GLY 120 401 Lecture 10 Groundwater

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Groundwater Lecture 10 GLY 120-401


The major source of groundwater is precipitation Other sources include Water that infiltrates from surface waters (lakes, rivers) Surface water deliberately injected into the groundwater system – artificial recharge Storm water retention or recharge ponds Agricultural irrigation


Aquifers Aquifer - A zone of earth material capable of supplying groundwater at a useful rate from a well A zone of earth material that will hold water but not transmit it fast enough to be pumped from a well is called an aquitard Aquitards often form a confining layer through which little water moves


An aquifer is called an unconfined aquifer if there is no confining layer restricting the upper surface of the saturated zone If a confining layer is present the aquifer is called a confined aquifer


Groundwater recharge – any process that adds water to an aquifer Can be natural or human induced Groundwater discharge – any process that removes water from an aquifer Can be natural – natural springs or human induced – pumping water from a well


Cone of depression – the depression in the water table that develops around a well from which water is being pumped


Water table – the upper limit of the zone of saturation Knowing where the water table is important in: Predicting the productivity of wells Explaining the changes in flow of springs and streams Accounting for fluctuations in the levels of lakes

Potential Problems Affecting Groundwater: 

Potential Problems Affecting Groundwater Groundwater depletion Pore collapse and land subsidence due to groundwater withdrawal Saltwater intrusion into aquifers Groundwater pollution


Groundwater Mining When groundwater removed from an aquifer exceeds the amount produced (i.e., demand > supply). NOT MINING-RELATED! Too much demand and too little rainfall. Northern and Eastern states - adequate groundwater supplies Southwest U.S. = inadequate supplies Groundwater Depletion


Case Study - High Plains Aquifer The High Plains Aquifer is a very large confined aquifer formed in the Ogallala Sandstone that underlies much of the High Plains (NE, KS, OK, TX) and is used for irrigation. It was filled (recharged) following the Ice Age (~10,000 years ago) The aquifer contains “fossil” water, the product of a wetter ancient climate associated with the end of the last ice age There is no sufficient contemporary source for water to recharge the whole aquifer Today = little recharge and demand greatly outweighs supply.


In the 1940's (before groundwater pumping occurred) the average saturated zone was > 60 meters thick. By 1980 the average saturated zone ~3 meters thick. 30-60 meter decline in some areas


Problems: Currently water is being extracted at 100 times the natural replacement rate Poor irrigation practices Losing substantial water to evaporation Little incentive to conserve water Government prices support the farming of water-hungry crops Lose water-use rights if a minimum amount of groundwater is NOT extracted each year


In the 1980's, the situation improved due to: better water management (well meters, waste water reuse) heavy rains new technologies - new irrigation nozzles that decrease evaporation loss by up to 98% over previous methods. Projection - 25% of the water in the High Plains aquifer will be used by 2020. UNSUSTAINABLE USAGE


SUBSIDENCE Subsidence - a sinking or downward settling of the earth's surface Typically irreversible Subsidence is observed in 45 US states, estimated damage is $125 million per year. Usually not associated with loss of life


Deep Subsidence Gradual compaction of sediment caused by withdrawal of fluids (groundwater or oil) from the subsurface over large regions usually human-induced late recognition tends to make the problem worse example regions: San Joaquin Valley, CA; Houston / Galveston, TX; Venice, Italy; and Mexico City, Mexico


Pore Collapse and Land Subsidence Pore Collapse When groundwater fills the pore space of a rock, it holds the grains of the rock or regolith apart Water cannot be compressed The extraction of water from a pore eliminates the support holding the grains apart The air that replaces the water can be compressed As a result, the grains pack more closely together This pore collapse permanently decreases the porosity and permeability of a rock Porosity = the ability to hold fluid Permeability = the ability to allow fluids to pass through


Land Subsidence Pore collapse can also decrease the volume of the aquifer The result is that the land above the aquifer sinks This land subsidence may cause fissures at the surface to develop and the ground to tilt The Leaning Tower of Pisa, in Italy, tilts because the removal of groundwater caused its foundation to subside Effect is most severe for clay- and organic-rich sediments


In coastal areas, land subsidence may even make the land surface sink below sea level The flooding of Venice, Italy, is due to land subsidence accompanying the withdrawal of groundwater


Damaging Effects of Subsidence Construction damage (buildings, roads, dams, etc.) Alteration of landscape Reduction of aquifer Increased risk of flooding (lowered land surface)


Case Study - Groundwater extraction San Joaquin Valley, CA (1925-1975) Location - flat basin between two mountain ranges - Sierra Nevada Mountains (E) and Coast Range (W). > 5000 km2 in central California subsided up to 8.93 meters (29.3 ft) Cause - overuse of groundwater (for agriculture) over a period of 50 years, linked to declining water table


Effect - subsidence was greatest on the west side (underlain by finest-grained sediment - easiest to compact) Reduced porosity and permeability (loss of aquifer!) Mitigation - greater use of surface water for agriculture Conclusion - region is still subsiding today (but much more slowly)

Saltwater Intrusion: 

Saltwater Intrusion In coastal areas, fresh groundwater lies in a layer above saltwater that entered the aquifer from the adjacent ocean Saltwater is denser than fresh water, so fresh groundwater floats above it If water is pumped out too quickly, the boundary between the saline water and the fresh groundwater rises If this boundary rises above the base of the well, the well will start to yield useless saline water

Groundwater pollution: 

Groundwater pollution Pollutants and sources that contaminate surface waters can also pollute groundwater. Examples: leaky landfills, industrial waste lagoons, agricultural activities, and underground storage tanks. Groundwater pollution may go unnoticed and undetected for a long time period. WHY? Groundwater moves very slowly.


Groundwater Contamination Rocks and sediment are natural filters capable of efficiently removing suspended solids (mud and solid waste) from groundwater These solids get trapped in the tiny pathways between pores Clay flakes can remove certain ions from water They have electrically charged surfaces However, invisible organic and inorganic chemicals may be carried along with flowing groundwater Some dissolved chemicals are toxic – arsenic, mercury, lead Others are not – salt, iron, lime, sulfur


In recent decades, human activity has increasingly introduced contaminants into aquifers These contaminants include Agricultural waste – fertilizers, pesticides, and animal wastes Industrial waste Effluent from “sanitary” landfills and septic tanks Petroleum products Radioactive waste – from weapons manufacture, power plants, and hospitals Acids leached from sulfide minerals in coal and metal mines


Some of these contaminants Seep into the ground from subsurface tanks Infiltrate from the surface Are intentionally forced through injection wells The cloud of contaminated groundwater that moves away from the source of contamination is called a contaminant plume


Contaminant Plume


3-D image of a contaminant plume


Most groundwater contamination occurs in shallow unconfined aquifers located near a contaminant source.


Common sources Leaking underground storage tanks (gasoline stations) before 1980, these tanks were made of metal and tended to corrode and leak pollutants into the soil and groundwater. Now, leak detectors are required, old storage tanks are being dug up and replaced, and the soil has to be cleaned up or destroyed (incinerated). In Denver, 80 liters of organic solvents contaminated 4.5 trillion liters of groundwater, affecting an area 5 km in length!


Septic tanks - bacterial/chemical pollution can be released when there are heavy rains and clay-rich soils cause the water to float the raw sewage to the surface extremely permeable or fractured bedrock does not effectively filter the waste as it passes through


Light and heavy immiscible contaminants Heavy: TCE (trichloroethene) Light: gasoline


Groundwater Treatment Reduce or stop input and then wait for nature to remove or destroy pollutants Easiest and least costly But, SLOW and a long-term hazard to the environment Extract groundwater Pump out the contaminated water and try to treat it Remove the soil and treat or destroy it Problem – extraction is not very efficient or productive In-situ Add chemicals to immobilize heavy metals Add oxygen or nutrients along with microorganisms to stimulate them to munch on the pollutants Common approach for oil spills

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