1. Types of Aquifers
Unconfined aquifer open to Earth’s surface and to infiltration
Confined aquifer overlain by less permeable materials
EXPLANATION
Unconfined aquifer is the most common type of aquifer
A confined aquifer is separated from Earth’s surface by rocks with low permeability
Perched water sits above the main water table
Artesian does not have to rise all the way to the surface
Artesian has no implication about water quality or taste (some artesian wells are salty water)
Perched aquifer underlain by low-permeability unit
Artesian aquifer: water rises in pipe (maybe to surface)
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2. How Does Water Move Between the Surface and Subsurface?
Water can soak into subsurface and become groundwater
EXPLANATION
Precipitation, snowmelt, and surface water can soak into subsurface and become groundwater
Water table is generally below surface, providing unsaturated zone into which water can seep
Where water table intersects surface, water can flow out into spring, lake, or river
Water table generally below surface, so water can seep in
Where water table intersects surface, water can flow out
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3. How Are Lakes and Wetlands Related to Groundwater?
Observe the settings of lakes compared to the water table
How Are Lakes and Wetlands Related to Groundwater?
Perched above water table
Wetlands at water table or perched
Water table intersects surface
INSTRUCTIONS TO STUDENTS
Observe how the water table relates to these lakes and wetlands
(After explanation) How do you think you could determine which of these settings a lake would be in?
Possible answers
determine elevation of water table by drilling
see whether a lake was losing or gaining water by modeling inflow, outflow, and evaporation
compare the chemistry of the lake water with runoff and with groundwater
EXPLANATION
Perched above water table with outflow to subsurface
Where water table intersects the surface (normal setting for lakes)
Lake on bench (flat area) in topography
Lake on bottom of valley, where water table is flat
Wetlands can be at water table or perched above an impermeable zone
On topographic bench
Valley bottom
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4. How Do Streams Interact with Water Table?
Observe how each stream relates to water table and flow of groundwater
Stream lower than water table: gains water
INSTRUCTIONS TO STUDENTS
Observe how each river relates to the water table and whether water flows from the river to groundwater or from groundwater to the river
How do you think you could determine which situation was occurring?
Possible answers
determine elevation of water table by drilling
see whether a lake was losing or gaining water by modeling inflow, outflow, and evaporation
compare the chemistry of the lake water with runoff and with groundwater
EXPLANATION
River lower than water table can gain water from inflow of groundwater
River higher than water table can lose water to groundwater
Mound of groundwater can form below river due to outflow of river water into groundwater
Stream higher than water table: loses water
Mound of groundwater below stream from water flowing into subsurface
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5. How Do Caves Form? Most in limestone (soluble)
Groundwater dissolves material
Above water table, cave may be dry
Features widen to cavities and caves
INSTRUCTIONS TO STUDENTS
(after explanation) How might we determine whether a cave is present at depth but not expressed at the surface?
Possible answers
see if the rock limestone at the surface
drilling
geophysical surveys, such as ground-penetrating radar, if shallow
EXPLANATION
Most caves form in soluble rocks, such as limestone
Groundwater in fractures and bedding planes dissolves material
Dissolution widens bedding planes and fractures until they become cavities and caves
Part of cave above water table may be mostly dry
Cave below water table will contain water that further dissolves material, enlarging cave
A sinkhole develops over the collapsed part of the cave on the right side
A cave may not have any expression at the surface, as on the left side
Below water table water further dissolves material
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6. What Features Accompany Caves?
Roof collapse can form sinkhole on surface
Dripping water evaporates, precipitates calcite
MEDIA
1706a2_Plateau_Sinkholes.kmz
1706a2_Florida_Sinkhole_Lakes.kmz
1706b1_Karst_WV.kmz
EXPLANATION
Roof of a cave can collapse, forming sinkhole on surface (especially a cave above water table)
Water dripping off roof evaporates, precipitating calcite and other minerals
Minerals build up cave formations from roof, floor, and walls
Surface with sinkholes, limestone pillars, disappearing streams = karst topography
If the cave is filled with water, it is less likely to collapse because the water is helping hold up the roof
If the cave is filled with water, typical cave formations are not forming because there are no drips or evaporation
Some caves may have been partially filled with water or were once dry, and have since been refilled when the water table rose
Water in caves can flow or be stationary
Cave formations on roof, floor, and walls
Surface with sinkholes, limestone pillars, disappearing streams = karst topography
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7. How Do We Study Groundwater?
Field studies: sequence and character of rocks
Measure discharge, losses/gains, and water quality
EXPLANATION
Explain a pumping test
Field studies
Types of rock and sediment
Estimate of overall permeability
Abundance and orientation of fractures
Orientation of layers
Presence or absence of cavities
Characteristics of the material, such as clast size and degree of sorting
Geophysical surveys
Gravity surveys to determine depth to bedrock
Seismic surveys to determine geometry of layers, units, and faults
Electrical surveys to determine location of water table and geometry of units
Drilling
Describe the sequence of materials versus depth
Characteristics of the materials (clast size)
Abundance and dip of fractures
Lowering special video cameras down the drill hole
Geophysical surveys done within a drill hole, such as how much radioactivity each layer gives off
Geophysical surveys: depth to bedrock and water
Drilling: verify geology, depth to water table, provide samples, pump tests
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8. How Do We Depict the Water Table?
Numbers show elevations of the water table: what is the pattern?
Compare water table to other features
Contour: water table at same elevation; blue arrows show flow
MEDIA
1707b3_groundwater_map.mov
Relates the sloping groundwater surface in the aquifer to the land surface and a groundwater contour map
INSTRUCTIONS TO STUDENTS
Examine the map on the upper left, which shows elevations of the water table
What is the pattern of elevations, and which way do you think groundwater would flow?
EXPLANATION
Contours mark where water table is at same elevation; blue arrows show flow
We visualize system to compare water table to other features, such as farms with wells that pump high volumes (this is a different area)
EXERCISE
Have students contour a simple map showing only the elevation of points, and then draw arrows perpendicular to contours and in a downslope direction
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9. Effects of Overpumping Groundwater
Before overpumping
After pumping: cone of depression
EXPLANATION
Before over-pumping, water table is fairly flat and water flows to left, past small well
Larger well causes cone of depression in water table, changing direction of flow
A cone of depression forms because water cannot flow fast enough to replenish the amount of water that is withdrawn from the well
In many cones, the water table becomes steeper near the well because water is withdrawn too fast
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10. Other Problems of Overpumping
Water table drops, aquifer compacts, land subsidence
Earth fissures
MEDIA
1708b4_Picacho_Peak_Fissures.kmz
Google Earth file of Picacho Peak area, Arizona, shows earth fissures crossing desert, canal, and I-10
Central Arizona Canal (up against the mountain) was built close to bedrock to try to avoid the fissures
EXPLANATION
As water table drops, the aquifer compacts, causing subsidence of land surface
Fissures form along boundaries between more and less subsidence
Along coast, freshwater floats on saltwater, so overpumping can draw saltwater into bottom of coastal wells
Areas can drop meters to tens of meters in elevation
Detect subsidence with repeated surveying with conventional surveying equipment, GPS, and other satellite measurements
Once compaction occurs, the pore space cannot be regained if the water table is allowed to rise (lost permeability, porosity, and storage capacity of the reservoir)
Fissures related to subsidence can be tens of meters wide, kilometers long, and tens to over a hundred meters deep
Fissures provide easy access for contaminant into the groundwater without being filtered
Along coast: freshwater floats on saltwater, so draw in saltwater
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11. How Can Water Become Contaminated?
Identify possible sources of surface water and groundwater contamination
Factory wells, spills, emissions
Mining and naturally mineralized rock
Fuel storage tanks
Landfill
Dry cleaner
Farm
INSTRUCTIONS TO STUDENTS
Observe this scene and identify all possible sources of water contamination
Households
Septic tank
Brought by river
Leaking tank in gas station
Truck fuel spill
Brought from outside area by groundwater
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12. How Does Groundwater Contamination Move?
With groundwater down water table
Can be filtered if flows slowly, like through sands
Not filtered if flows rapidly, like in limestone caves
EXPLANATION
Contamination largely moves with groundwater down the slope of the water table
Can be naturally filtered out with enough time: flows slowly and in contact with material like sand
Not filtered if flows rapidly through a rock, such as a limestone with open cavities
Importance for considering the direction of groundwater flow when sighting the relative positions of a water well and contamination sources, such as a septic tank
In general, put the well uphill of a septic tank, but be aware of what is farther up the hill
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13. How Do We Track and Remediate Groundwater Contamination?
Spreads out due to diffusion and mixing, forming contamination plume
Contour water table and contamination
EXPLANATION
Contamination spreads out due to diffusion and mixing, forming a contamination plume
Contour water table and concentration of contaminant to track plume
Drill wells to intercept plume, pump out and treat water
Dispose of contaminated water or treat it with activated charcoal or certain geologic materials
Drill wells to intercept plume, pump and treat water
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14. Setting of the Ogallala Aquifer
Unconfined aquifer; water levels dropping in south; some parts may run dry (graphs below)
MEDIA
The next slide has two files about the aquifer, including thickness, precipitation, and water-level change
EXPLANATION
Most important aquifer in the U.S.
Also called the High Plains Aquifer
Provides water for many important agricultural areas of the high plains
Unconfined aquifer in Cenozoic sediments and sedimentary rocks
Water levels are dropping in south, and some parts are predicted to dry up due to overuse
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15. Ogallala Water Supplies and Usage
MEDIA
1711a_Ogallala_Aquifer_maps.kmz
This file shows the location of the Ogallala Aquifer. Turn on other layers in Google Earth, from bottom to top, to look at 1) thickness of the aquifer, 2) precipitation, and 3) water-level change
1711a_Ogallala_Aquifer.mov (QTVR)
Click and drag down and up to change from one map to another, from shaded relief to precipitation, thickness of the aquifer, and water-level change
EXPLANATION
Eastern part receives more rain than western part
Northern part has greatest saturated thickness (supply)
Water table decline greatest in south (Kansas & Texas)
Thickest in north
More rain in east
Water table decline greatest in south (Kansas & Texas)
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16. Investigation: Who Polluted Water in This Place?
NOTES
Students can do this exercise on a worksheet or in class without a worksheet; need to use with a printout of this figure, or its map-view equivalent, or a printout of the data table
May want to make a special PowerPoint of these last three slides that cycles continuously in class
INSTRUCTIONS TO STUDENTS
Observe the geology and other features on the surface to try to infer which way water would flow in this area
Use the data from the geologic section to determine which units will be aquifers and which ones will limit any vertical flow of groundwater
Use the water table elevations and analyses of contamination to determine which areas are contaminated and from where such contamination most likely came
EXPLANATION
Note that water can flow through the layers at depth as well as on the surface
Surface water can infiltrate into groundwater and groundwater can flow to the surface in springs
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17. 17.12.a2 17

18. Investigation Data Table18