GroundwaterGroundwater

HydrogeologyHydrogeology (hydro- meaning water, and -geology meaning the study of the Earth)

It is the area of geology that deals with the distribution and movement of groundwater in the soil and rocks of the Earth's crust (commonly in aquifers).

The hydrologic cycle begins with the evaporation of water from the surface of the ocean. As moist air is lifted, it cools and water vapor condenses to form clouds. Moisture is transported around the globe until it returns to the surface as precipitation.

•PrecipitationPrecipitation and and infiltrationinfiltration contribute to contribute to groundwatergroundwater, which is stored , which is stored in underground reservoirs in underground reservoirs until it surfaces as a spring until it surfaces as a spring or is drawn from a well.or is drawn from a well.

Infiltration:Infiltration:• process of water process of water

moving into the moving into the soil/groundsoil/ground

• to penetrate, to penetrate, enter or gain enter or gain accessaccess

I. Water Beneath the I. Water Beneath the SurfaceSurface

Groundwater:Groundwater: underground underground water that fills almost all of water that fills almost all of the the pores/voids pores/voids in rocks and in rocks and sedimentsediment

AquiferAquifer: : a body ofa body of rockrock that can that can store water and also let water store water and also let water

flow throughflow through

Think of an AQUIFERAQUIFER as a…• large, horizontal spongesponge that

absorbs and transports water along its length

An aquifer is an underground layer of water-bearing permeable rock, rock fractures or unconsolidated materials (gravel, sand, or silt) from which groundwater can be extracted using a water well.

It is porous & permeable to the extent of maintaining a steady supply of sufficient amount of water to the wells or springs.

Occurrence of Groundwater in different terrains –

•Weathered – pore spaces & voids/openings

•Hard and Stratified rocks – cracks & fractures

Aquifer types

• Aquifer• Aquiclude• Aquitard• Aquifuge

Unconfined aquiferConfined aquifer( artesian)Perched

Aquifer : A saturated, permeable, geologic unit / formation or group of formations that can yield a significant amount ( sufficient quantity) of groundwater to be called as a source of supply.Aquiclude :A saturated geologic unit which yields inappreciable quantity of water to drain & through which there is inappreciable movement of water.

Aquitard : A saturated, permeable, geologic unit / formation or group of formation which yield water in appreciable quantity to drain completely to an aquifer but through which appreciable leakage of water is possible.

Aquifuge : A formation that does not store any water.

Unconfined

Unconfined aquifer• Ground water possesses a free

surface open to the atmosphere • • Upper surface of the zone of

saturation is called water table

Confined

Confined aquifer( artesian)

• It is one in which ground water is confined under pressure by overlying & underlying aquitard or aquicludes.

• Water level in wells rises above the top of the aquifer

Potentiometric Surface or Piezometric surfaceRepresents the areal variation of the head of an aquifer

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City water systems can be considered to be artificial artesian systems.

ParametersParameters Porosity

Permeability

Specific yield & retention

Transmissibility and Storage Coefficient

porosityPorosity “n” is the ratio of the voids to the total

volume of an unconsolidated or consolidated material

percentage of open spaces in a rock or sediment

Porosity:

FACTORS THAT INFLUENCE FACTORS THAT INFLUENCE POROSITY:POROSITY:

(a) SORTING: amount of uniformity in particle size

HIGH pore space LOW pore space

(b) SHAPE:(b) SHAPE: rounder particles = more porous

(c) PACKING:(c) PACKING: loosely packed = more porous

Permeability: It is the property by virtue of which the rock/ geological formation allows water to travel through its pore spaces or other openings

2. Permeability: how freely water flows through open

spaces in a rock (the pores must be connected)

Impermeable – water can NOT flow through

It is the measure of the quantity of water passing through a unit cross section in a unit time & 100 % hydraulic gradient.

Can something be porous Can something be porous but impermeable?but impermeable?

• The cork end is The cork end is red because the red because the grape juice went grape juice went into it. Does this into it. Does this mean it is porous?mean it is porous?

• If you turn the If you turn the bottle upside bottle upside down will it leak? down will it leak? Is the cork Is the cork permeable?permeable?

A permeable rock is always porous but a porous rock may not always be permeable

Porosity & permeability depend on width of fracture, frequency, distribution & orientation .

Fine grain sediments have the tendency to hold up water against the force of gravity

Coarse grained well sorted sediments yield water readily

In case of consolidated rocks, dimensions of secondary openings, joints, cavities largely control the specific yield.

Specific retention depends on total interstitial surface in the rock.

B. Zones of GroundwaterB. Zones of Groundwater1. 1. Zone of Saturation:Zone of Saturation: the layer the layer

of ground where all the pores of ground where all the pores are filled with water.are filled with water.

2.2. Water Table: Water Table: upper surface upper surface of the zone of saturationof the zone of saturation

3. 3. Zone of Aeration: Zone of Aeration: upper region, upper region, where water is both in the soil and where water is both in the soil and seeping down to the water table.seeping down to the water table.

Zone of Aeration

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• Capillary Action: ability of water to rise UP the soil.

• Smaller particle size = greater capillarity

Capillary Fringe:Capillary Fringe: middle layer, middle layer, between the water table and the between the water table and the

zone of aeration.zone of aeration.

C. MOVEMENT OF C. MOVEMENT OF GROUNDWATERGROUNDWATER

• Depends on:1.Permeability of the aquifer

• ↑ permeability = ↑ velocity2.Gradient of the water table

• Gradient = steepness of the slope

• ↑ gradient = ↑ velocity

II. Wells and SpringsII. Wells and Springs• Water Table:Water Table: upper layer of upper layer of

rocks with all pores filled with rocks with all pores filled with water.water.

A. Ordinary Well:A. Ordinary Well: A hole dug A hole dug below the water table which below the water table which fills up with water.fills up with water.

Cone of Depression: Cone of Depression: lowered lowered area of the water table due to area of the water table due to

pumpingpumping

B. Artesian Wells: water flows freely with no pumping

• Underground water under pressure (between two impermeable rock layers) rises to the surface

• Water is naturally filtered as it passes through porous rock and seeps to surface

III. Groundwater and III. Groundwater and Chemical WeatheringChemical Weathering

A.A. Caverns:Caverns: rocks rich in calcite rocks rich in calcite (limestone) slowly dissolved (limestone) slowly dissolved by carbonic acid.by carbonic acid.

• Hard water = many dissolved Hard water = many dissolved minerals (Ca, Mg, Fe)minerals (Ca, Mg, Fe)

• Soft water = few dissolved Soft water = few dissolved mineralsminerals

Luray Caverns, VA

•Formation of a Cave

1. Stalactites:1. Stalactites: water with water with dissolved calcite runs from a dissolved calcite runs from a cave’s cave’s ceilingceiling and solidifies and solidifies downward.downward.

2. Stalagmites: 2. Stalagmites: calcite rich calcite rich water drips on the cave’s water drips on the cave’s floorfloor and builds upward. and builds upward.

If the If the two two connect, connect, then it then it forms a forms a columncolumn..

3. Natural 3. Natural Bridge:Bridge: when when the roof of a the roof of a

cavern cavern collapses in collapses in

several places several places leaving an leaving an

arch- shaped arch- shaped rock.rock.

B. KARST TOPOGRAPHYB. KARST TOPOGRAPHY• regions where chemical weathering by

groundwater is now visible at the surface

• regions where it is humid and there are limestone formations near the surface

• Virginia, Kentucky, Tennessee, Indiana, and Florida have Karst Topography

1. Sinkholes 1. Sinkholes (the main (the main feature of Karst feature of Karst

Topography)Topography)

How do sinkholes form?How do sinkholes form?• Rain seeps through soil and absorbs Rain seeps through soil and absorbs

carbon dioxide and reacts with carbon dioxide and reacts with decaying vegetationdecaying vegetation

• Acidic water is createdAcidic water is created• Water moves through spaces and Water moves through spaces and

cracks undergroundcracks underground• Limestone is slowly dissolved, Limestone is slowly dissolved,

creating network of cavities and voidscreating network of cavities and voids• Pores and cracks enlargePores and cracks enlarge• Land surface above collapses or sinksLand surface above collapses or sinks

2. Streams that disappear 2. Streams that disappear in the groundin the ground

Like Lewis Creek in Staunton

3. Caverns and Caves3. Caverns and Caves

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