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Lecture 11. groundwater hydrology

Date post: 01-Sep-2014
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Groundwater Hydrology
  • Groundwater Hydrology
  • What is Groundwater?
  • - Water that comes from the ground
  • - Water beneath the surface of the earthwhich saturates the pores and fractures of sand, gravel, and rock formations.
  • GROUNDWATER Agriculture
  • GROUNDWATER Industry
  • GROUNDWATER Drinking
  • Groundwater is stored in, and moves slowly through, moderately to highly permeable rocks called aquifers. Aquifers literally carry water underground. An aquifer may be a layer of gravel or sand, a layer of sandstone or cavernous limestone, a rubbly top or base of lava flows, or even a large body of massive rock, such as fractured granite, that has sizable cracks and fissures. In terms of storage at any one instant in time, groundwater is the largest single supply of fresh water available for use by humans.
  • An estimated one million cubic miles of the worlds groundwater is stored within one-half mile of the land surface.Only a fraction of this groundwater, however, can be practicably tapped and made available on a perennial basis through wells and springs.The amount of groundwater in storage is more than 30 times greater than the nearly 30,000 cubic- miles volume in all the fresh-water lakes and more than the 300 cubic miles of water in all the worlds streams at any given time.
  • How does groundwater occur?
  • Groundwater is replenished by precipitation and, depending on the local climate and geology, is unevenly distributed in both quantity and quality. When rain falls or snow melts, some of the water evaporates, some is transpired by plants, some flows overland and collects in streams, and some infiltrates into the pores or cracks of the soil and rocks.
  • The first water that enters the soil replaces water that has been evaporated or used by plants during a preceding dry period. Between the land surface and the aquifer water is the unsaturated zone. In this unsaturated zone, there usually is at least a little water, mostly in smaller openings of the soil and rock; the larger openings usually contain air instead of water.
  • After a significant rain, the zone may be almost saturated; after a long dry spell, it may be almost dry. Some water is held in the unsaturated zone by molecular attraction, and it will not flow toward or enter a well.
  • All geologic material beneath the earths surfaceis either a potential aquifer or a confining bed.An aquifer is also a saturated geologic formationthat will yield a usable quantity of water to awell or spring. A confining bed is a geologic unitwhich is relatively impermeable and does notyield usable quantities of water. They restrict themovement of ground water into and out ofadjacent aquifer.
  • Recharge is the process by which ground water isreplenished. A recharge area is where water fromprecipitation is transmitted downward to anaquifer.Most areas, unless composed of solid rock orcovered by development, allow a certainpercentage of total precipitation to reach thewater table. However, in some areas moreprecipitation will infiltrate than in others. Areaswhich transmit the most precipitation are oftenreferred to as "high" or "critical" recharge areas.
  • How much water infiltrates depends on vegetationcover, slope, soil composition, depth to the watertable, the presence or absence of confining bedsand other factors. Recharge is promoted by naturalvegetation cover, flat topography, permeable soils,a deep water table and the absence of confiningbeds.
  • AQUIFERConfined Aquifer Unconfined Aquifer- Overlain by a confining bed -Has no confining bed - Are recharged through - Open to infiltration fromcracks in impermeable layer surface
  • The water level in a confined aquifer does not riseand fall freely because it is bounded by theconfining bed. Being bounded causes the water tobecome pressurized. In some cases, the pressurein a confined aquifer is sufficient for a well tospout water severalfeet above the ground.Such wells are called flowing artesian wells.Confined aquifers are alsosometimes called artesianaquifers.
  • Discharge areas are the opposite of rechargeareas. They are the locations at which groundwater leaves the aquifer and flows to the surface.Ground water discharge occurs where the watertable or potentiometric surface intersects theland surface, or when we apply mechanical workto pump out the water (wells). Where thishappens, springs or seeps are found. Springs andseeps may flow into fresh water bodies, such aslakes or streams, or they may flow into saltwaterbodies.
  • A water well is an excavation or structurecreated in the ground, by digging, or drillingto access water in underground aquifers.
  • The four most important parts of a water wellCasing is used to maintain an Screen keeps sand and gravelopen access in the earth while out of the well while allowingnot allowing any entrance or groundwater and water fromleakage into the well from the formations to enter into thesurrounding formations. The well. The most popular screenmost popular materials used for used are the stainless steel andcasing are black steel, slotted PVC pipe. Screen isgalvanized steel, PVC pipe and used when wells are drilledconcrete pipe. into unconsolidated materials.Grout is a sealant that is used to fill in Gravel pack is placedthe spaces around the outside of the around the outside of thewell. It protects the well against the screen to prevent sandintrusion of contaminants. A grout from entering the well ormixture can be made of cement, or clogging the screen and toconcrete. stabilize the well assembly.
  • When water is withdrawn from a well, its water leveldrops. When the water level falls below the waterlevel of the surrounding aquifer, ground water flowsinto the well. The rate of inflow increases until itequals the rate of withdrawal.The movement of water from an aquifer into a wellalters the surface of the aquifer around the well. Itforms what is called a cone of depression. A cone ofdepression is a funnel-shaped drop in the aquiferssurface. The well itself penetrates the bottom of thecone. Within a cone of depression, all ground waterflows to the well.
  • As ground water flows downwards in an aquifer, its uppersurface slopes in the direction of flow. This slope is knownas the hydraulic gradient and is determined by measuringthe water elevation in wells tapping the aquifer.For confined aquifers, the hydraulic gradient is the slope ofthe potentiometric surface. For unconfined aquifers, it isthe slope of the water table.
  • A well can easily be contaminated if it is notproperly constructed or if toxic materials arereleased into the well. Toxic material spilled ordumped near a well can leach into the aquiferand contaminate the groundwater drawn fromthat well.Contaminated wells used for drinking water areespecially dangerous. Wells can be tested tosee what chemicals, pathogens and othercontaminants may be in the well and if they arepresent in dangerous quantities.
  • Things you can do to protect your groundwater and water well:1. Maintain your well and test the water quality annually.2. Keep household chemicals, paint and motor oil away from your welland dispose of them properly by taking them to a recycling center orhousehold hazardous waste collection site.3. Limit your use of pesticides and fertilizers.4. Install a well cap and keep it clear of leaves, mulch, dirt, snow andother materials.5. Use caution when mowing around your well so you dont damage thewell casing.6. Practice water conservation measures in your home and install lowwater use appliances.7. Learn more about well testing.
  • Groundwater Treatment Technology
  • Extraction, Treatment, Reinjection
  • Re-circulating Well Technology
  • Natural Attenuation Natural attenuation refers to the strategy of allowing natural processes to reduce contaminant concentrations to acceptable levels. Natural attenuation involves physical, chemical and biological processes which act to reduce the mass, toxicity, and mobility of subsurface contamination These processes are always occurring and in many cases may reduce risk to human health and the environment to acceptable levels.
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