GROUNDWATER

Ground-water Usage in Texas (1992)

Projected Texas Water Use

Projected Texas Water Use

Why Study Groundwater?

• 50% of U.S. drinking water
• Will we run out?
• Are there drawbacks to using groundwater?
• Pertinent web-sites
• Texas Water Development Board http://www.twdb.state.tx.us/
• U.S. Environmental Protection Agency http://www.epa.gov/

Hydrogeology

• Distribution of Earth's water (Fig. 10.2)

Hydrologic Cycle (cont.)

• Ocean - giant reservoir of water
• Water evaporates from ocean, condenses in atmosphere, and falls as rain on ocean (75%) and land (25%)
• Water falling on land as precipitation becomes
• Runoff - flows over land (streams and rivers)
• Groundwater - infiltrated into ground - process called recharge
• Per cent infiltrated depends on slope and permeability of surface.

Groundwater

• Permeability
• The ability to allow fluids to flow
• Varies with material
• Gravel - very high
• Sand - moderate to high
• Silt - moderate to low
• Clay - low to very low
• Igneous rocks, metamorphic rocks, cemented sandstones, shales - low permeability; flow through fractures
• Limestones - variable permeability; depends on dissolution; fractures

• Ability of rock to store water depends on porosity
• Porosity - volume of pore space per volume rock
• In summary:
• Flow rate depends on permeability
• Storage depends on porosity

Aquifer Systems

• Water infiltrates down or into permeable water-bearing rock bodies called aquifers.
• Good aquifers - sand, gravel, sandstone, porous limestone, highly fractured bedrock
• Poor aquifers - shale, mudstone, clay, unfractured igneous and metamorphic rock
• Less permeable rocks serve to separate aquifers and are called aquatards or aquacludes.

• Groundwater infiltrates down through the unsaturated zone (vadose zone) until coming to rest on the saturated zone.
• Process =recharge.
• Area of recharge = recharge zone.
• The top of the saturated zone = water table.
• Level varies depending on rainfall.
• Water table is a subdued replica of surface topography (Fig. 11.2)
• Less permeable rocks serve to separate aquifers; called aquitards (or aquicludes)
• Aquifer systems consist of aquifers and aquitards
• Aquifers in Brazos County ( cross-section )
• Aquifers in Texas ( map )

Movement of Groundwater (p. 272)

• In the saturated zone, groundwater will flow from where the water table is high to where it is low
• Differences in water table heights cause pressure gradient (pressure change/distance)
• Hydraulic head - elevation difference between water table at recharge and discharge (where groundwater leaves aquifer such as at a spring or well)
• Hydraulic gradient
• Equals hydraulic head divided by distance (length)

• Darcy's Law
• Velocity v = K x head/distance

                 = K x hydr aulic gradient

      where K = hydraulic conductivity (dependent mostly on permeability)

• v in our sand aquifers roughly 1 m/yr
• In permeable gravel, v can be 50 m/yr

Gaining vs. Losing Streams

• Gaining stream (Effluent stream)
• Gains water from aquifer (Fig. 11.3)
• Humid climates
• Losing stream (Influent stream)
• Loses water to aquifer  (Fig. 11.3)
• Source of recharge in arid environments
• What are ours?

Confined and Unconfined Aquifers

• There are two types of aquifers
• Unconfined - not overlain by aquitards
• Direct recharge from above
• No build-up of pressure
• Confined - overlain by aquitards
• May be recharged far from site
• Hydrostatic pressure can build up

• Artesian well
• Groundwater rises above the level of the aquifer
• Groundwater under pressure
• Recharge area is at a higher elevation than ground surface at well site
• Springs (Fig, 11.5, 11.6)
• Occur where water table intersects ground surface
• Can develop from perched water tables (caused by an elevated aquiclude)

Groundwater Resources

• What makes a good aquifer?
• Quality
• Quantity

• What controls quantity?
• Porosity - determines amount of groundwater storage
• Size of aquifer - thickness, areal extent
• Permeability - affects recharge rate, ability to pump

• What happens to water table when groundwater is pumped?
• Drawdown - Lowering of water table with pumping
• Cone of Depression
• Cone-shaped subsidence of water table (Fig. 11.11)

Mining Groundwater

• Mining groundwater - removal of groundwater faster than it can be recharged
• Ozarka
• Ogallala aquifer (p. 282)
• 450,000 km²
• Tertiary and Quaternary sediments
• Averages 60 m thick (up to 180 m)

Groundwater Quality

• What Controls Quality?
• Total dissolved solids (ions), gases, pollutants; related to:
• Presence of soluble minerals (NaCl, CaSO ₄, CaCO₃
• Proximity to sources of contaminants
• Age

Groundwater Quality
Why our water tastes bad (yes, more chemistry).

• College Station drinking water chemistry

        CONSTITUENT    CONCENTRATION (mg/L)

sodium (Na+)                    212

calcium (Ca2+)                     3

bicarbonate (HCO3-)        442

chloride (Cl-)                     54

sulfate (SO42-)                    10

Total dissolved solids    509

• Specific conductance demonstration

Groundwater Quality
Why our water tastes bad (cont)

• Three-step process
• Oxidation of organic matter in soil and aquifer
• Organic matter + oxygen   ->  carbonic acid
• CH₂O + O₂  ->   H₂CO₃
• Dissolution of CaCO₃ (calcite)
• calcite shell + acid   ->  calcium ion + bicarbonate
• CaCO₃ + H₂CO ₃  ->   Ca²⁺ + 2HCO₃ ^-
• Cation exchange on clays (natural water softener)
• Ca²⁺ + 2Na⁺ (on clay)  ->   Ca²⁺ (on clay) + 2Na ⁺

• Demonstration of specific conductivities of different waters

• Overpumping may cause saline water downdip in the aquifer to move toward well

• Point source - specific site of pollution
• Examples: gasoline spill, septic tank, pipeline rupture
• Non-point source - pollution from broad area
• Examples: fertilizers, pesticides, road salt

• Treating a contaminated aquifer
• Discovery
• Elimination of source
• Monitoring
• What has been the impact?
• Modeling
• What will be the impact? Prediction
• Remediation
• Pump-and-treat, bioremediation

Natural versus artificial remediation

• Age estimates from Darcy’s Law
• Age = distance from recharge site (km) / flow velocity (km/yr)
• Example:
• Our groundwater is recharged about 40 km northwest of the well
• Darcy’s law yields a velocity of roughly 1.5 m/yr for this aquifer
• Age = ________________

•  Radiometric techniques
• Tritium (³H)
• Natural + nuclear bomb product
• Half life = 12.3 years
• Determines if age less than 50 years
• Carbon-14 (¹⁴C)
• Date dissolved inorganic carbon (mostly bicarbonate)
• Up to 50,000 years

• Groundwater-related Hazards
• Subsidence - pumping of groundwater may cause land to subside
• San Joaquin Valley, CA
• Houston

Groundwater-related Hazards (cont.)

• Sinkholes - depression caused by dissolution of limestone by groundwater (Fig. 11.23); typical of karst topography

• Karst topography - terrain characterized by dissolution of limestone by groundwater (actively forming in humid climates).
• Features of karst topography
• Caverns - dissolution of limestone in saturated zone; when water table drops, cavern remains
• Speleothems - CaCO₃ deposits formed in caves (Fig. 11.21)
• Stalactites - travertine pendants hanging from the roof of cavern or cave
• Stalagmites - travertine "pedestal" built up from floor