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Planet earth groundwater_lecture_outline
 

Planet earth groundwater_lecture_outline

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    Planet earth groundwater_lecture_outline Planet earth groundwater_lecture_outline Document Transcript

    • Groundwater HydrologySummary of Important Concepts • The hydrologic cycle refers to the natural movement of water between various places on earth: the atmosphere, inside living things, the earth’s surface (the ocean, rivers, and lakes), and underground. • Humans can realistically tap only a small fraction of earth’s total fresh water supply. The most ready source of abundant fresh water is ground water -- all the water that naturally occurs underground in open spaces (pores and fractures) within rock. • The water table is a contact underground between the zone of aeration, where open spaces are filled mostly with air, and below that the zone of saturation where open spaces are filled with water. The water table often slopes due to uneven recharge and discharge. This slope creates a pressure gradient called hydraulic head, causing ground water to flow from higher areas to lower areas of the water table.Ground water occurs in AQUIFERS -- large bodies of rock that have both high porosity and highpermeability.Porosity refers to the percentage of the rock made up of open spaces that can hold water.Permeability refers to the ease with which water flows through the rock.WATER is our most important natural resource. The hydrologic cycle refers to the natural movement ofwater between various places on earth: the atmosphere, inside living things, the earth’s surface (the ocean,rivers, and lakes), and underground.The vast majority of the earth’s water lies in the oceans. But sea water is not readily usable by humans.(Desalination -- extracting fresh water from sea water -- is an expensive and difficult process and currentlyproduces only a tiny fraction of fresh water used.)Of the earth’s fresh water the vast majority lies trapped in ice caps and glaciers -- again, not readily usableby humans.Of the earth’s liquid fresh water, some occurs on the earth’s surface in lakes, rivers and streams. But farmore occurs underground in open spaces within rock.Places that lack local supplies of fresh water adequate to suit demand must resort to importation -- bringingin water from elsewhere. In southern California, high human population, extensive irrigation agriculture, andgenerally arid conditions means that the majority of water is imported. Irrigation is the single largest userof fresh water in the U.S. and in the rest of the world.The Water TableGoing into the ground, one passes through the zone of aeration - the region where open spaces in the rockare filled mostly with air - and passes into the zone of saturation, where the open spaces are filled withwater. The contact between these zones is the water table.The water table does not remain fixed in position but•Fluctuates with the quantity of recent precipitation.•Mimics topography.
    • •Flows.The water table is not stagnant. It flows and it rises and falls in elevation depending on rainfall. Duringdrought situations the water table drops and water from streams may filter through the aerated zone downto the water table. When there is abundant rainfall, the groundwater table rises and recharges the stream. Ground water occurs in AQUIFERS -- large bodies of rock that have both high porosity and highpermeability.Porosity refers to the percentage of the rock made up of open spaces that can hold water.Pore spaces can be the spaces between sedimentary grains, gaps made when rock dissolves, or cracks andfractures.• Well-rounded coarse-grained sediments usually have higher porosity than fine-grained sediments, because the grains do not fit together well.• Well sorted sediments and rocks hold greater amounts of water because there is a constant space between grains even when closest packed. Poorly sorted sediments usually have lower porosity because the fine-grained fragments tend to fill in the open space.• Since cements tend to fill in the pore space, highly cemented sedimentary rocks have lower porosity.• In igneous and metamorphic rocks porosity is usually low because the minerals tend to be intergrown, leaving little free space. Highly fractured igneous and metamorphic rocks, however, could have high porosityPermeability is a measure of the degree to which the pore spaces are interconnected, and the size of theinterconnections. Permeability refers to the ease with which water flows through the rock.• Rock makes a good aquifer when it can hold a lot of water (high porosity) and allow that water to flow easily (high permeability). Common aquifer-forming rocks are: sandstone, conglomerate, limestone, and fractured igneous rock (rock that has been cracked by weathering or by stresses in the crust).• Bodies of rock that have low permeability (do not allow water to flow through easily) are called an aquicludes. Such rocks form barriers to the movement of ground water. Common aquiclude-forming rocks are: shale and unfractured igneous rock.Porosity and Permeability influence movement and storage of groundwater.•Poorly sorted material have little open spaces between the grains (small grains fill little holes).•Well sorted sediments/rock is more porous.•Larger grains have the same % pore space as smaller grains but there is less friction between water and thegrains in larger grained material. Larger grain sizes have higher permeability.•Fractures also greatly increase the permeability.Impermeable layer of rock form an aquiclude. Because downward percolating water cannot go through thislayer, a small water table forms on top of the shale bed, above the level of the main water table. Thiscondition is called a perched water table.Water underground obeys gravity, just like water above ground. Both above and below ground, water flowsdownhill! If the water table has a slope, there will be pressure on the water to flow from higher areas tolower areas of the water table. The slope of the water table is called the hydraulic gradient.If the water table is flat there is no hydraulic gradient, and the water won’t flow. But if pressure is appliedat the pump the removal of water near the pump causes the water table to lower, forming a cone ofdepression in the water table. This creates a hydraulic gradient, and water flows “downhill” toward the well.The difference in elevation of the water table and the water level in the well is called the drawdown. The
    • amount of drawdown depends on the rate of pumping and the permeability of the rock.The water table will rise and fall as a function of water being added (recharge) and water leaving(discharge). A common pattern is for the water table to rise in the rainier season and lower in the drierseason. A well that is not drilled deep enough may run dry during the season when the water table is low.The concept of water flowing downhill underground (i.e. from high to low areas of the water table) is wellillustrated by these two types of streams.• In the influent stream (a), water flows away from the stream into the ground; the stream loses water to the water table.• In the effluent stream (b), water flows into the stream from the ground; the stream gains water from the water table.GROUNDWATER MOVEMENT/DARCYS LAW•Water flows from high head to low head due to gravity. Unless under pressure.•Slope of the water table. The steeper the slope, the faster the water will flow downhill.•Permeability of the aquifer. Groundwater flows faster in highly permeable aquifers.•Pressure. If the aquifer is under pressure, water flows faster.•The rate at which groundwater moves through the saturated zone depends on the permeability of the rockand the hydraulic gradient. The hydraulic gradient is defined as the difference in elevation (h1-h2) dividedby the distance between two points on the water table (L). –Velocity, V, is then: V = K(h2 - h1)/L•where K is the coefficient of permeability (Hydraulic conductivity); depends on the type of aquifer; it’s aconstant for a particular aquifer type.•If we multiply this expression by the area, A, through which the water is moving, then we get thedischarge, Q. –Q = AK(h2 - h1)/L, which is Darcys Law.Springs are places where water naturally flows out from the ground onto the surface. Springs are createdwhen geologic conditions cause the water table to intersect the earth’s surface.An aquifer can be either confined or unconfined. Unconfined aquifers are not bound by impermeable rocklayers. An artesian well results when there is an area of recharge to a confined aquifer (an aquifer bound bytwo impermeable layers). Pressure builds up within this layer.Some aquifers are confined aquifers, in which the water is “sandwiched” in between aquicludes. In this case,if the confined aquifer is sloping, the water becomes pressurized by the difference in elevation. Thispressure difference is called hydraulic head. In wells drilled into such aquifers the water rises upward dueto hydraulic head, and may even flow out onto the surface. Because the aquifer is confined, the water isunder pressure and will flow up to its potentiometric surface (imagine a U shaped pipe. Applying air pressureto one side might prevent water from ‘evening’ out in the pipe. Once the pressure is released, the water willflow up to a certain point….that’s the potentiometric surface).Artesian wells are wells in which the pressure (hydraulic head) in a confined aquifer causes the water to riseupward in the well. In some cases the pressure is enough to cause the water to “erupt” out of the ground!Once you tap into a confined aquifer, the pressure is released, and the water flows to its potentiometric
    • surface even if there is no land at that surface. The result is the water pumps itself right out of theground! This is an artesian well, and you’d rather have one of these pump itself than have to exert energy topump water from a regular well.Water towers take advantage of the physics of artesian well systems. Water companies will expend energypumping water into the tank once (as opposed to pumping water to each individual home). By creating apotentiometric surface at a higher altitude, the water, now under pressure, will flow through the pipe systemwithout pumping.Underground caverns near intrusive igneous heat sources forms hot springs and geysers. The groundwaterheats up, steam forms, pressure increases and then water eventually is released out of the chambersfollowed by a steam blast. Cold water enters the caverns and the process repeats. That’s why eruptions areintermittent. Water, steam and pressure need to build up.GROUND WATER PROBLEMS & MANAGEMENTYou now have a background into the main principles of ground water formation and movement. Now we willreview the following major issues related to ground water problems and water management:Ground Water Problems and Water Management1. Overdraft. The amount of water a particular aquifer can produce on a day-today basis indefinitely iscalled the sustained yield. If pumping exceeds the sustained yield, an overdraft situation exists, which willgradually lower the water table in the aquifer. This drives up pumping costs, causes shallow wells to go dry,and may cause subsidence problems.2. Salt water encroachment occurs in coastal areas where sea water rises up underneath areas where freshwater is being pumped, producing salty water. Mitigation requires maintaining high fresh water tables to pushthe denser sea water down to lower levels underground.3. Surface collapse. Solution of rock (particularly limestone) by acidic groundwater creates caverns,sinkholes, and karst terrane. Surface collapse in such areas causes damage. Collapse problems occur morefrequently where pumping lowers the water table.Dissolved materials and water quality. The amounts and types of dissolved ions in water determines itspotability (its purity and “drinkability”).5. Ground water pollution. Many types of pollutants can impair groundwater quality and cause healthhazards. Various methods to mitigate ground water pollution are used, depending on the problem.6. Conservation. Accomplished by personal voluntary conservation; by recycling of waste water; by water-saving irrigation devices; and through artificial recharge of aquifers (by trapping excess water during wetyears).Irrigation is the single largest user of fresh water (about 82% of water use world-wide), and the majority ofthat comes from ground water. Establishing conservation measures for irrigation is the single best way toconserve our fresh water resources.OverdraftAn aquifer that gets recharged regularly with new water by nature can sustain a certain amount of pumpingindefinitely. We define the sustained yield as the amount of water an aquifer can produce on a day-to-daybasis without being depleted. If an aquifer is pumped at rates that exceed the sustained yield, we have an
    • overdraft situation. In this case the water table will fall from year to year, as shown in this figure.An overdraft situation is often referred to as “ground water mining”, because the resource is being removedfaster than it can naturally form.The main problems caused by overdraft:- Increased costs in electricity to pump the water the extra distance up to the surface.- Shallower wells begin to run dry as the water table becomes lower.- Subsidence and/or surface collapse may occur as the water table becomes lower.Some of the worst overdraft problems occur in the High Plains Aquifer of the middle states of the U.S.,stretching from Texas to South Dakota. Groundwater in this region has been heavily pumped since the 1950sto support agriculture…(see next slide…)One solution to overdraft is to use artificial recharge to resupply an aquifer. Water may be brought in fromelsewhere and allowed to percolate into the ground to refill the aquifer.Salt Water EncroachmentIn coastal areas near the ocean, fresh ground water “floats” on denser seawater underground. The freshwater occurs in a curving, lens-shaped area. The thickness of this lens depends on the height of the freshwater table: the higher the fresh water table, the thicker the lens, and the farther down the area of saltywater.Ground water pumping that lowers the fresh water table too much allows sea water to rise up into wells,creating salty drinking water. This problem of salt water encroachment occurs in many heavily populatedcoastal areas, such as Long Island, NY (see figure below).Mitigation of salt water encroachment requires maintaining high fresh water tables to push the denser seawater down to lower levels underground.Surface CollapseGround water is slightly acidic, and over time it can dissolve away large amount of rock, particularlycarbonate rock like limestone. This forms caves and caverns underground, and sinkholes (collapsed caverns)above ground. This type of landscape is known as karst terrane.Karst topography is the landforms produced by groundwater. Specifically, groundwater that readilydissolves limestone. Active movement of groundwater with dissolved carbon dioxide/carbonic acid,effectively dissolves limestone beneath the ground.Sinkholes are circular shaped depressions that form when an underground cavern collapses. Water collectsin fractures in limestone and dissolves it. Fresh water enters and dissolution continues. As water travelsthrough joints and fractures in the underground limestone, large caverns, caves form. The overlying rockand soil (and whatever is atop that) becomes too heavy and it collapses. If the water table is high enough,the sinkhole will fill with water.Solution of limestone rock by ground water formed this cave, and precipitation of calcium carbonate byground water created the dramatic cave features shown here: stalactites, stalagmites, and columns.Land that has been completely perforated with sinkholes has the rugged and otherworldly appearance shownand is referred to as karst terrane.Because of the many holes that form from dissolution of limestone, streams that flow on the surface may‘disappear’ right into the ground. Conveniently they are called Disappearing Streams.Water Quality
    • Ground water is never completely pure. Natural water contains a certain amount of dissolved substances.The U.S. Public Heath Service has defined the maximum amount of dissolved materials public watersupplies can contain to be considered potable (safe and drinkable). The amounts are expressed in parts permillion, or “ppm”. For example, if water contains 2.3 ounces of dissolved iron for every 1 million ounces ofwater, we say the concentration of iron is 2.3 ppm.Some materials dissolved in water are important for health. Our bodies need calcium, magnesium, fluoride,and other substances. Some materials, such as arsenic or lead, can be hazardous even at low concentrations.Some materials, while not necessarily hazardous in small amounts, can give water an unpleasant taste (iron,zinc, copper), or make it less able to do useful things like lather soap and remove dirt (calcium, magnesium).Ground Water PollutionA pollutant is any kind of chemical, physical or biological substance that negatively affects water’s safetyand usefulness.There are, unfortunately, many sources of ground water pollution. In urban areas, ground water iscontaminated by leaking sewer systems, fuel storage tanks, runoff of pesticides, fertilizers, and highwaysalts, and wastes.In rural areas, ground water is polluted by runoff of pesticides, herbicides, and fertilizers from farm fields,and by leaking septic tanks and leaching of fecal matter from animal feed lots.Environmental consulting firms use groundwater modeling computer simulated programs to ‘predict’ the flowof groundwater (and its contaminants). They can estimate the direction of the ‘plume’, the time it takes fortravel and if or when it will resurface in other fresh water systems (or water wells). These data can help todetermine if remediation is necessary or not.Water ConservationWater is a renewable resource, but all too often it is used faster by people than it is naturally replenished.Water conservation must be a fundamental part of any sustainable water policy. What are the main wayswater can be conserved?1. Recycling of waste water. Treatment of waste water can be done to different levels of purity. Waterdoes not have to be potable to be useful. So called “gray water” can be used to irrigate public parks,roadsides, and certain crops.2. Water-saving irrigation devices. Irrigation is the single largest user of fresh water (about 82% ofwater use world-wide), and the majority of that comes from ground water. Conservation devices forirrigation represent the single best way to conserve our fresh water resources.3. Artificial recharge of aquifers. During wet years, water that would run off down rivers can be trappedand allowed to sink into the ground to recharge local aquifers.4. Personal voluntary conservation.SubsidenceUp to this point, we have discussed MASS WASTING -- the downward movement of material due directly togravity. Another type of “downward movement” that causes problems is SUBSIDENCE-- the sinkingdownward of the earth’s surface.Subsidence is not dangerous, but it does cause major economic problems in the form of earth fissures (largecracks in the ground), and damage to structures, pipelines, drainage systems, and sewer systems.Subsidence can be caused by natural processes. For example, large earthquakes commonly cause vast areasof land to sink downward in some places, and rise upward in other places.
    • But most problems involving subsidence are caused by human activities.The major causes of subsidence are from pumping of water, crude oil, or natural gas from deep underground.