Aqueous Geochemistry 101

• water is constantly being recycled in the hydrologic cycle (Fig)
• it is an excellent solvent
• it is vital in Earth's geochemical cycles
• water contains dissolved gases, ions, inorganic and organic molecules, organisms,....
  
• dissolved solids tend to be dominated by ions (an atom or a group of atoms that has either an excess or a deficiency of electrons)
• major ions in water, positive charge (cations), negative charge (anions)
• H⁺, Na⁺, K⁺, Ca²⁺, Mg²⁺, NH₄⁺, F^-, Cl^-,  SO₄^2-, NO₃^-, HCO_3^-, CO₃^2-, PO₄^3-
• we typically express concentrations in mmol/L or mg/L
• Mole: unit of amount of substance. 
• By definition, the mole is the amount of substance of a system that contains as many elementary entities as there are atoms in 0.012 kilogram (12g) of carbon-12. The elementary entities, which must be specified, may be atoms, molecules, ions, electrons, other particles, or even specified groups of such particles.
• 1 mole = 6.022*10²³ molecules or atoms (or other units)
• conversion from mg/L to mmol/L
  
• On-line periodic table
• Equivalent: another unit of amount of substance. An equivalent or Eq is the number of moles divided by the number of electrons lost or gained by each molecule in a reaction
• we'll follow a water droplet in the hydrologic cycle:
  

Rainwater chemistry

• atmospheric moisture makes it into the atmosphere by evaporation and transpiration, shouldn't it be distilled water?
  
• there is lot's of stuff in the atmosphere (gases, particles) that dissolve in cloud droplets and increase the anion concentrations
• these substances are eventually removed from the atmosphere by wet and dry deposition
  

Particles in atmosphere

• sea salts from the ocean surface (35%)
  
• soil and mineral dust from the continents (27%)
  
• forest fires (6%)
  
• volcanic ash (3%)
• particles from direct anthropogenic emissions (3%)
• particles from gaseous emissions
  
• sulfate from H₂S
• sulfate from SO₂
• nitrate from NO_x
• hydrocarbons, biogenic
• hydrocarbons, fossil fuel

Sources of major anions in precipitation (marine; terrestrial; pollution)

• Cl^-: sea salt; industrial HCl, roadsalt
  
• SO₄^2-: sea salt, marine gases (DMS, Dimethylsulfide); H₂S from biological decay, volcanoes, soil dust; burning of fossil fuels to SO₂, forest burning
• NO₃^-: N₂ plus lightning; NO₂ from biological decay and N₂ from lightning; combustion of fossil fuels, forest burning, nitrogen fertilizers
• PO₄^3-: soil dust, biogenic aerosols absobed on sea salt; burning vegetation, fertilizer
• HCO₃^-: CO₂ in air; CO₂ in air, soil dust
• (NH4⁺ (cation!):: biological activity, bacterial decay, fertilizers, human, annimal waste decomposition)
  

• Anions in NE US precipitation, mg/L
• Anions in NE US precipitation, mmol/L
• Comparison between precipitation and oceans
  
• NADP network US precipitation
• static maps
• trends
  

• dry deposition
• evaporation
  

Rivers 

• precipitation will eventually hit the surface and may take up substances that have deposited by dry deposition on surfaces, such as leaves
  
• rainwater interacts with soils & rocks => weathering; e.g.
  
• H₂CO₃ + CaCO₃ => Ca²⁺+2HCO₃^-
  
• Mg₂SiO₄ + 4H₂CO₃ => 2Mg²⁺+ 4HCO₃^-+H₄SiO₄
• weathering releases other compounds (e.g. Cl^-, SO₄^2-)
  
• concentrations of most anions higher than in precip due to dissolution of dry deposition particles and weathering, some are lower (e.g. nutrients, which may be taken up by plants)
• anion concentrations may increase due to
  
• leakage from sewers and septic systems (NO₃^-, NH₄⁺, PO₄^3-)
  
• roadsalt use in the winter time (Cl^-)
• Anions in average US river, mmol/L

Groundwater

• groundwater anion concentrations increase further due to additional weathering
  

Sea water

• rivers eventually flow into the ocean
• oceanwater is fairly concentrated solution, affected by tectonic and biological activity
  
• Anions in oceans, mmol/L

Summary of major ion compositions, mmol/L

Electrical conductivity as a measure of dissolved ions

• definition of water conductivity
• conversion into total dissolved solids (actual ions) depends on the composition of the ions
• rule of thumb: 1000 micro S/cm ~ 640 mg/L TDS (varies from 500 to 1000 mg/L)
  

Drinking water

• drinking water in the NYC area comes from
  
• reservoirs north of the city (mostly Croton, Delaware, and Catskill systems)
• groundwater (northern NJ, Long Island, parts of Queens, Rockland Country)
• the NYC drinking water is currently unfiltered surface water, chlorine and fluoride and a few other substances are being added
  
• NYC water quality reports
  

Resources

• Berner, E.K., and Berner, R.A. (1987) The global water cycle. Prentice-Hall, Englewood, NJ, 397pp.
• Berner, E.K., and Berner, R.A. (1996) Global Environment. Prentice-Hall, Englewood, NJ, 376pp.
• Drever, J.I. (1988) The geochemistry of natural waters. Prentice-Hall, Englewood Cliffs, NJ,  437pp.
  
• National Atmospheric deposition program
• NYC Drinking Water Supply and Quality Report