Free water distribution:

 • 96% Oceans
 • 3% ice
 • 1% ground water
 • 0.01% streams and lakes
 • 0.001% atmosphere

Hydrologic cycle is the way that water moves through these reservoirs.

Volume per time is the flux of water

 
Flux can describe motion of any material (water, solute, students in room)

Another way to look at it is the Residence time of the material in the reservoir.
 

Definition:
 
 

Example:
 
 
 

 
Residence time:

 (1) “average” time that material in reservoir
 (2) only applicable in steady state

Steady state = when concentration of reservoir does not change; i.e. input = output
 

Response time:

 • When the reservoir is not in steady state
• Represents the time to reach certain concentration (e.g. doubling time).
 
Water composition (atmospheric water)

• Water in contact with other materials (air, rocks)
• consequence: no natural water “pure”
 

Important because hydrologic cycle causes flux of material
 
Water also has temporal variation in composition at any one location:

• Initial rainfall from individual storm has higher solute concentrations
• Initial snow melt has higher salt concentrations
 

Other mechanisms of deposition:

 • Dry deposition
 • Occult deposition
 
Dry deposition:

(1) uptake of gases and aerosols by vegetation and wet surfaces
(2) sedimentation of large aerosols

Aerosol: suspension of fine solid or liquid in gas

Occult deposition:

• Dry deposition plus deposition from fog and mist
 
Hydrology – water on/in the earth

Variety of processes:

 • evaporation
 • transporation (evaporation through leaves)
 • evapotraporation
 • underflow – flow through soil zone
• overland flow – heavy flow on land surface
• percolate into ground water
 
Chemical changes:

• Plants – provide solutes, neutralize acidity, extract N and P species
 • Soil/minerals – dissolve providing solutes
• Evaporation – increase overall solute concentrations
 
Water volume in streams:

• Streams flow even in droughts – sourced by baseflow (from ground water)
• Baseflow augmented by interflow, overland flow, direct precipitation
• During floods, water temporarily stored in stream banks – bank storage
 
Water composition in streams

• Generally little changes downstream (compared with flow off land)
 - short residence time
 - little contact with solids
• Changes usually biological
 - nutrients (N, P, Si)
 - pollutants

 
Ground water (unconfined)

• At depth below earth’s surface all voids filled with water
 - porosity: fraction of total rock that is void
• Boundary between water filled and air-filled voids is water table
• Separates phreatic (saturated) and vadose (unsaturated) zones
 
• Flow through the rocks controlled by permeability
• Water flows from high areas to low areas
• Consequently, water table mimics the surface topography
• Flow rates depend on gradient and permeability
 
Ground Water (confined)

• upper boundary of ground water in contact with impermeable rock
• level that water flow up wells is piezometric surface
• water above the impermeable rock is perched
 
Non-meteoric water

• most water is meteoric – i.e. derived from atmosphere (rain, snow, fog).

Other types of water:
• water buried with sediments – formation, pore, interstitial waters/fluids
 - generally geologically “old” water, greatly altered composition
• Dehydration of hydrated minerals phases: clays, amphiboles, zeolites Metamorphic water
• Water contained in mantle minerals, original with earth formation: juvenile water
 
Units

• Many ways to express concentrations
 • Primarily two:
  - weight (mass)
  - moles (number)
 • Chemistry is done with moles
 • Geology by mass
 

 
Mass/mole conversion:

 • 1 mole is Avogadro’s number of stuff- anything, atoms, molecules

 • Definiton: number of atoms in 12 g of 12C.
 • Value: 6.02 x 10²³

 
Why do this?
 • Reaction stoichiometry is written in terms of moles, not of mass.  e.g.

 • Simple conversion between mass (easily weighed) and moles
 
 

 
• Molar concentration units:

 (1)  Molality- number of moles of substance per kilogram of solvent.
  Symbol "m"

 (2)  Molarity - number of moles of substance per liter of solution.
  Symbol "M"

Note: 1 L = 1000 cm³ water at 4ºC
 
(3) Equivalents:

 • Another molar unit based on charge
 • generally meq/l (1/1000 of an equivalent)
 • Defined:

(milli-molarity of ion) x (charge of ion)

 • used to determine charge in electrolyte solution
 

Importance: all electrolyte solutions must be neutral
- sum of cations = sum of anions.
 
 

 
Mass concentration units

 • Generally used as weight ratios
 • e.g. percent, per mil, ppm, ppb, etc.
• e.g. ppm is g/10⁶ g, or mg/kg or µg/g
 

 
Some terms:

 TDS - total dissolved solids- weight of solid that remains following evaporation.
  - bicarbonate converted to carbonate

 Salinity – similar to TDS except Br and I to Cl.
 - Br and I titrated with Cl

 Fresh water - potable water, generally less than 1000 mg/l TDS

 Brackish water - non-potable, but less saline than seawater

 Seawater - water found in the ocean.  97% of all seawater has salinities that range from ~34‰ to 37‰

 Saline water - from seawater salinities to higher concentrations

 Brines - more concentrated than saline waters.

 Conductivity - a measure of salinity.  Current carried by solution is proportional to dissolved ions

 Refractive index - another way to measure salinity