Groundwater is

1
Groundwater is

• An important component of the hydrologic cycle
  (4 by volume, residence times 2
  weeks-10,000years).
• An important natural resource 51 of homes in
  Tennessee rely on wells for their water supply.
• Highly variable in quality and quantity.
• Sensitive to anthropogenic contamination.

2
To be of high quality, an aquifer must have

• High permeability.
• High recharge (area with high precipitation
  rate).
• A short flowpath (limited contact with soluble
  minerals).

3
General considerations

• Groundwater flow generally follows topography
  Groundwater regions analogous to surface drainage
  basins.
• Groundwater regions have distinct physical and
  hydrologic characteristics resulting from a mix
  of structural, climatic, and petrologic controls.
• Temperature of groundwater close to average
  annual atmospheric temperature.

4
What happens to water when it enters the ground?

• Terminology Unsaturated (vadose) zone, Saturated
  (phreatic) zone.
• Comparison of typical soil and surface water
  compositions what controls composition in soil
  zone?

5
Carbon Dioxide in the soil zone and weathering

• Water in the soil zone may have PCO2 of 10-3 to
  10-1 bar (equilibrium pH 4.3-4.5), higher than
  the atmospheric value of 10-3.5 (pH 5.7).
• Typical weathering reactions involving H2CO3 are
• Carbonate CaCO3 H2CO3 Ca2 2 HCO3-
• Silicate NaAlSi3O8 (albite) H2CO3 9/2 H2O
  Na HCO3- 2 H4SiO4 1/2 Al2Si2O5(OH)4(kaolini
  te)
• Oxidation of organic matter in the soil zone
  replensihes H2CO3 consumed in weathering
  reactions.

6
Major-ion evolution sequence (Chebotarev
sequence)

• Groundwater composition changes progressively
  along a flowpath toward the composition of
  seawater (increase in TDS, concentration of
  dissolved salts, etc.), so that major ions change
  in the following sequence
• HCO3- ? HCO3- SO42- ? SO42- HCO3- ? SO42-
  Cl- ? Cl- SO42- ? Cl-
• Question why is SO42- the dominant sulfur
  species?
• Also get decrease in Eh and flow rate, increase
  in age and residence times.

7
Three main zones of groundwater flow

• In large sedimentary basins get three zones with
  distinct flow and composition characteristics
• Upper zone high flow rate, short residence
  times, low TDS, high Eh, HCO3- dominant anion
  (calcite and dolomite abundant, moderately
  soluble, dissolve rapidly).
• Intermediate zone SO42- dominant anion (gypsum
  and anhydrite uncommon, but very soluble).
• Lower zone low flow rate, long residence times,
  high TDS, low Eh, Cl- dominant anion (salts
  halite and sylvite found only at depth, typically
  in marine evaporite deposits, but are extremely
  soluble).

8
Groundwater contamination

• Contamination introduced by deep-well injection,
  leakage from landfills, disposal ponds and spills
• Contaminants include heavy metals, carcinogenic
  or disease-carrying organic wastes, nutrients or
  oxygen-demanding wastes, and radioactive
  materials.
• Transport may be slowed by adsorption, ion
  exchange, precipitation and coprecipitation (next
  chapter), or bacteria may consume waste.

9
Conclusions

• Composition of groundwater mainly controlled by
  the availability of minerals along groundwater
  flowpaths and their solubilities.
• With time, groundwater evolves in composition to
  that of seawater. Evaporation of seawater
  returns pure H2O to the atmosphere and leaves
  dissolved solids behind as evaporite deposits.
  Precipitation and infiltration renews the cycle.