainfall climate, with abundant vegetation --Extensively jointed limestone formations --Appreciable (noticeable) hydraulic gradient

Tapping groundwater

--Oases in the Sahara develop from spring flow --Water from recharge areas flows to oasis discharge points --These locations have been culturally important for millennia

Water Table Topography

--The water table is not flat; it is a sloping surface --The water table is a subdued replica of the topography ----The water table is high where the land is high ----The water table is low where the land is low --Water flows from higher elevations to lower elevations --Topography is useful for estimating groundwater flow --Typical rate of groundwater flow—0.00002 km/hour

The most common aquifers are

--Unconsolidated sands and gravels --Permeable sedimentary rocks such as sandstones and fractured limestones --Heavily fractured volcanic and crystalline rocks

How water flows through soil and rock

--We know that water moves into and through the ground --There are no large open spaces for pools or rivers of water underground --The only space available for water is the pore space between grains of sand and other particles that make up the soil and bedrock and the space in fractures --Some pores, however small and few, are found in every kind of rock and soil, but large amounts of pore space are most often found in sandstones

Lowering the water table

--When we extract groundwater from wells at a rate faster than it can be resupplied, the water table drops. First, a cone of depression forms locally around the well; then the water table gradually becomes lower in a broad region. ----Depletion—severe water table decline can alter surface water flow. Before groundwater withdrawal, the water table is high, discharging to a swamp and permanent stream. After protracted groundwater withdrawal, the water table falls and no longer intercepts the surface at the swamp and stream, which dry up. --The water table can also drop when people divert surface water from the recharge area. ----E.g. the Everglades of southern Florida—a huge swamp where, before the expansion of Miami and the development of agriculture, the water table lay at the ground surface. "river of grass" Diversion of water from the Everglades' recharge area into canals has significantly lowered the water table, causing parts of Everglades to dry up. The decrease in the supply of freshwater has also led to saltwater (saline) intrusion along the coast.

Karst topography is characterized by

--sinkholes --springs --disappearing streams

we can see

-Rivers—water moving from one place to another on Earth's surface -Lakes and Oceans—water stored on the surface

Excessive pumping causes

1. Lowering the water table 2. Reversing the flow direction of groundwater 3. Saline intrusion 4. Pore collapse and land subsidence 5. Tarst topography

permeability of poorly sorted sandstone

15%

permeability of vesicular basalt

20%

permeability of limestone (with solution cavities)

25%

permeability of conglomerate unconformity

30%

permeability of well-sorted eolian sandstone

30%

permeability of shale

5%

percentage salt water on Earth

95.96% (ocean and seas, 1.40 x 10^9 km^3)

permeability of granite (intruding marble)

<1%

Unconfined aquifers

An aquifer that intersects the surface --In contact with the atmosphere --Easily contaminated --The water travels through beds that extend with more or less uniform permeability to the surface in both discharge and recharge areas --The level of the reservoir in an unconfined aquifer is the same as the height of the water table

Pore collapse and land subsidence

As water in some sediments is removed, the sediments undergo compaction; the loss of volume is manifested in the lowering of the surface, ~1 m every 3 years. --Examples—Mexico city, Venice, Italy, San Joaquin Valley, CA ----The Leaning Tower of Pisa's foundation was destabilized by groundwater removal ----Evidence of land subsidence in the San Joaquin Valley, CA. Former ground elevations are marked on the pole.

percentage fresh water on Earth

Fresh water—4.04% --Glaciers and polar ice—2.97% (4.34 x 10^7 km^3) --Atmosphere—0.001% (1.5 x 10^4 km^3) --Lakes and rivers—0.009% (1.27 x 10^5 km^3) --Underground waters—1.05% (1.54 x 10^7 km^3) --Biosphere—0.0001% (2 x 10^3 km^3)

Confining unit

a hydrogeologic unit of impermeable or distinctly less permeable material bounding one or more aquifers and is a general term that replaces aquitard, acquiclude.

Reservoir

a place in which water is stored

Confined aquifers

an aquifer bounded above and below by confining units (aquicludes) of distinctly lower permeability than that of the aquifer itself --Isolated from the surface --Less susceptible to pollution --Water is contained under pressure ----At any point in the aquifer, the pressure is equivalent to weight of all the water in the aquifer above that point. A confined aquifer is a completely saturated aquifer bounded above and below by aquicludes. The pressure of the water in confined aquifers is usually higher than atmospheric pressure, which is why when a well is bored into the aquifer the water rises up the well tube, to a level higher than the aquifer.

Potentiometric surface

an imaginary surface that defines the level to which water in a confined aquifer would rise were it completely bored with wells. --Analogue of the water table for a confined aquifer --Determined by hydraulic head elevation in recharge area --A well casing below this surface will flow without pumping

Stalagmite

an inverted icicle-like deposit on the floor below the stalactite. formed by dissolution of limestone.

Surface runoff

collected and is stored in natural lakes and artificial reservoirs created by the damming of rivers. Wetland areas, swamps, and marshlands also act as storage depots for runoff.

Artesian wells

drilled into a confined aquifer at a point where the elevation of the ground surface is lower than that of the potentiometric surface, the water will flow out of the well spontaneously. --Artesian wells tap confined, tilted aquifers --Upland recharge pressurizes the aquifer --Water rises in artesian wells to the potentiometric surface

Tarst topography

erosion by groundwater: Dissolution of limestone (or rarely, other soluble rocks such as evaporates) by groundwater --Dissolution of limestone is enhanced by the atmospheric CO2 contained in rainwater. Waters that infiltrate soils may pick up even more of the gas from the CO2 given off by plant roots, bacteria, and other soil-dwelling organisms. ----As this CO2-rich water moves down to the water table, through the unsaturated zone to the saturated zone, it creates openings as it dissolves carbonate minerals --Dissolution mostly in the saturated zone

Hydrology of runoff

flash floods after torrential rains

Saline intrusion

in coastal areas, fresh water lies in a layer above saltwater that entered the aquifer from the adjacent ocean --Normally, the pressure of fresh water keeps the saltwater margin slightly offshore. ----Pressure of fresh water = 1.00 g/cm^3 ----Pressure of salt water = 1.02 g/cm^3 --Excessive pumping can lead to a saltwater intrusion, where the coastal well causes a cone of depression and causes the saltwater margin to rise. Well then pumps salty water

Porosity and permeability of fine-grained sand and silt

moderate porosity, moderate to low permeability

Porosity and permeability of sandstone, moderately cemented

moderate to low porosity, low permeability

Droughts

periods of months to years, when precipitation is much lower than normal

springs

places where water flows naturally from the ground (from spaces in the bedrock)

Unsaturated zone

pores contain some air and are not completely filled with water (aka- "vadose zone"). Water and air both occupy pore spaces

Saturated zone

pores of the soil or rock are completely filled with water. Water fills all pore spaces

Infiltration

process by which water that falls on land soaks into the ground. Water enters rock or soil through joints or small pore spaces between particles.

Aquicludes

relatively impermeable beds. A hydrogeologic unit from which, although porous and capable of storing water, does not transmit it at rates sufficient to furnish an appreciable supply for a well or spring.

Tower Karst

ridges or walls between adjacent sinkholes tend to be steep-sided, for they were originally joint controlled. Over time, the walls erode, leaving only jagged, isolated spires—a karst landscape dominated by such spires is called tower karst. --Tower Karst Landscape in the Guilin region of China

Aquitard

sediment or rock that hinders water flow. A confining bed that retards but does not prevent the flow of water to or from an adjacent aquifer; a leaky confining bed.

aquifer

sediment or rocks that stores and transmits groundwater in sufficient quantity to supply wells

Hydrology

study of the movement and characteristics of water on land and within land

sinkholes

surface depressions formed by the collapse of caves or other underground void spaces. A small, steep depression in the land surface above the cavernous limestone formation. In some places, dissolution may thin the roof of a limestone cave so much that it collapses suddenly. --The Arecibo Radio telescope in Puerto Rico was built in a sinkhole --Winter Park, Florida—a large sinkhole formed by the collapse of a shallow underground cavern. Such collapses can occur so suddenly that moving cars are buried. (Florida has a LOT of sinkholes)

Permeability

the ability of a solid to allow fluids to pass through it

Porosity

the amount of pore space in rock, soil, or sediment. The ratio, usually expressed as a percentage, of the total volume of voids of a given porous medium to the total volume of the porous medium.

Reversing the flow direction of groundwater

the cone of depression that develops around a well creates a local slope to the water table. The resulting hydraulic gradient may be large enough to reverse the flow direction of nearby groundwater. Such reversals can lead to the contamination of wells. --Before pumping, septic effluent is carried by the regional groundwater flow away from the home well --A large irrigation well creates a large cone of depression that reverses the hydraulic gradient and causes septic contamination of the home well

Cone of depression

the downward-pointing, cone-shaped surface of the water table in a location where the water table is experiencing drawdown because of pumping at a well

Strongest influence on local hydrology

the geology of the land and climate (temperature and precipitation)

groundwater

the mass of water stored beneath Earth's surface ---rain that sinks into the ground

Runoff

the sum of all rainwater that flows over the surface

Porosity and permeability of gravel

very high porosity, very high permeability

Porosity and permeability of unfractured shale

very low porosity, very low permeability

unfractured shale

very small amounts of pore space between clays and silt grains.

Recharge

water enters the saturated zone --Infiltration of rain or snow meltwater from the surface --Through the bottom of a stream, where the stream channel lies at an elevation above that of the water table

Discharge

water leaves the saturated zone --Exit of groundwater to the surface