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)  ↔ H₂CO_{3 (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.