Water Lecture 2

GEOL 142: Water


Lecture 2: What is dissolved in water? Why is the ocean salty?


The Global Water Cycle

Ruddiman, 2014, Box 2-3.



  • Bookkeeping

    • Typically, the volume of water referenced is a liter (1000 grams-1 kg by mass), and the dissolved component mass is in grams (or milligrams, mg). Milligrams/Liter (mg/L; 0.001g/L) or milligrams/kg (mg/kg, 0.001g/1000g) is also called parts per million (ppm) concentration. Concentrations of a dissolved species in water are commonly expressed as:  mg/L or ppm. Total dissolved solids (TDS) is everything dissolved in the water added together, also expressed as mg/L or ppm.

  • Water volume is typically measured in liters (1000 grams = 1 kg by mass).

  • Dissolved component mass is measured in grams (g) or milligrams (mg).

  • Concentration units:

  • Milligrams per Liter (mg/L) = 0.001 g/L

  • Milligrams per Kilogram (mg/kg) = 0.001 g/1000 g

  • Parts per million (ppm) is equivalent to mg/L or mg/kg.

  • Concentrations of dissolved species in water are usually expressed as mg/L or ppm.

  • Total Dissolved Solids (TDS): The sum of all dissolved substances in water, also expressed as mg/L or ppm.

  • Rainwater “Meteoric Water”

    • Rainwater Composition:

      • Nearly pure water, with <10 ppm Total Dissolved Solids (TDS).

      • Contains dissolved CO₂(g) from the atmosphere 

        • PCO₂ = 10⁻³.⁴ atm= 420 ppm

      • Dissolution reaction:   

        • CO2(g)   CO2(aq) + H2O(l H2CO3 (aq) H+(aq)  + HCO3-( )

    • Minor Sources of Dissolved Constituents:

      • Sea-salt aerosols

      • Mineral dust particles

      • Particulate industrial pollutants

    • Variability in Rain & Snow Composition (Selected Locations):

      • Measured Constituents:

        • Silicic acid (H₄SiO₄), Calcium (Ca), Magnesium (Mg), Sodium (Na), Potassium (K), Bicarbonate (HCO₃⁻), Sulfate (SO₄²⁻), Chloride (Cl⁻), Nitrate (NO₃⁻), Total Ions (Σ), pH

      • pH Variation:

        • Ranges from 4.6 to >10.14, indicating some samples are acidic while others are alkaline.

      • Regional Differences:

        • Some regions (e.g., Northern Europe, Baltimore) have higher sulfate and nitrate concentrations, likely due to industrial pollutants.

        • San Diego, Australia, and Colorado show higher total ion concentrations, possibly due to sea spray or dust influences.

        • New Mexico snow and rain exhibit lower ion concentrations, suggesting minimal atmospheric contamination.

  • Chemical Weathering: The Great Titration

    • Rocks formed at high temperatures and pressures are altered by interaction with meteoric waters (rain, snow, groundwater).

      • Chemical weathering occurs through reactions between oxygenated, acidic waters and Earth's materials.

    • Sources of Weathering Agents:

      • Water Source: Primarily from meteoric precipitation (rain, snow, etc.).

      • Acid Source: Mostly from atmospheric CO₂, which dissolves in water to form carbonic acid (H₂CO₃).

    • Effects of Weathering:

      • Creates landforms, soils, new minerals, and sediments.

      • Produces dissolved substances transported by rivers to the oceans.

      • Plays a key role in regulating atmospheric CO₂ over geological time scales.


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