groundwater and hydrogeology -...
TRANSCRIPT
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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