Chapter FifteenChapter FifteenGroundwater Groundwater

GroundwaterGroundwater Earth’s hydrosphere extends from top of atmosphere to ~

10 km (6 mi) below the Earth’s surface. Groundwater, 0.6% of world’s water, accounts for 97% of Earth’s supply of unfrozen water.

Surface waters from rivers, lakes or precipitation enter the ground under the influence of gravityThe amount of water that enters the ground depends on

the topography, geologic composition and vegetation The surface water will percolate downwards until it

reaches a zone where all the pore spaces are saturated with water

Factors affecting infiltrationFactors affecting infiltration

Moisture through treeMoisture through tree

Surface distribution of waterSurface distribution of water

Movement and Distribution of GW Movement and Distribution of GW

Water bound to clay mineralsZone of aeration: unsaturated zoneZone of SaturationWater Table: Upper surface of the zone of

saturationCapillary Fringe: Lower part of the aeration zone –

range from few tens of cm to several meters

Groundwater MovementGroundwater Movement For groundwater to flow- need porous and permeable

materials Porosity is the percent of pore spaces in relation to the total

soil/rock/sediment volume (Primary Porosity: Porosity that develops as a rock forms; Secondary Porosity: Develops after a rock has formed)

Permeability is a measure of the ability of rock/sediment to transmit fluid

Groundwater flow occurs when there is a hydraulic gradient Hydraulic conductivity is the ability to transmit groundwater.

Typical groundwater flow is slow ~0.5 – 1.5 cm/day (0.2 - 0.6 in/day). The slow movement stores the water making it available to humans.

The subsurface distribution of waterThe subsurface distribution of water

Primary porosity of sedimentary rocksPrimary porosity of sedimentary rocks

Pore connection and PermeabilityPore connection and Permeability

Pore connection and Permeability – contd.Pore connection and Permeability – contd.

Hydraulic gradientHydraulic gradient

Variations in water table depthVariations in water table depth

Water Table configurationWater Table configuration

Perched water poolsPerched water pools

Tapping Groundwater aquiferTapping Groundwater aquifer Groundwater depth is a reflection of local topography

and prevailing climate Aquifers are permeable water-bearing bodies of

geologic materialsUnconfined and Confined aquifers

Aquiclude and Aquitard Artesian flow- water flows to the surface from wells

without pumpingPotentiometric surfaceNatural SpringsGeysers- intermittent surface emission of hot

water/steam

How groundwater flowsHow groundwater flowsGroundwater flow potential

– Gravity– Pressure from overlying water and rocks– Hydraulic Gradient: Difference in potential / distance– Hydraulic conductivity: Measure of conductivity –

size, shape & degree of sorting of its grains – coarse, well-rounded, well-sorted and gravel – high conductivity

– Darcy’s principle (19th Century): Rate at which water flows is difference in potential

hydraulic conductivity

Groundwater flow – contd.Groundwater flow – contd.Mesuring the Rate of GW Flow:

– Dye injection method– Carbon –14 dating method

Groundwater Flow Rate:

Average: 0.5 to 1.5 cm/day

Fastest: 100 m / day

Composite LandscapeComposite Landscape

Artesian aquiferArtesian aquifer

Desert OasesDesert Oases

Municipal water towerMunicipal water tower

Natural springsNatural springs

Natural springs – contd.Natural springs – contd.

GeysersGeysers

Locating groundwater and features associated with pumpingLocating groundwater and features associated with pumping

Locating GroundwaterMapping

Water table from well records, lake, river levels, drilling test holes

Features Associated with PumpingCone of depression- local depression in water around

pumping well Subsidence- compressed land due to aquifer depletionSalt water intrusion- due to over pumping along coast line

Greater demand for water- leads to groundwater depletion and introduction of pollutant

Solutions- enhance recharge, water transfer, conservation, desalinization, and use of iceberg

Search for GroundwaterSearch for GroundwaterCharacteristics of the Water Table:

– Depends on Typical depth– Relationship to topography– Response to Climatic Fluctuations

AQUIFERS:– Aquifer: Permeable, water-bearing bodies of

geological material– Unconfined aquifer: Not overlain by impermeable

cap tock– Confined Aquifer: Found in greater depths, between

aquiclude (impermeable rock layers) and Aquitard

Search for waterSearch for water

Overuse of groundwaterOveruse of groundwater

Water table around a wellWater table around a well

The effect of development on a water tableThe effect of development on a water table

Subsidence in a coastal areaSubsidence in a coastal area

Subsidence in a coastal area – contd.Subsidence in a coastal area – contd.

Groundwater contamination / purificationGroundwater contamination / purification

Natural occurring solutes- Chlorine, Arsenic, Mercury, and Selenium

Manufacturing contaminants- from household products: cans of paint, solvent, cleanser etc.

Natural Groundwater PurificationFiltration- adhere to clay particlesDecomposition- some decompose completely

by oxidationBacteria action- organic solids consumed by

microorganis

Saltwater intrusionSaltwater intrusion

Salt water intrusion – contd.Salt water intrusion – contd.

Stratigraphy of Yucca MountainStratigraphy of Yucca Mountain

Landfill sitesLandfill sites

Landfill sites – contd.Landfill sites – contd.

Products of GroundwaterProducts of Groundwater Karsts

Water reacts with carbon dioxide to form weak carbonic acid which then attacks limestone

H2O + CO2 => H2CO3 + CaCO3 => Ca++ HCO3 –

Caves- natural underground cavities and most common geological product of limestone dissolution

Cave deposits- spelotherms are deposits on cave surfaces in a variety of forms: travertine, Stalactite, Stalagmite, banded draperies or drip curtains

Growth of spelotherm- depends on the solution and porosity of surface material, climate, topography, and vegetation

State 1 of cave formationState 1 of cave formation

State 2 of Cave formationState 2 of Cave formation

Stalactite, stalagmite and travertineStalactite, stalagmite and travertine

Cave formationCave formation

Speleothem growth and climateSpeleothem growth and climate

Rise and fall of sea levelRise and fall of sea level

Karsts TopographyKarsts Topography

Surface expression of the geology of dissolved limestone and work of near surface water

Cave and Karsts landscapes are extremely sensitive- so need to be protected

LandformSinkholes-circular surface depressionDisappearing Streams- flow through sinkholes may

emerge as spring several kilometers awayNatural Bridge- series of neighboring sinkholes

expand and join together

Worldwide distribution of karst landformWorldwide distribution of karst landform

Typical landforms associated with karst topographyTypical landforms associated with karst topography