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Soils, Hydrogeology, and Aquifer Properties
Philip B. Bedient
2006
Rice University
Basin Hydrologic Cycle
Charbeneau, 2000.
Global Water Supply Distribution
• 3% of earth’s water is
fresh - 97% oceans
• 1% of fresh water in
lakes, streams, rivers
• 29% of the world’s
fresh water exists in
aquifers and 70% in
glaciers
1. Hydrogeology: What is it?
• Hydrogeology examines the relationships of geologic materials and flowing water
• Volume, water fluxes, and water quality are important
Basin Hydrologic Cycle – GW/SW Interaction
U.S. Hydrogeologic Units
Southeast
Coastal Plai n
Columbia Lava Plateau
Hawai i
Alask aNon-Glaciated Central Regio n
Atlantic and Gulf Coastal Plai n
Colorado
Platea u
Alluvial Basin s
Western Mountain Range s
High Plain s Northeast and Superior Upland s
Glaciated Centra l
Non-Glac iated Central Regio n
Piedmont and Blue Ridg e
Artesian Zone of Edwards Aquifer
• The artesian zone is a complex system of interconnected voids that can discharge water to the surface - stream
• Located between two relatively less permeable layers that confine and pressurize the system
• Underlies 2100 square miles of land
2. Soil Horizons2. Soil Horizons2. Soil Horizons2. Soil Horizons
Charbeneau, 2000.
porosity
Figure 15.1
3. Porosity and Density3. Porosity and Density3. Porosity and Density3. Porosity and Density
Types of Porous Media
Freeze and Cherry, 1979.
n = VV / VT = Vol Voids / Total Vol
Bedient et al., 1999.,
Porosity - not good indicator of flow
4. Hydraulics & Wells4. Hydraulics & Wells4. Hydraulics & Wells4. Hydraulics & Wells
K represents a measure of the ability for flow through porous media:
• K is highest for gravels - 0.1 to 1 cm/sec
• K is high for sands - 10-2 to 10-3 cm/sec
• K is moderate for silts - 10-4 to 10-5 cm/sec
• K is lowest for clays - 10-7 to 10-9 cm/sec
Hydraulic Conductivity KHydraulic Conductivity KHydraulic Conductivity KHydraulic Conductivity K
4. Hydraulic Conductivity4. Hydraulic Conductivity4. Hydraulic Conductivity4. Hydraulic Conductivity
Darcy’s Set up for Analysis
Pressure and Elevation Heads - Laboratory
Freeze and Cherry, 1979.
ψ = pressure head
z = elevation head
h = total head
Freeze and Cherry, 1979.
ψ = pressure head
z = elevation head
h = total head
Pressure and Elevation Heads - Field
Two Confined Aquifers with Different Heads
Charbeneau, 2000.
Groundwater will tend to
flow from the top aquifer
to the bottom aquifer.
We can’t make any
conclusion about
horizontal head gradients
from this picture.
Horizontal and Vertical Head Gradients
Freeze and Cherry, 1979.
Occurrence of Ground Water
• Ground water occurs when water recharges
the subsurface through
cracks and pores in soil and rock
• Shallow water level is called the water table
Example Layered Aquifer System
Bedient et al., 1999.
Geology & Pumping Impacts
Other Aquifer Features
5. Aquifers Types
Definition: A geological unit which can store and supply significant quantities of water.
Principal aquifers by rock type:
Unconsolidated - Gulf Coast
Sandstone - oil reserves
Sandstone and Carbonate
Carbonate-rock - Edwards
Volcanic - Hawaii
The Major Aquifers of Texas
Carrizo-Wilcox
Edwards
Gulf Coast
Ogallala
Recharge
Natural
• Precipitation
• melting snow
• Infiltration by streams and lakes
Artificial
• Recharge wells
• Water spread over land in pits, furrows, ditches
• Small dams in stream channels to detain and deflect water
6. The Edwards Aquifer
Formation History
Hydrologic Features
Pumpage to Date: 33,035.30 mg
Average Daily Pumpage: 144.26 mg
Historic minimum level (1956): 612.5’
Historic maximum level (1992): 703.3’
Minimum Edwards Level for 2000: 649.7’
Maximum Level for 2002: 690.5’
The Edwards Limestone
• When the limestone was exposed, it was extensively eroded creating cavities and
conduits making it capable of holding and
transmitting water
• Then it was covered over with relatively
impermeable sediments forming a confining unit
Geology of Edwards Aquifer
• Primary geologic unit is Edwards Limestone
• one of the most permeable and productive aquifers in the U.S.
• The aquifer occurs in 3 distinct segments:
-The drainage zone
-The recharge zone
-The artesian zone
Artesian Wells
• A well whose source of water is a confined aquifer
• The water level in artesian wells is at some height above the water table due to the pressure of the aquifer
Water Table Well
Artesian Well
Flowing Well
Pot. Surface
Confined Aquifer and
Confining Unit Above
Bedrock Zone
Drainage Zone of Edwards Aquifer
• Located north and west of the aquifer in the region referred to as the Edwards Plateau or Texas Hill Country
• Largest part of the aquifer spanning 4400 sq. miles
• Water in this region travels to recharge zone
Recharge Zone of Edwards Aquifer
• Geologically known as the Balcones fault zone
• It consists of an abundance of Edwards Limestone that is exposed at the surface
-provides path for water to reach the artesian zone
Artesian Zone of Edwards Aquifer
• The artesian zone is a complex system of interconnected voids varying from microscopic pores to open caverns
• Located between two relatively less permeable layers that confine and pressurize the system
• Underlies 2100 square miles of land
The Edwards Group
The Edwards Group
• The Edwards limestone is 300-700 ft. thick
• Outcrops at the surface is tilted downward to
the south and east and is overlain by younger
limestone layers and thousands of feet of
sediment
• The immense weight of this sediment layer
caused faulting in the region
Typical Dip Section
Regional Dip Section
Flowpaths of the Edwards Aquifer
The Ogallala Aquifer
• Deposition of the Ogallala formation began in the late Miocene to early Pliocene
• During the Pliocene eroded sediment from the Rockies was deposited on the existing surface of Triassic and Permian rocks until the whole surface was covered with Ogallala sediments up to 900 ft. thick
Characteristics
• Consists primarily of heterogeneous sequences of coarse-grained sand and gravel grading upward to fine clay, silt and sand
• Saturated thickness ranges from a few feet to more than 525 ft.
Aquifer Size
• The Ogallala supplies water for eastern New Mexico, much of west Texas, as well as parts of Colorado, Oklahoma, Kansas, Nebraska, Wyoming and South Dakota
• Approximately 170,000 wells draw water from the aquifer
• Contains 3.3 billion acre feet of water (1 acre foot = 326,000 gallons)
Water Level Change before 1980
Water Level Change - 1980 to 1994
The Ogallala Problem
• Water level declines of 2-3 feet per year in some regions are drawing concern of the aquifer drying up - first one in the U.S.
This is due to pumping more water than is replaced through recharge, and only 10% is restored by rainfall.
Large issues exist over contamination and salt water intrusion problems as well.
Sample Hydrograph
The Ogallala Future
• Drawdown from 1998 to 2050 is predicted to
be more than 150 feet in some areas given
the forecast amount of pumping.
• By 2010, parts of the aquifer in Oklahoma
and Kansas are simulated as going dry.
Dutton, A.R., Reedy, R.C., and Mace, R.E. (2001). Saturated Thickness in the
Ogallala Aquifer in the Panhandle Water Planning Area: Simulation of 2000
through 2050 Withdrawal Projections.
http--www.twdb.state.tx.us-gam-ogll_n-OG-n_report.pdf.url