Groundwater and SubsidenceChapter 9Sinkholes, Land Subsidence, and Swelling Soils.Groundwater and SubsidenceChapter 9Sinkholes, Land Subsidence, and Swelling Soils. 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 (California and Texas). The Water TableGoing 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. 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. 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 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. OverdraftAn 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.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. Overdraft and Salt Water EncroachmentIn 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 ofsalt water encroachment occurs in many heavily populated coastal areas, such as Long Island, NY (see figure on the next slide). Water ConservationWater 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 downrivers can be trapped and allowed to sink into the ground to recharge local aquifers. 4. Personal voluntary conservation.Surface CollapseGround 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 landscapeis known as karst terrane. - Solution of limestone rock by ground water can form caves, and precipitation of calcium carbonate by ground water create dramatic cave features: stalactites, stalagmites, and columns. - When an underground cavern collapses it forms a sinkhole.Water QualityThe 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). Ground Water PollutionA pollutant is any kind of chemical, physical or biological substance that negatively affects water’s safety and
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