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Natural Disasters Groundwater and Subsidence Presentation
 

Natural Disasters Groundwater and Subsidence Presentation

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    Natural Disasters Groundwater and Subsidence Presentation Natural Disasters Groundwater and Subsidence Presentation Presentation Transcript

    • Groundwater and Subsidence Chapter 9 Sinkholes, Land Subsidence, and Swelling Soils.
    • Subsidence SUBSIDENCE-- the sinking downward of the earth’s surface. • Subsidence is typically not dangerous, but it does cause major economic problems in the form damage to structures, pipelines, drainage systems, and sewer systems. • Subsidence can be caused by natural processes, but most problems involving subsidence are caused by human activities. Some of the most severe subsidence problems in the U.S.A. occur in agricultural areas with intensive groundwater pumping.
    • Subsidence This map shows water consumption by state. California and Texas are #1 and #2 users of water respectively. Both states have extensive irrigation agriculture.
    • The Water Table Going into the ground, one passes through the zone of aeration - the region where open spaces in the rock are filled mostly with air - and passes into the zone of saturation, where the open spaces are filled with water. The contact between these zones is the water table.
    • Aquifers Ground water occurs in AQUIFERS -- large bodies of rock that have both high porosity and high permeability. 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 (a, b), gaps made when rock dissolves (c ), or cracks and fractures (d ). Permeability refers to the ease with which water flows through the rock.
    • Hydraulic Gradient Water underground obeys gravity, just like water above ground. Both above and below ground, water flows downhill! The hydraulic gradient is the slope of the water table or aquifer . If the water table is flat there is no hydraulic gradient, and the water won’t flow. But if pressure is applied at the pump shown here….(next slide).
    • Hydraulic Gradient …the removal of water near the pump causes the water table to lower, forming a cone of depression in the water table. This creates a hydraulic gradient, and water flows “downhill” toward the well.
    • Cone of Depression This figure illustrates nicely in 3-D how cones of depression form in the water table around pumping wells.
    • Overdraft An aquifer that gets recharged regularly with new water by nature can sustain a certain amount of pumping indefinitely: sustained yield. 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.
    • Subsidence Subsidence is caused by pumping of water, as well as other materials such as crude oil, or natural gas from deep underground. When fluid (water, oil, natural gas) is pumped out of the ground pressure on the surrounding rock particles is reduced, the particles settle closer together, and the ground sinks.
    • Subsidence Long Beach, CA, has experienced as much as 30 vertical feet of subsidence as oil has been pumped from deep underground. The building and parking lot at the left are several feet below sea level. Walls hold back the ocean, and boat owners walk uphill from the parking lot to get onto their boats!
    • Subsidence This dog is thinking “How can I use that hydrant way up there?!” The hydrant was at ground level when it was installed. The hydrant was held in place by the piping system as the ground subsided around it. (This is in Long Beach, CA; same as previous slide).
    • Subsidence Areas of Mexico City have subsided as ground water has been pumped out from the sedimentary layers beneath the city. This church’s foundation was built half on firm bedrock and half on sedimentary layers that subsided as water was withdrawn. Its pretty easy to tell here which side is which!
    • Overdraft and Salt Water Encroachment In coastal areas near the ocean, fresh ground water “floats” on denser seawater underground. 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 populated coastal areas, such as Long Island, NY (see figure on the next slide).
    • Overdraft and Salt Water Encroachment
    • Water Conservation Water is a renewable resource, but all too often it is used faster by people than it is naturally replenished. What are the main ways water can be conserved? 1. Recycling of waste water. Treatment of waste water can be done to different levels of purity. Water does not have to be potable to be useful. So called “gray water” can be used to irrigate public parks, roadsides, and certain crops. 3. Artificial recharge of aquifers. During wet years, water that would run off down rivers can be trapped and allowed to sink into the ground to recharge local aquifers. 4. Personal voluntary conservation.
    • Surface Collapse Ground water is slightly acidic, and over time it can dissolve away large amount of rock, particularly carbonate 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 Terrain Land that has been completely perforated with sinkholes has the rugged and otherworldly appearance shown here. This region of southern China is classic karst terrain.
    • Karst Terrain Solution of limestone rock by ground water formed this cave, and precipitation of calcium carbonate by ground water created the dramatic cave features shown here: stalactites, stalagmites, and columns.
    • Sinkholes When an underground cavern collapses it forms a sinkhole, such as the one shown here in Winter Park, FL. Several homes and cars were destroyed (“swallowed” really!) by this sinkhole.
    • Water Quality The U.S. Public Heath Service has defined the maximum amount of dissolved materials public water supplies can contain to be considered potable (safe and drinkable). Some materials dissolved in water are important for health: 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 or smell (iron, zinc, copper. sulfur), or make it less able to do useful things like lather soap and remove dirt (calcium, magnesium).
    • Groundwater Pollution A pollutant is any kind of chemical, physical or biological substance that negatively affects water’s safety and usefulness.