Chapter 17 - Groundwater

It’s only 0.006% of the Hydrosphere, but…

Ground water is:

•94% of (liquid) fresh water.

•40% of all water usage except power generation and cooling.

•50% of U.S. population’s drinking water

•25% of Industry’s needs

After rainfall – evaporation, runoff, or infiltration.

Rainfall infiltration - affected by slope, intensity & duration of rainfall event, soil characteristics, vegetation, etc.

Groundwater helps store freshwater that sustains streamflow. Excess rainfall during and after storms (runoff) leaves the system quickly. Groundwater system stores water, releases it gradually to streams.

Ground water plays a role in shaping some landforms, esp. in “karst terranes”. The removal of groundwater can modify landforms by subsidence.

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O Layer A Layer

Water Table

Saturated Zone

Belt of Soil Moisture

Unsaturated Zone

Capillary Fringe

Bottoms of O layer, A layer are not flat, but are usually transitional into the next layer. The top of the water table is usually not flat, either. It is often a subdued replica of the ground surface. See Fig. 11.2, pg. 304

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Belt of Soil Moisture – water held by molecular attraction to individual particles

Unsaturated Zone – water percolates through pore spaces

Saturated Zone – water completely fills pore spaces in sediment or rock

Capillary fringe – just above the water table, where moisture is held by surface attraction in minute pore spaces

Water table – Upper limit of saturated zone

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Stream/surface water interactions –

Gaining stream – water table slopes toward stream.

Losing stream – water table slopes away from stream.

Aquifer incision – Stream has eroded downward into a confined aquifer

Pressure changes – In a gaining stream, increased pressure during flood conditions can be transferred through pore spaces in aquifers to affect nearby wells.

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Groundwater Movement and Storage is affected by –Porosity - % of open pore spacePermeability – degree of connection between pore spacesGradient of water table – Down-gradient = down-stream

Aquifer – a body of rock or sediment that can hold and transmit usable quantities of ground water. Aquitard – impermeable zones or layers of rock or sediment that do not pass water easily. Also called a “confining” bed.

Recharge area – Areas where rainfall infiltrates and percolates downward to aquifer

Groundwater movement is in response to gravity and/or hydrostatic or lithostatic pressure. Fig. 11.5, pg. 307

Groundwater velocity is predicted by D’Arcy’s Law

V = Kh/l V = Velocity, K = permeability coefficient, h = head (pressure due to elevation change) l = length of pathway.

Hydraulic gradient = h/l

Measuring groundwater flow rates –

Tracer dyes - Radioactive isotopes – Already in the water, not added, e.g., Tritium – short half-life.

Typical flow rates – about 5 ft./yr., may be locally faster through karst fracture systems.

Springs occur where the water table intersects the ground surface or where downward percolating water encounters an aquitard and then flows laterally.

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A “perched water table” exists where an aquitard locally inhibits downward flow of water, above the main water table (Fig. 11.7, pg. 308).

Local aquitard

Main water table

Perched water table

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Hot springs – 10-15 degrees F hotter than annual air temperatures.

• Usual source is cooling igneous rock body or a deep source.

Geysers – Intermittent hot springs or fountains in which heated water and/or jets of steam emanate from a vent. Yellowstone geysers lie within a caldera-type volcano. Sub-surface heating pressurizes water, resulting in steam/hot water eruptions, see pp. 309-310 & Fig. 11.10.

Effluent – discharge from a spring. When the springwater is mineralized (especially when heated), when water discharges and changes its chemistry and/or cools rapidly, minerals may be deposited around the spring, usually silica-rich geyserite or calcium carbonate – travertine. Sometimes sulfur compounds may also be released.

Influent – recharge to a spring.

Wells are drilled into the zone of saturation, i.e., the aquifer. This may be a sand or gravel bed or perhaps a fracture zone in a crystalline (igneous or metamorphic) or sedimentary rock, or even saprolite, in some cases. Piedmont Piedmont Coastal Plain

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Concrete Surface pad

Ground surface

Borehole

PVC Casing

Water table

Screen zone

Aquifer

Well diagram

Annular space

Concrete grout

Clay grout

Gravel pack

Typical water well construction and terms Water Well Construction Terms

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The importance of proper well completion – Apparent lack of near surface grouting and concrete surface pad = potential contamination of unconfined (shallow) aquifer.

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Unconfined aquifer – Aquifer that is “open” to the un-saturated zone, i.e., it is not overlain by an aquitard. The water must be pumped to reach the surface. Is directly recharged by rainfall.

Confined aquifer – Overlain by an aquitard and pressurized by “up-gradient” water. Wells into confined aquifer are called artesian wells. Water from Non-flowing Artesian Wells rise above the level of the aquifer, but not to the surface. Water from Flowing Artesian Wells rises above the land surface.

Artesian Springs occur when water from a confined aquifer rises by way of natural rock fractures, sometimes called “Boiling Springs”.

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Aquifers in layered sediments or sedimentary rocks, similar to Coastal Plain.

Blue – Aquifers – usually sand (sand-stone) - updip or limestone (downdip)

Brown – Aquicludes (confining beds) – usually clay.

Unconfined aquifer Confined aquifers

Well

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Groundwater flow to Atlantic or Gulf

Drinking water for Savannah, Brunswick, and other coastal Ga. and Florida Peninsula cities comes primarily from the Floridan Aquifer. In the coastal areas & Florida, the aquifer largely consists of Eocene-aged limestones, like the Ocala Limestone. Some of the Recharge Zone Floridan aquifer is exposed Eocene sands and limestones near the Georgia Fall Line and other exposed limestones midway to the coasts (Screven County, Dougherty County).

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Basin and Range Province aquifer types.

Old river sediments

Silt and clay

Alluvial fan deposits

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Hueco Bolson

Franklin Mts.

Hueco Mts.

PROBLEMS WITH GROUNDWATER WITHDRAWAL

Slow recharge, salinization of land, draw-down, cones of depression, subsidence, saltwater intrusion (incursion), contamin-ation.

Slow recharge – failure to understand the origin, history, and “plumbing” of groundwater resources, “Tragedy of the Commons”, i.e., the perception that groundwater is “free”, other issues lead to overuse. See Box 11.1, pg. 316 concerning Ogallala aquifer.

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Salinization of soil – In arid or semi-arid climates, evaporation following irrigation leads to precipitation of salts. This also an issue with the Ogallala aquifer.

Drawdown – Over pumping of aquifer, i.e., faster than recharge “draws down” the water table.

Cone of Depression – If pumping is dominated by one large well or well field, a local, exaggerated depressing of the water table may occur.

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Subsidence – Over-withdrawal of aquifer water leads to compaction of aquifer material and subsidence of overlying land surface, see Box article 11.2, pg. 317.

Salt Water Intrusion (Incursion) – (Fig. 11.18, pg. 315), in Coastal areas, pressure in aquifer prevents inland migration of salt groundwater in sandy sediments. When overpumping lessens pressure, salt groundwater boundary moves inland.

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Contamination – Leakage from sewage (septic tanks, outhouses, animal waste lagoons), buried toxic wastes, the past (or illegal) use of wells as dumping stations, leakage from surface runoff, leakage through aquitards…

Organic wastes may be naturally broken down if ground-water flow rate is slow between recharge and discharge points, generally, slow flow occurs in sand/sandstone aquifers.

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Ocean

Salt Water incursion in coastal areas

Coastal Plain/Continental Shelf sediments (aquifer)

Under normal conditions, aquifer pressure causes freshwater to leak into ocean. Pressure prevents leakage of salt water into aquifer. When overpumpage lessens pressure, salt water may leak into aquifer.

Pressure

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Overpumping may reverse groundwater gradient (Fig. 11.20, pg. 318) and “draw” plume toward new well. It may also increase leakage through aquitards.

Because groundwater rate is usually slow, problem may not be apparent until a number of people become ill.

Aquifer may have to be locally abandoned, if remediation efforts are not immediately successful.

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Groundwater Modification

Rain + carbon dioxide = carbonic acid

Additional acids released by organic decay, mineral decay, sulfuric acid (see inset, pg. 319)

Calcium carbonate (calcite) easily dissolved, removed as calcium bicarbonate solution.

Downward percolation through limestone bedding planes, joints, fractures leads to development of caverns, especially as water table drops due to downcutting of nearby stream valleys.

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In temperate areas, continual infiltration results in the development of stalactites,

Excessive dissolution and/or decline of water table leads to cavern collapse – sinkholes and modification of landsurface – karst terranes (Fig. 11.25, pg. 322)

Karst terranes – “Bluegrass region of Kentucky”, Dougherty Plain (SW Ga.), Screven, Bulloch Counties (SE Ga.), Central Florida Peninsula (Orlando), Guadalupe Mts., Texas, New Mexico (Carlsbad Caverns).

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At right – Diagram of Burke County wells showing comparison of sand aquifer and limestone aquifer wells.

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Example of a threat to surface and ground water.

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