Groundwater and Subsidence Chapter 9 Sinkholes Land Subsidence and Swelling Soils Subsidence Refer to pages 231 238 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 Refer to pages 231 238 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 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 Refer to pages 231 238 Case in Point page 242 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 Refer to pages 231 238 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 Refer to pages 231 238 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 Refer to pages 231 238 Case in Point page 244 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 Encroachm ent 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 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 Surface Collapse Refer to pages 226 231 Karst Terrain Refer to pages 226 231 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 Refer to pages 226 231 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 Refer to pages 226 231 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 Groundwater Pollution A pollutant is any kind of chemical physical or biological substance that negatively affects water s safety and usefulness
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