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Chapter 17

Groundwaterand

Hydrogeology

Importance of groundwater

� Groundwater: water in the saturated pore space of soil, sediment, or narrow fractures in bedrock

• Largest reservoir of fresh water that is readily available to humanity

• Crucial resource for agriculture (irrigation) and drinking water

Fresh Water BudgetImportance of groundwater

� Geologic role of groundwater� As an erosional agent:

• Groundwater can dissolve rock!• Sinkholes• Caverns and Caves

� During drought, groundwater is an important connection and supplier to stream flow

“Gaining” Stream

Distribution of groundwater

� Soil moisture– water held by molecular attraction (surface tension) on soil particles� Limited to root zone of plants, trees

� Unsaturated Zone: “Zone of Aeration”� Below soil moisture but above the water

table� Cannot be pumped by wells due to air

flow within pore space� Includes capillary fringe and soil

moisture

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� Capillary Fringe :� Groundwater that seeps upward from

the water table via capillary action� Due to molecular attractive forces

between polar water molecules� Groundwater is held by surface tension

in tiny passages (pores) between grains of soil or sediment

Distribution of groundwater

Distribution of groundwater

� Saturated Zone: Groundwater� Water not held as soil moisture

percolates downward (infiltration )� Zone where all of the pore space in

sediment and rock are completely filled with water (no air)

� Water within the pores is called groundwater

The water table

� Water table: Upper limit of the zone of saturation

� Regional: extends over large areas and is generally continuous

� Variations in the depth of water table� Varies seasonally and annually

• High in spring, low at end of summer • Drought, flood can affect water table level

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The water table

� Surface of water table is a subdued replica of the surface topography� Water tends to “pile up” beneath high areas

• BUT at greatest depth beneath high areas

� Closest to ground surface in topographically low areas (rivers, wetlands, springs)

� Variations in rainfall (drought) will change depth to water

� Variations in permeability spatially

Variations in Water Table DepthStreams and the water table

� Streams and Groundwater Are Connected!� Gaining stream– stream gains water from

inflow of groundwater through the streambed• Groundwater flows toward and into stream• Water level in stream regional ground water

� Losing stream– stream loses water by outflow through the streambed to gw

• Stream water is not in contact with groundwater• Water table is below stream bed

Gaining stream: Water level in stream reflects regional water table elevation

Losing streams: water level in stream is higher than regional water table

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� Streams can change from gaining to losing streams

� Seasonally

• Gaining stream in wet season (spring)

• Dry in late summer, losing stream

• May go completely dry in late summer

� Regionally (as geology changes along stretch of stream)

� Temperature of groundwater can indicate if stream is gaining/losing

Streams and the Water Table

Groundwater TemperatureGroundwater

Storage and Movement

� Porosity – percentage of pore space in a volume of rock or sediment that consists of� Determines total storage of groundwater� Variations can be considerable over

short distances as geology changes

GroundwaterStorage and Movement

� Permeability – the ability of a geologic unit to transmit a fluid

� Aquifer – permeable rock strata or sediment that freely transmits groundwater� Good aquifers = sands and gravels, sandstone

� Fractured bedrock = sometimes a good aquifer• Depends on connectivity of fractures

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GroundwaterStorage and Movement

� Aquitard – an impermeable layer that hinders or prevents water movement � Clay or shale

� Can act as “Confining Unit ,” trapping GW beneath aquitard

� High porosity, low permeability

� “ Perched” groundwater can accumulate on top of an aquitard

• Above disconnected from groundwater table

Perched Water on Aquitard GroundwaterStorage and Movement

� Movement of groundwater� Very slow – typically a few centimeters

per day� Gravity and pressure provide energy for

groundwater movement� Darcy’s Law – Describes and predicts

movement of groundwater (both velocity and direction)

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Movement of Groundwater

� Darcy’s Law� As slope of the water table increases, the

velocity of groundwater increases

� Hydraulic head – vertical difference in gw elevation between two measured points

� Hydraulic gradient (i) – the water table slope• rise/run• divide hydraulic head by the distance between the

measuring points

Movement of groundwater

� Hydraulic Head: (H1 – H2)

� Hydraulic gradient (i) : hydraulic head divided by horizontal distance (L)� Slope: rise over run � (H1 – H2)/L = i (symbol for hydraulic gradient)

� The rate of groundwater movement can be measured directly using

• Dye tracers• Carbon-14 or Tritium dating

Hydraulic gradient

Movement of Groundwater

� Hydraulic Conductivity (K) : a measure of the ability of a rock unit to transmit water� Units = distance/time (e.g., ft/d, cm/s)

� Different types of rock have very different values of K

� Can range across 10 orders of magnitude (or 10,000,000,000 times)

� Gravel (very high K) to clay (low K)

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Darcy’s Law

� Darcy’s Law: Discharge rate (Q) of aquifer is proportional to: (1) Hydraulic gradient (i)(2) Hydraulic conductivity of aquifer (K)(3) Area of saturated zone of aquifer (A)

• “Saturated Thickness”

� Darcy’s Law: Q = KiA� Velocity of GW: v = (Ki)/n

where n = porosity

Hydraulic Conductivity (K)

� How can we get a value for K?� Lab measurements: permeameter

� Field measurements: well pump tests, slug tests, ring permeameters

� Estimate from published values (if geology is known)

Field Permeameter

Testing

Aquifer Pump Test

Ground Water Flow Maps

� GW elevations are measured at individual wells (water table)

� GW elevations are plotted on a map

� Elevations are contoured (exactly like a topo map)

� GW will flow down-slope direction perpendicular to contours � shortest distance “down the hill”

� “Flow Lines” trace a particle of water along its flow path

GW Flow Maps

� “Potentiometric Surface” is elevation of water table surface

� Can be mapped (like ground surface elevation) like a topographic map

� Used to estimate flow directions and velocities� Flow lines are always perpendicular to gw

elevation lines

� Very important for contamination investigations� Where is contamination going?

� How fast is it traveling?

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Springs

� Springs� Occur where the water table intersects

Earth’s surface� Natural outflow of groundwater to

surface� Can be caused by an aquitard creating a

localized zone of saturation which is called a perched water table

Spring resulting from a perched water table

SPRING

Hot springs and geysers

� Hot Springs� Water is 6-9 oC warmer than the mean

annual air temperature of the locality� The water for most hot springs is heated

by nearby cooling igneous rock� As water cools, minerals are deposited

� Solubility of dissolved minerals decreases as hydrothermal water cools

Distribution of hot springs and geysers in the U.S.

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Mammoth Hot Springs

Yellowstone NP

Mammoth Hot Springs

Yellowstone NP

Hot springs and geysers

� Geysers= Intermittent hot springs� Need extensive underground chambers

within hot igneous rock• Groundwater heats beyond boiling point,

expands to steam, and erupts in upper chamber

• Pressure relieved from overlying gw causes deep water to boil to steam

• Water erupts with great force

Old Faithful

Yellowstone NP

Shoshone Geyser Basin

Yellowstone NP

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Wells

� For a continuous supply of water, a well must penetrate below the water table� During all seasons/conditions

� Supply wells (pumping) � Pumping of wells can cause

• Drawdown (lowering) of the water table• Cone of depressionin the water table

� Monitoring wells � Monitor water quality or water level� Not actively pumped

Drawdown in the water table

Wells

� Artesian well – groundwater under pressurerises above the level of the aquifer� Types of artesian wells

• Nonflowing – water surface within well is above water table but below ground level

• Flowing – ground water surface is above ground level

� Not all artesian systems are wells• Artesian springs also exist

Artesian well from an inclined aquifer

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Artesian Well: Bellefonte, PA Ground Water Withdrawal

� Groundwater is a nonrenewable resource

� If amount being withdrawn > amount recharging aquifer

� “Groundwater mining”

� Significantly depressed water table� Cone of depression at pumping wells

�Subsidence� GW is part of “skeleton” of aquifer� Ground level compresses and sinks when

water pumping >> natural recharge � San Joaquin Valley, CA; Houston, TX� Ogallala Aquifer; central plains USA

• Huge and critical water supply to agriculture over much of Great Plains

• In semi-arid terrain

Ground Water Withdrawal

Subsidence due to Groundwater Mining

San Joaquin Valley, CA

Dewatering of the Ogallala Aquifer

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Problems associated with groundwater withdrawal

� Saltwater Intrusion� Freshwater is less dense and floats on salt

water in coastal areas

� Excessive groundwater withdrawal causes saltwater to be drawn up into wells

� Contaminates the freshwater supply

� Primarily a problem in coastal areas and islands (L.I., NY)

Salt water and ground water interface

Salt water intrusion into pumping well

Groundwater contamination

� Common source: sewage from septic systems� Septic systems purify sewage naturally as it

passes through an aquifer• Biologic action naturally decomposes septic waste

� Permeable aquifers, like coarse gravel, have large pore spaces and high gw velocities

• GW (and septic wastes) may travel long distances very quickly without being cleaned

� E coli, phosphates, nitrates, pharmaceuticals

� Can impact water supply wells, water bodies

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Groundwater contamination

� Pumping wells can change ground water flow directions

� Water supply wells can draw in contamination

� Sources and types of contamination include substances such as:

Highway Salt L.U.S.T.Fertilizers/Pesticides Chemical WastesIllegal Dumping Buried wastesLandfills Septic Systems

Groundwater Landforms

� Groundwater dissolves rock� Groundwater is often mildly acidic

• Contains weak carbonic acid• Forms when rainwater dissolves carbon dioxide

from the air and from decaying plantsH2O + CO2 ���� H2CO3

• Rainwater reacts with sulfur trioxide from burning fossil fuels

H2O + SO3 ���� H2SO4

� Acidic gw reacts with calcite (CaCO3) in limestone and dissolves the rock

CaCO3 + H2CO3 ���� CO2 + H2O + CaO

Caves

� Caves: created by acidic groundwater dissolving soluble rock (limestone)

� Composed of dripstone (travertine)� Calcite deposited as dripping water

evaporates, leaves behind solids

� Stalactites: hanging from the ceiling of cavern

� Stalagmites: form on the floor of a cavern

Carlsbad Caverns, NM

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“Soda straws” in Carlsbad Caverns National Park

Groundwater Landforms

� Karst topography� Landscapes shaped mainly by the

dissolving power of groundwater� Some common features:

• Sinkhole or “sink” - formed by gw slowly dissolving the underlying bedrock

• Often accompanied by collapse of bedrock

• Irregular, hummocky terrain

• Lack of surface drainage (streams)

Developmentof karst

Karst Topography

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Northern Florida

Karst Topography

Winter Park, FL Winter Park, FL

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Karst Landforms

� Disappearing streams� Also called “sinking streams”� Streams flowing in karst terrain will

occasionally encounter a sinkhole or enlarged fracture

� The entire stream disappears, flowing into underground chambers

Disappearing Stream

Disappearing Stream