GROUNDWATER

Hydrogeology

• Distribution of Earth's water (Fig. 14-1)

Hydrologic Cycle (cont.)

• 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 (Fig. 15-1)
• 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 called recharge.
• Area of recharge called recharge zone.
• The top of the saturated zone of is called the water table.
• Level varies depending on rainfall.
• Water table roughly follows topography.

Aquifer Systems 
Movement of Groundwater

• In the saturated zone, groundwater will flow from where the water table is high to where it is low
• different 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 hydraulic 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

Aquifer Systems (cont.)

• 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 (Fig. 15-7)
• Springs
• occur where water table intersects ground surface
• Can develop from perched water tables (caused by an elevated aquiclude)

Aquifer Systems
Gaining vs. Losing Streams

• Gaining stream (Effluent stream)
• gains water from aquifer
• humid climates
• Losing stream (Influent stream)
• loses water to aquifer
• source of recharge in arid environments
• What are ours?

"Mining" Groundwater 
Groundwater Quality

• What makes a good aquifer?
• Quality
• Quantity
• What Controls Quality?
• Total dissolved solids (ions), gases
• Presence of soluble minerals (NaCl, CaSO4, CaCO3
• 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

Groundwater Quality
Why our water tastes bad (cont)

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

Groundwater 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?
• Cone of Depression
• cone-shaped subsidence of water table

Dating Groundwater

•  Groundwater dating
• 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 - as much as 10 feet
• Houston - 30 feet?

Groundwater-related Hazards (cont.)

• Sinkholes - depression caused by dissolution of limestone by groundwater; 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.
• Stalactites - travertine pendants hanging from the roof of cavern or cave.
• Stalagmites - travertine "pedestal" built up from floor.