Chapter 19: Groundwater

Groundwater reactions

*-Dissolution/ precipitation:* If groundwater enters a new environment where it can contain more ions, it may dissolve the surrounding rock. If it enters an environment where it can not contain more ions, it may precipitate and form cement. *-Oxidation/reduction:* Oxygen may oxide the chemicals, thereby turning it into a harmless substance. *-Hydrolysis & Hydration:*

How does Karts landforms form?

*1) The formation of a cave network:* -Dissolution takes place near the water table. *2)A drop in the water table:* -If the water table drops, cave dry out. Downward-percolating groundwater emerge from the roof of the caves; dripstone precipitates. *3)Roof collapse:* Roof collapses. So sinkholes form, leaving behind hills and natural bridges.

Contaminant introduction

*1)Point source* Example: Septic system. *2)Nonpoint source* Example: Contaminated pond. Groundwater transports pollutants away from a source.

Pore collapse Land subsidence

*Pore collapse:* Pores collapse without groundwater. This permanently decreases the porosity and permeability of a rock and thus lessens its value as an aquifer. *Land subsidence:* Aquifer sinks due to the lack of groundwater. This may cause fissures at tilt the ground. -Example: The Leaning Tower of Pisa, Italy. -Can be avoided with a well or a recharge basin (drains in a city) *-Near the coast, land subsidence may cause flooding.*

Capillary fringe

*Surface tension*, the electrostatic attraction of water molecules to one another and to mineral surfaces, causes water to seep upward from the saturated zone.

Characteristics of the water table

*Unsaturated zone (vadose zone)* -The top of the unsaturated zone may include a soil. -May have a local aquitard with a *perched water table.* -Has the *capillary fringe* on top of the water table. *Saturated zone (phreatic zone)* -Contains the unconfined aquifer, aquitard, and the confined aquifer. -All pore space is filled with water.

The High Plain Aquifer

-Erosion of the Rocky Mountains lead to the depostion of huge alluvial fans over what if now the High Plains region. -Forms the Ogallala Formation (A sheet of porous and permable strata that remains just below the graound surface) -Water can infiltrate this unconfined aquifer over a broad area. -Lies over the Dakota sandstone aquifer.

Groundwater Flow

-Flow paths are not straight lines, instead, flow paths are curved *(concave-up)* -Recharge areas: The location where surface water enters the ground (downward trajectory). -Discharge areas: The location where groundwater flows back up to the surface. -Drainage Divide: A highland or ridge that separates one watershed from another.

Geologic settings in which springs form

-Groundwater reaches the ground surface in a *discharge area.* -Groundwater reaches a *fault*, it has an impermeable barrier, so it rises. -groundwater seeps where a *perched water table* intersects a slope. -A network of interconnected *fractures channels* water to the surface of the hill. -Groundwater seeps out of a cliff face at the top of a relatively *impermeable bed.* -Water under pressure lies below an aquitard, a crack may provide a pathway for an *artesian spring.*

Contaminated water intrusion

-In the image, the water was flowing from east to west. The large irritation well has changed the flow direction, so the *contaminated plume* from the septic tank flows from west to east and goes up the small well.

Dissolved ion concentrations depend on..

-Temperature: Warmer water, the more ions it carries. -Pressure: The more pressure, the deeper it is, so the more ions it has. -pH: A measure of the hydrogen ion concentration in solution and is also referred to as the degree of acidity or *alkalinity.* -Time: The longer it was underground, the more ions it has.

Phoenix basin Aquifer

-The city of Phoenix resides in the Basin and Range Province, a continental rift. -During rifting, downward slip of crustal blocks on normal faults produced deep wedge-shaped basins that filled with gravels and sands eroded from adjacent ranges. -These sediments serve as quifers.

Topography of the water table

-The water table will mimic the overlying topography, but the relief (the vertical distance between the highest and lowest elevations) of the water table is not as great as that of the overlying land. -The elevation of the water table varies because groundwater moves so slowly through rock and sediment that it cannot quickly assume a horizontal surface. When it rains, the water table rises before the groundwater had time to sink very far.

Mahomet Aquifer

-When glaciers melted during the Pleistocene Ice Age, meltwater streams carried coarse gravel southward. -This gravel filled stream valleys that had previously been cut into the bedrock of central Illinois. -Younger glacial sediment later buried the filled valleys. -The porous and permeable gravel of the largest buried valley comprises the Mahomet aquifer.

Groundwater

95% of all the fresh water on Earth. ~33% drinking water needs. Threatened

Sinkholes

A large surface crater caused by the collapse of an underground channel or cavern; often triggered by groundwater withdrawal.

Drawdown

A lowering of the groundwater level caused by pumping water out of the well faster than it flows in from the surrounding aquifer. -Drawdown is additive. *-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.

Darcy's Law

A mathematical equation stating that a volume of water, passing through a specified area of material at a given time, depends on the material's permeability and hydraulic gradient.

Springs

A natural source of water formed when water from an aquifer percolates up to the ground surface.

Karst landforms

A region underlain by caves in limestone bedrock; the collapse of the caves creates a landscape of sinkholes separated by higher topography. *Types of landforms:* -Sinkholes -Disappearing streams. -Natural Bridges. -Tower karst -Caves. -Speleothems. -Springs.

Hot springs

A spring of naturally hot water, typically heated by subterranean volcanic activity. -High concentration of dissolved minerals since it dissolves minerals from the rock as it passes through. Distinctive geological features: -Bubbling mud pits (hot water + ash) -Travertine terraces (water passed thorough limestone) -Sulfur minerals

Disappering streams

A stream that intersects a crack or sinkhole leading to an underground cavern, so that the water disappears into the subsurface and becomes an underground stream. *A losing stream*

Caves

Develop when abundant, acidic (water absorbed CO2) groundwater dissolves soluble limestone bedrock. *Speleothems:* The various intricately shaped formations that grow in caves by the accumulation of dripstone. *Dripstone:* Limestone (travertine in a cave) formed by the precipitation of calcium carbonate out of groundwater.

Artesian wells

Drilled hole in a confined aquifer releases pressure and pushes water up. *Potentiometric surface:* The elevation to which water in an artesian system would rise if where not confined. -Where the potentiometric surface *lies underground, the well will be *non-flowing.* -Where the potentiometric surface *lies above the grounds*, the well will be *flowing.*

Natural bridges

Form from natural limestone caves, where paired sinkholes collapse and a ridge of stone is left standing in between, with the cave passageway connecting from sinkhole to sinkhole.

Groundwater time scale

Groundwater flow occurs on a variety of scales. Water can stay underground for... *-Local:* Hours to weeks. *-Intermediate:* Weeks to years. *-Regional:* Centuries to millennia.

Wells

Holes dug or drilled deep into the ground to reach a reservoir of groundwater (Either unconfined/confined/ or perched aquifer)

Geysers

Hot springs that shoot jets of steam and heated water into the air. How geysers work: -Network of irregular fractures in the rocks, one connects to the surface. -Groundwater fills these fractures and becomes really hot. -The boiling temperature of water increases with growing pressure, so water remains liquid. -As it begins to rise, pressure in it decreases until some of the water transforms into steam and expands. -The resulting expasion cause water higher up to spill out, so the pressure deeper in the conduit decreases. -Superhot water turns into steam instantly and rapidly rises as an eruption.

Sources of Groundwater Contamination

Human activities: -Dissolved and pure organic and inorganic compounds. -Dissolved metals. -Failing septic systems. -Agricultural wastes. -Animal feedlot runoff. -Industrial wastes. -Petroleum storage. -Landfill leachate -Mining wastes Pollution is often not recognized until damage occurs. Groundwater cleanup is slow, expensive, and limited.

Hydraulic head

Hydraulic head: The potential energy available to drive the flow of a given volume of water at a location. -The level to which water rises in a drill hole is the hydraulic head. -This statement generally implies that groundwater, regionally, flows from high water table to low water table. *Potential energy comes from elevation (due to gravity) and pressure.* -Hydraulic head at p1 > p2. -Same elevation. -The weight of water over p1 is greater than that over p2.

Water table of a permanent stream

If the water table lies at or above the ground level, we have a surface of a permanent stream such as a lake. In such places, there is no unsaturated zone, and the soil itself can be saturated, so we can't really distinguish soil moisture from groundwater. -*Permanent lakes or streams cannot exist in arid regions* unless fed by a water supply from elsewhere.

Limestone column

Limestone column: How does limestone columns form? When the downward-percolating groundwater containing dissolved calcite re-enters the air in the cave, it evaporates a little and releases some of its dissolved carbon dioxides. As a result, calcite precipitates out the water and produces a dripstone. Eventually, the dripstone accumulates on the bottom and on top; forming limestone column.

Perched aquifer

Localized zone of saturation above the regional water table created by an underlying layer of impermeable material.

Salt water intrusion

Near the coast, over-pumping of groundwater causes saltwater to move into the aquifer. -This can be solved by putting a *recharge pump well* to increase the water table.

Secondary porosity

New pore space in rocks, created some time after a rock first forms. -Weathering can cause *dissolution* and *fractures* that creates pores within the rock.

Groundwater Quality

Often high quality. -porous media filters particulates. *-Clay minerals absorb some ions.* May contain unwanted substances... *-Hard water:* Passed through limestone or dolomite and contains calcium and magnesium. -Dissolved iron, manganese, and hydrogen sulfide gas and arsenic.

Porosity

Percentage of open spaces between grains in a rock. *High Porosity:* Well-sorted, poorly cemented. Example: Sandstone. *Low Porosity:* Poorly-sorted, well-cemented. Example: Shale, Granite

Aquitard

Sediment or rocks that have low permeability and do not transmit groundwater easily. Generally, they do not hold much groundwater because they have low porosity, too. Examples: -Shales -Evaporites -Very well-cemented sandstones *Low porosity and permeability*

Aquifer

Sediment or rocks that hold a lot of groundwater and transmit it easily because they have both high porosity and permeability. *Unconfined:* Reaches the ground surface. *Confined:* Lies beneath an aquitard. Examples: -Coarse gravels -Poorly cemented sandstones -Highly fractured and partially dissolved limestones *High porosity and permeability*

Permeability

The ability of a rock to allow water to pass through a rock or soil layer. *High Permeability:* Pores well-connected, so water flows readily. Example: Fractured Granite, loosely cemented gravels *Low Permeability:* Pores are poorly connected, so water flows slowly. *No permeability:* Pores are not connected. Determined by... -The number of available conduits. -Size of the conduits. -Straightness of the conduits.

The Water Table

The horizon that separates unsaturated zone above from the saturated zone below. -The water table is the top of the groundwater. On the other hand, the bottom of the groundwater depends on the geothermal gradient. But usually, groundwater lies at a depth of between 12 and 20 km. Thus, *groundwater occurs on in the upper crust.*

Infiltration

The process in which water from the land surface percolates into the subsurface.

Groundwater depletion

The removal of groundwater at a rate exceeding the natural resupply of groundwater in a region. Types of depletion... -Irrigation: Cultivating in dry areas requires tons of water. -Divergence of water: Urbanization has removed water from recharge areas, disrupting flow paths. Can leads to... -Contaminated water intrusion -Salt water intrusion -Land subsidence

Primary porosity

The space that remains between solid grains or crystals immediately after sediment accumulates or rock forms. -Primary porosity tends to decrease with increasing burial death. The deeper, the more consolidated they are and the lower their porosity. *Chemical sedimentary rocks*: Mineral crystals do not grow snugly against each other during precipitation. *Crystalline igneous rocks/Metamorphic rocks:* Grains do not interlock perfectly. *Fine-grained of glassy igneous rocks:* Vesicles.

Groundwater Flow Rates

The velocity of groundwaters depends on... *-Permeability:* Flows faster though high-permeability rocks. *-Hydraulic gradient:* Flows faster in regions where the water table has a steep slope. Groundwater moves slower than surface water for two reasons: *-Surface tension:* Water sticks to the solid material around it and slows down as it escapes from pores. -Water does not follow a straight path because it must percolate through a *complex network of tiny conduits.*

Water table seasonal fluctuations

Water table rises during rainy season due to recharge received from rainfall and falls back during the summer season due to lack of recharge and continuing extraction of groundwater.