I. Units
    A. Mass/volume

    B. Mass/mass

    C. ppm

    D. ug-atoms
        sometimes see measurements reporting compounds vs. elements; (e.g., drinking water standards = 10 mg NO₃^- - N/L)

                                  molecular weight

                                   molecular weight

        oceanographers in older literature used ug-atoms/L:

         1 ug-atom/L of PO₄-P = 31 ug PO₄^--P/L (since 31 is the atomic weight of P)

    E. molarity

    F. molality

    G. normality

 

II. Activity - concentration available to react with other compounds or ions

        Debeye-Huckel Theory – you can estimate the activity coefficient by the electrostatic interactions of ions in the solution
 

III. Origins of Major Ions
    A. Major ions in river water

        World average of river water (ppm)
                   CATIONS                                                          ANIONS
           Ca²⁺   Mg²⁺   Na⁺   K⁺                           HCO₃^-     SO₄^2-   Cl^-     SiO₂
             13.4      3.4        5.2      1.3                               52         8.3         5.8     10.4
 

     B. Where do these ions come from?  Why is this the composition of surface waters?
            1. Atmospheric inputs
            2. Rock weathering
            3. Evaporative concentration
    C. Atmospheric inputs, rain

    Picks up salts from ocean spray (sodium, chloride, magnesium, sulfate) and from dust over the continents
        (calcium and potassium); also gases over volcanoes

     D. Rock weathering – interaction of rocks and water
          1. 
          2. example:

          3. analyzing weathering contributions -- 3 approaches
            a. mass balance
            b. statistical correlations
            c. thermodynamics

    D. Evaporation - can concentrate ions until you get the precipitation of minerals
          general equation; equilibrium constant = K_eq = K_sp (solubility product)

        final composition depends on amounts and ratios of ions initially present

        but if the ion product is greater than Kso, then the calculated oversaturation does not necessarily mean you
            will get precipitation

    E. Biological considerations

        Conservative ions – concentrations undergo no or minor changes due to organisms (either direct use or
            indirect conversion) [Na, K, Cl]
        Vs.
        Dynamic ions – concentrations are strongly influenced by the metabolism of organisms
 
          Examples:

             1. uptake of N and P by algae
             2. nitrogen fixation by cynaobacteria (bluegreen algae)
             3. reactions mediated by microbes; decomposition
             4. sphagnum moss as an ion exchanger
             5. anthropogenic alterations

         ‘biogeochemistry’

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