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Geology 110


GROUNDWATER


Learning Objectives

 



Water is an important natural resource and is used in many ways.

Groundwater is primarily used as a resource for drinking and irrigation, irrigation is by far the largest user of groundwater.

Largest source of water on the planet is the ocean, next is glaciers and third is groundwater, a resource that is often taken for granted.



Groundwater: water that flows beneath the surface.  It is usually at a constant temperature and usually free of contaminants (silts, clays and pollutants).  Groundwater is locally chemically uniform unlike surface water that can vary in chemical and contaminant content over short distances.
Minimal treatment is needed but often groundwater will carry large concentrations of dissolved mineral ions.

Hard Water: water that contains large concentrations of Calcium, Magnesium and Iron.  These dissolved ions form a "scum" around bath fixtures and causes soap to not be as "sudsy".  Iron in groundwater will stain concrete red if used to irrigate a lawn.  (Go out the main entrance of the building.)
Soft water: water that is treated to remove the dissolved ions.  Usually Sodium is exchanged for Ca, Mg and Fe through exchange columns.


Groundwater flows and is held in pore spaces between individual grains in rocks and sediment.  It flows by gravity and capillary action.

Porosity: the total amount of pore space contained in a material.

Permeability: the ability of a fluid to flow through that material.

A fine grained rock or sediment has small pore spaces but lots and lots of them.  A coarse grained rock or sediment has larger pore spaces but many fewer.  In a square meter of fine grained material and coarse grained material the fine grained material actually has more total pore space (empty space in which fluids can exist).  Finer grained material has a higher POROSITY.

Fluids flow through larger pore spaces much more readily than in material with finer pore spaces.  Therefore coarse grained material is more PERMEABLE.

Water at the Earth's surface infiltrates into the surface and moves downward by gravity.  When is hits a nonporous or low permeability layer it moves laterally.

Zone of Aeration, Vadose Zone: Material is unsaturated, it contains both water and air

Zone of Saturation, Phreatic Zone: material is saturated, pore spaces contain only water

Water Table: the boundary between the vadose and phreatic zone, water table roughly follows the topography.

Aquifer:  permeable layer of rock or sediment that contains water that is free flowing.  Aquifers can occur at many depths under the surface.  In the Hampton Roads area there are a series of aquifers at varying depths because the subsurface material is interbedded sands, silts and clays or alternating layers of permeable and nonpermeable material.

Wells: tap into the aquifer

Cone of depression: area surrounding the well head where the water table becomes depressed.  The larger the well pump the more extensive the cone of depression.

Artesian wells: when an aquifer that is under pressure is tapped by a well water flows out of the well without pumping.  If a well is not an artesian well the water must be brought to the surface using a pump.  "Artesian" DOES NOT infer anything about the quality.

Spring: where an aquifer intersects the land surface water will flow out onto the surface.  "Spring water" DOES NOT infer anything about the quality.

Thermal spring: water heated by  a thermal energy source in the Earth.

Geyser: a thermal spring that erupts.  In some instances water flowing underground pools and heats to steam and then erupts suddenly to the surface through a spring.  After the heated water erupts cooler water starts to pool underground and there is a lag time between eruptions as the cooler water is heated to steam.

Recharge: the term used to describe the replenishment of groundwater.  Water recharges aquifers when it infiltrates from the surface.  It is imperative that water sit on the surface as infiltration is often very slow.  If water drains off a surface too quickly it cannot recharge the aquifer below.  Wetlands are an important source of aquifer recharge.  Vegetation slows water drainage and increases infiltration.  Retention ponds are now required in many areas during construction to allow water that would runoff a surface to runin instead.  Look around campus.



Problems with groundwater usage:

Overuse: some areas recharge slowly and overpumping will remove the majority of the water.  This causes the aquifer to compress and land subsidence can result.  Also if the aquifer compresses it often cannot recharge to its former capacity.
Venice, Italy is sinking NOT because of sea level rise but because of over pumping of its aquifers.

Salt water intrusion: areas near the ocean, if overpumped, can cause saltwater to infiltrate an aquifer.  Once saltwater infiltrates an aquifer it is very difficult to be removed because it is heavier than fresh water.  Eastern Shore has had some problems as well as Long Island, New York and parts of Florida.

Ground water pollution:  Pollutants can seep into an aquifer from landfills, underground storage tanks and septic tanks.  Once an aquifer is contaminated remediation is very expensive and difficult.


Karst Topography: groundwater can alter a landscape. In areas underlain by limestone water mixes with carbon dioxide to form carbonic acid.  This slowly dissolves the limestone forming caves and sinkhole.

Cave: empty space underground formed from dissolution of rock material by water.  As water flows through cave it acts to enlarge the cave through dissolution.  Water present within the cave can precipitate calcium carbonate structures.

MOST caves form in limestone material but under certain circumstances cave can form in rock material other than limestone.

Speleotherm: general term for a mineral deposit formed in a cave.
Stalactite*: water dripping from ceiling of cave forms pointed feature.  VERY slow growth
Stalagmite*: water that drips onto cave floor forms pointed feature.  VERY slow growth.
Stalactites and stalagmites take thousands of years to form,  over time they may merge to form columns*.
 

Sinkholes: A rounded hole on the surface formed by collapse of a cave roof when cave is close to the surface.  VERY common in Florida and usually form during periods of drought.  High water table fills the cave with water, during a drought the water recedes from the cave weakening support of the ceiling and the weight of overlying material causes collapse forming a sinkhole.