Soils, Hydrogeology, and Aquifer Properties Philip B. Bedient 2006 Rice University
Charbeneau, 2000. Basin Hydrologic Cycle
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 Northeast and Superior Upland s Glaciated Centra l Non-Glaciated CentralRegio n Piedmont and Blue Ridg e Alluvial Basin s Hawai i Colorado Platea u Southeast CoastalPlai n Alask a Non-Glaciated CentralRegio n Atlantic and Gulf CoastalPlai 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
Charbeneau, 2000. porosity
Figure 15.1
3. Porosity and Density
Types of Porous Media n = V V / V T = Vol Voids / Total Vol Freeze and Cherry, 1979.
Bedient et al., 1999., Porosity - not good indicator of flow
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 ψ = pressure head z = elevation head h = total head Freeze and Cherry, 1979.
Pressure and Elevation Heads - Field ψ = 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.
Freeze and Cherry, 1979. Horizontal and Vertical Head Gradients
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
Bedient et al., 1999. Example Layered Aquifer System
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 Artesian Well Water Table 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