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

SoutheastCoastal Plain

ColumbiaLavaPlateau

Hawaii

Alaska Non-GlaciatedCentral Region

Atlantic and GulfCoastal Plain

ColoradoPlateau

AlluvialBasins

WesternMountainRanges

High Plains Northeast and Superior Uplands

GlaciatedCentral

Non-GlaciatedCentral Region

Piedmont andBlue Ridge

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

Charbeneau, 2000.

porosity

Figure 15.1

3. 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 & 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 K

4. Hydraulic ConductivityDarcy’s Set up for Analysis

Pressure and Elevation Heads - Laboratory

Freeze and Cherry, 1979.

= pressure headz = elevation headh = total head

Freeze and Cherry, 1979.

= pressure headz = elevation headh = 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 CoastSandstone - oil reservesSandstone and CarbonateCarbonate-rock - EdwardsVolcanic - Hawaii

The Major Aquifers of Texas

Carrizo-Wilcox EdwardsGulf 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