Chapter 19 GROUNDWATER MAJOR GLOBAL RESERVOIR EXTRACTING GROUNDWATER Is this a renewable resource DESCRIBING WATER IN SOIL most of what exists is not accessible Either too deep or brackish There is always some level of water in soils Usually some fraction of voids or spaces in soil Saturated zone pore spaces are lled with water GROUNDWATER ZONES POROSITY PERMEABILITY Porosity how much void space there is Ranges of different porosities in different soils and rocks POROSITY PERMEABILITY Permeability how connected voids are Top impermeable soil Bottom permeable soil AQUIFERS NOT underground rivers Aquifers are formations soils rocks that transmit water Aquitards do not transmit water water easily Con ned aquifers separated from the surface by an aquitard ARTESIAN WELLS These are wells that ow at the surface without pumping How can this happen THE HIGH PLAINS AQUIFER a k a Ogallala Aquifer HIGH PLAINS AQUIFER Dakota sandstone makes up most of this aquifer Ogallala has been signi cantly WATER TABLE VARIABILITY Fluctuations due in part to surface water changes During rainy season water table is depleted higher WATER TABLE VARIABILITY In some regions groundwater height is driven by rainfall and in ltration Thus groundwater can be a renewable resource NOT ALWAYS A RENEWABLE RESOURCE Travel times depends on the material porosity permeability and pressure Flow is from areas of high pressure to low pressure Pressure is characterized by GROUNDWATER AND TOPOGRAPHY The groundwater table somewhat follows topography The water table can be above sea level GROUNDWATER FLOW PATHS hydraulic head POTENTIOMETRIC SURFACE The height to which water rises in a standpipe Water tower gives water height and potential energy Friction in pipe causes potentiometric surface to diminish Wells are artesian when potentiometric surface is above ground Surface ARTESIAN WELLS GROUNDWATER FLOW Flow is controlled by Hydraulic gradient Hydraulic conductivity related to permeability Hydraulic gradient is change in the potentiometric surface divided by distance SUBSURFACE CAN BE COMPLEX Perched water table zone of saturation perched on an aquitard below which is an unsaturated zone SOUTHERN CALIFORNIA GROUNDWATER MODELINGWHAT HAPPENS In places we have pumped until the water table is out of reach in a few decades Often water has been stored for millions of years DURING PUMPING Con ned aquifer hydraulic head lowers Uncon ned aquifer Water table lowers PUMPING FASTER THAN RECHARGE OGALLALA DEPLETION Began pumping for irrigation in the 1930s Around 13 million acres of irrigated land Has declined over 150 feet in some places Estimate we have used up 0 of the water here SUBSIDENCE Major problem in Central Valley GROUNDWATER FROM SPACE GRACE INDIA AND GROUNDWATER Massive amounts of pumping have lowered reservoir levels In some places it is no longer economical to pump INDIA GROUND WATER AND THE GREEN REVOLUTION High yield rice helped avoid a food crisis in the 1960s High yield rice requires more water SALTWATER INTRUSION Originally water recharged aquifers beneath Florida from Lake Okeechobee to feed the Everglades Pumping has disrupted groundwater ow One result is that salty sea water intrudes into aquifers Also a problem in Southern California SALTWATER INTRUSION Puri cation systems are not capable of dealing with the saltwater We end up using additional pumps to push the saltwater back in order to solve this GROUNDWATER CONTAMINATION Once contaminants get into groundwater how do they move Pumping can signi cantly change hydraulic head and piezometric surface