groundwater!
hydrologic cycle! 2
global distribution of water! Source Volume Percent Ocean 97.2000 Glaciers and other ice 2.1500 Ground Water 0.6100 Lakes fresh 0.0090 saline 0.0080 Soil Moisture 0.0050 Atmosphere 0.0010 Rivers 0.0001 need 400 million gallons for basic biological needs per day for US! need 1.8 trillion gallons per day for other purposes in US! "---domestic, agriculture, industry!
water use in the US from 1950-2000! (ground water in red)!
ground water withdrawal in 2000 from selected states! (data from US Geological Survey)! Alabama " 440! Arkansas "6,920! California 15,400! Colorado "2,320! Florida "5,020! Kansas "3,790! Missouri "1,780! Nebraska "7,860! Texas " "8,970! (in millions of gallons per day)! Arkansas use is quite high--state dependent on ground water!
groundwater! lies beneath the surface! fills pore (empty) spaces and fractures in rocks! is resupplied by infiltration/percolation! is generally cleaner than surface water! is accessed by wells!
ground water definitions! porosity! percentage of rock by volume that consists of voids or openings! permeability! --measure of the rock s ability to hold water--! capacity of a rock to transmit fluid through pore space/fractures! --rock is permeable if fluid flows easily--! porous = permeable! same porosity, but different permeability! importance of connectivity of pore space!
loose sand has 30%-50% porosity! compacted sand has 10%-20% porosity! crystalline rock (e.g. granite) has 0%-5% porosity!
water table! top of saturated zone! saturated zone! zone in subsurface where! all pore space is filled! with water! vadose zone! zone in subsurface above! water table that! is unsaturated! water level of most! lakes and rivers! corresponds to! local water table! capillary fringe: downward flow is! counteracted by upward attraction of molecules (like paper towels absorbing water)!
perched water table: water table above zone of saturation! " " " " " isolated!
water table!
ground water movement! movement through pores and fractures is slow (cm/m per day)! (flows in limestone caverns may be much higher)! velocity depends on slope of the water table and permeability of rock!
groundwater movement! water table! hydraulic head (h) = elevation + pressure! no pressure at points A and B on water table! --on water table so no water is above them! elevation A > elevation B! hydraulic head of A > B! hydraulic gradient =! (h at point 1) - (h at point 2)! distance between 2 points! h. g. = h/l! water flows from zones of high! high hydraulic gradient to! low hydraulic gradient! h at C > D! (greater pressure at C)! h at F > G! ( elevation of F, G < pressure of F, G!
remember that water flows to lower hydraulic gradient! A! B! black lines show regions of equal hydraulic head! water flows perpendicular to the black lines and in! the direction of downward slope of water table! blue dots at A and B have higher hydraulic gradient! than associated red dots (more pressure)!
groundwater recharge! replenishment of groundwater by precipitation! water infiltrates from surface! infiltration increases! "cracks and pore space! "vegetation (roots enhance infiltration),! "shallow slopes (more time for water to sink),! "greater precipitation (if fine particles do not wash into pores)! water flows down through soil and rock in response to gravity! some water emerges as groundwater discharge! some water remains stored in ground for long periods of time!
groundwater: through what does it flow?! aquifer! body of saturated rock or sediment through which! water can move easily! examples: sandstone, conglomerate, sand, gravel, fractured rock! fractured rock! aquitard! body of rock with low or no permeability, which! retards, or slows, flow of water across it! examples: shale, clay, unfractured crystalline rocks!
aquifers: confined and unconfined! aquitard:! is impermeable! confined aquifer:! is filled with water! is separated from surface by an aquitard! unconfined aquifer:! has a water table that rises and falls during wet and dry seasons! recharges quickly through precipitation,! has rapid movement of groundwater through it!
well! hole drilled or dug into ground to access water in aquifer! in an unconfined aquifer, water level! before pumping is the water table! pumping draws water from pore space! and may cause drawdown! drawdown results in! cone of depression! (lowering of water table)! if cone of depression grows too large,! water table may fall below well bottom!
example: Mississippi Alluvial Aquifer system!
central Arkansas! Stuttgart map shows growth of region! where drawdown has! produced a! cone of depression! at a depth of 110 feet! DeWitt when pumping began! in 1930 s,! depth to water table! was about 50 feet!
artesian well! well in confined aquifer yields water that rises above surface! water rises to water table of same aquifer where it is unconfined! rocks must be tilted to do this!
artesian well!
how does an artesian well work?! potentiometric surface! level to which water would rise in a pipe in absence of friction! consider water tower and building (operates like Artesian well):! live up here: low water pressure! potentiometric surface! live down here: no problem! water flow!
springs! where water flows naturally from rock or sediment! water table is! on the surface! dependent on precipitation!
spring along the Buffalo River!
streams: gaining vs. losing! gaining stream! gets water from saturated zone! and top of stream is! water table! losing stream! loses water to saturated zone! and creates! mound on water table! continued loss of water! results in dry stream!
groundwater in limestone: caves, karst, sinkholes! caves! sinkholes! karst! naturally formed underground chambers! acidic groundwater dissolves limestone! (rainwater becomes acidic as it passes through air and soil)! caves near surface that collapse! landscape characterized by dissolution features!
caves! dripstones (speleothems):! stalactites (ceiling)! stalagmites (floor)! most caves in limestone! --other soluble rocks also form caves--! Alabaster Caves, Oklahoma (gypsum)!
sinkholes! collapse of near surface, underground cavities!
sinkholes may fill with water -- provide resources for humans! Mayan civilization dependent upon them! common in Florida!
Arecibo Observatory is in giant sinkhole in Puerto Rico!
sinkhole lakes! sinkhole lakes can disappear when debris that clogs lower opening becomes unclogged 2002! Lake Jackson, Tallahassee, Florida! 2003! dried up and re-filled at least 4 times in 20th century! lake drains when material plugging hole in subsurface is removed!
karst! rainwater infiltration! cave development! limestone dissolution! groundwater acidification! voids in limestone! precipitation of calcite! (dissolved limestone)! lowering of water table! dripstone development! very sensitive to water table!
karst features! disappearing streams! natural bridges!
karst topography! southern China! north coast of Puerto Rico! Ozarks are karst! Jamaica (radar satellite image)!
groundwater contamination! infiltration of contaminants into water table! pesticides/fertilizers! landfill pollutants! industrial chemicals! heavy metals! bacteria/parasites from sewage! oil and gas! pharmaceuticals! animal waste!
groundwater contamination! expensive and difficult to clean up! contamination plume moves and grows!
groundwater contamination! different materials lead to different problems!
groundwater contamination! septic tanks! plume cleaned as it flows slowly through sandstone! well produces clean water! plume flows quickly through fractured crystalline rock! well is contaminated!
groundwater contamination! saltwater intrusion! created by over-pumping! and! movement of salt water! into aquifer!
contamination in karst! difficult to know where water goes! dump something in! hole here.! it may end up here! and be pumped to! surface here!
balancing withdrawal and recharge! pumping > recharge! lowers! water table! (cone of depression)! eventually yields! dry wells! and/or! subsidence!
subsidence! sinking of ground surface! in response! to removal of water! rock: pore space filled with water! (buoyant and holds rock up)! water pumped from pore space! pores collapse (nothing in them)! and rock loses volume (sinks)!
collapse of! confined aquifer! from removal! of water!
hot water underground! hot springs! water temperature is warmer! than that of human body! water heated by nearby magma! or by deep circulation to deep! crust, which is warmer! hot water less dense than cool! and rises on its own! geysers! hot springs that! periodically erupt! hot water and! minerals! geyser just! starting! to erupt! calcium carbonate deposits travertine! from hot water--forms terraced pools! steam! geyserite! deposits!
geothermal energy! produced using natural steam! or superheated water! no CO 2 or acid rain is produced! "( clean energy)! directly heats buildings! some toxic gases are emitted! "(e.g. sulfur)! superheated water can be! "corrosive to pipes! Iceland is a leader!