What are the key factors affecting the volatility and solubility of organic compounds?
Volatility increases with higher vapor pressure and usually lower molecular weight.
Solubility increases with more polar or hydrophilic functional groups, and decreases with higher molecular weight or increasing salinity.
Temperature increases solubility.
These factors influence how organic compounds behave in different environments.
What are functional groups and how do they affect the key physical properties of organic compounds?
Functional groups are specific atom arrangements (e.g., –OH, –COOH, –NH₂, –Cl) that determine reactivity, solubility, polarity, acidity/basicity, and volatility.
* Hydrophilic groups ↑ solubility
* Nonpolar groups ↑ hydrophobicity & Kow
* Halogens ↑ density & persistence
They influence solubility, boiling point, and acidity.
What are the most important physical parameters needed for assessing the fate and transport of organic compounds? What are Kow and KH?
Key parameters: vapor pressure, solubility, density, Henry’s constant (KH), octanol–water partition coefficient (Kow).
* Kow = Co/Cw → measures hydrophobicity
* KH = Pi/Cw → gas–water partitioning; high KH means the compound tends to volatilize.
These parameters help predict how organic compounds will behave in the environment.
How do you compute percentage distributions of an organic compound in water, air, and soil particles?
Use:
* Mw = Cw × (porosity × water saturation)
* Ma = Cw × KH × (air-filled porosity)
* Ms = 0.6 × foc × Kow × ρs × Cw
Then compute percentage = Mphase / (Mw + Ma + Ms).
This involves calculating the distribution based on the environmental conditions.
What are the common degradation pathways for organic pollutants? What is co-metabolism?
Degradation pathways: oxidation, reduction, hydrolysis, photolysis, biodegradation.
Co-metabolism: microorganisms degrade pollutants incidentally while consuming another primary substrate (pollutant isn’t the energy source).
Co-metabolism is a process where one organism degrades a compound while using another as a primary energy source.
What are the three common types of plastics and why are microplastics becoming a major environmental issue?
Plastics: PE (polyethylene), PP (polypropylene), PVC (polyvinyl chloride).
Microplastics are harmful because they persist, accumulate in ecosystems, absorb toxins, enter the food chain, and are found in water, soil, and organisms.
Microplastics are problematic due to their persistence in the environment and potential harm to wildlife.
What are the common uses of halogenated hydrocarbons and why?
Halogenated hydrocarbons are used as solvents, degreasers, pesticides (DDT, atrazine), refrigerants (CFCs), and insulators (PCBs) because they are stable, nonflammable, and chemically inert.
They are used for their stability and effectiveness in various applications.
What are PFAS and why are they considered forever chemicals?
PFAS: fluorinated compounds used in Teflon, firefighting foams, coatings; called “forever chemicals” because C–F bonds are extremely strong, making them persistent, mobile, and resistant to degradation.
Their persistence leads to accumulation in the environment and potential health risks.
How do density differences in organic pollutants affect DNAPLS and LNAPLs in groundwater?
- LNAPLs (lighter than water): float on the water table and spread laterally.
- DNAPLs (denser than water): sink through aquifers, accumulate on low-permeability layers, migrate downward through fractures, and are much harder to remediate.
This affects how contaminants spread and are remediated in groundwater.
What are the key factors affecting the suspended sediment loads in rivers?
Sediment load is controlled by relief/slope, basin size, water discharge, climate (monsoon, seasonality), basin geology, and presence of lakes/dams.
These factors influence how much sediment is carried by rivers.
Why does the Yellow River in China carry so much suspended sediment?
Because of deforestation, cultivation, loose loess soils, high erosion rates, and seasonal rainfall events that mobilize sediment.
The river’s geology and human activities exacerbate sediment transport.
How do human land use activities affect suspended sediment loads?
Increase loads: deforestation, overgrazing, construction, agriculture.
Decrease loads: dams, reservoirs, bank stabilization, soil conservation.
This leads to higher sediment loads in rivers.
Which regions of the world have high suspended sediment loads and why?
- Pacific islands & Himalaya → steep relief
- Monsoon regions → extreme seasonal rainfall
- Young tectonic basins → rapid erosion
- Areas with glaciers → glacial flour and meltwater transport
These conditions promote erosion and sediment transport.
Why are rivers a principal way to transport N to oceans, and how do humans affect this flux and estuary health?
Rivers carry nitrogen from soil organic matter, fertilizers, sewage, and atmospheric deposition.
Humans dramatically increase N via:
* Agriculture (fertilizer runoff)
* Wastewater
* Atmospheric deposition
This causes eutrophication, algal blooms, fish kills, and hypoxia in estuaries.
Human activities can significantly impact this nitrogen flux.
What are the dominant anions and cations in river water and why?
Dominant ions: Ca²⁺ and HCO₃⁻ from carbonate weathering; rivers drain continents rich in sedimentary carbonates.
Their relative abundance varies based on river type and environmental conditions.
How do Na⁺ vs Ca²⁺ and Cl⁻ vs HCO₃⁻ vary across Gibbs’ three river types, and why?
Precipitation-dominated:
* High Na⁺/Ca²⁺
* High Cl⁻/HCO₃⁻
* Low TDS
Because composition resembles rain.
Rock-weathering dominated:
* Low Na⁺/Ca²⁺
* Low Cl⁻/HCO₃⁻
* Moderate TDS
Ca²⁺ and HCO₃⁻ from carbonate dissolution dominate.
Evaporation-dominated:
* High Na⁺/Ca²⁺
* High Cl⁻/HCO₃⁻
* High TDS
Salt concentration increases downstream.
This classification helps understand river chemistry.
Explain the formation of dimictic lakes in temperate regions.
Temperate lakes freeze in winter and stratify in summer. Because water density is highest at 4°C, lakes mix twice per year during spring and fall when temperature is uniform top-to-bottom.
This mixing affects water chemistry and temperature stratification.
How does water chemistry differ in epilimnion vs hypolimnion of stratified lakes, and why?
- Epilimnion: warm, well-oxygenated, lower nutrients due to biological uptake.
- Hypolimnion: cold, often low O₂, higher nutrients from decomposition, potential for anoxia.
Stratification prevents mixing.
This stratification affects aquatic life and nutrient cycling.
What are the main factors causing lake eutrophication?
Excess phosphorus and nitrogen, mainly from:
* Fertilizers
* Sewage
* Detergents
* Animal waste
* Urban runoff
Eutrophication leads to algal blooms and oxygen depletion.
Why is phosphate added to detergents, and what are the environmental consequences?
Added because P improves cleaning efficiency, softens water, and binds minerals.
Consequences: cultural eutrophication, algal blooms, decreased oxygen, fish kills.
However, it can lead to environmental consequences like eutrophication.
What conditions favor the formation of saline lakes?
- Closed basins with no outflow
- Evaporation > inflow
- Arid climates
- Enough inflow to sustain water body
These conditions lead to increased salinity.
Why do saline lakes often have high pH?
Evaporation precipitates Ca²⁺ and Mg²⁺ as carbonates, leaving excess Na⁺ and HCO₃⁻, which increases alkalinity and raises pH (often > 10).
Bedrock weathering contributes to this phenomenon.
How do you perform geochemical box model calculations (single-box lake example)?
Use steady-state mass balance:
ΔM/Δt = CiFi − C Fo + Rd − Rp = 0
Solve for unknown (usually Ci or C).
If dissolution equals previously precipitated material:
Rs = Rp − Rd
Use:
Ci = (C Fo + Rs) / Fi
This model helps in understanding nutrient cycling in lakes.
What nutrient is limiting in lakes/soils vs estuaries/oceans, and why?
- Lakes/soils: Phosphorus (P) is limiting — no gaseous phase, strongly binds to sediments.
- Estuaries/oceans: Nitrogen (N) is limiting — denitrification removes N, and fixation is limited by light.
These nutrients limit biological productivity in their respective environments.
