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
Columbia
Lava
Plateau
Western
Mountain
Range s
High Plain s
Northeastand Superior Upland
s
Glaciated
Centra l
Non-Glaciated
Central Regio n
Piedmontand
BlueRidg e
Alluvial
Basin s
Hawai i
Alask a
Southeast
Coastal Plai n
Colorado
Platea u
Non-Glaciated
Central Regio n
Atlantic andGulf
Coastal Plai n
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 Horizons
porosity
Charbeneau, 2000.
Figure 15.1
3. Porosity and Density
Types of Porous Media
n = VV / VT = Vol Voids / Total Vol
Freeze and Cherry, 1979.
Porosity - not good indicator of flow
Bedient et al., 1999.,
4. Hydraulics & Wells
Hydraulic Conductivity K
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
4. Hydraulic Conductivity
Darcy’s Set up for Analysis
Pressure and Elevation Heads - Laboratory
 = pressure head
z = elevation head
h = total head
Freeze and Cherry, 1979.
Pressure and Elevation Heads - Field
 = pressure head
z = elevation head
h = total head
Freeze and Cherry, 1979.
Two Confined Aquifers with Different Heads
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.
Charbeneau, 2000.
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
Ogallala
Carrizo-Wilcox
Edwards
Gulf Coast
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
Pot. Surface
Flowing Well
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