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.