Comprehensive Introduction to Water Quality

Physical Properties of Water

• Molecular Structure: Water ($H_{2}O$) has a molecular weight of $18$. It is a dipolar molecule with an oxygen atom covalently bonded to two hydrogen atoms at an angle of $105^{\circ}$. The distance between nuclei is $0.96 \times 10^{-8} \text{ cm}$.

• Hydrogen Bonding: Due to its polarity, water molecules form hydrogen bonds. While weaker than covalent or ionic bonds, they are stronger than van der Waals forces, leading to high boiling/freezing points and temperature-dependent density.

• Specific Heat: Water has a high specific heat ($1.0 \text{ cal/g/}^{\circ}\text{C}$) compared to ice ($0.50 \text{ cal/g/}^{\circ}\text{C}$) and steam ($0.48 \text{ cal/g/}^{\circ}\text{C}$).

• Latent Heat:

  • Latent heat of fusion: $80 \text{ cal/g}$.

  • Latent heat of vaporization: $540 \text{ cal/g}$.

  • Latent heat of sublimation/deposition: $680 \text{ cal/g}$.

• Density: Maximum density is $1.000 \text{ g/cm}^{3}$ at $3.98^{\circ}\text{C}$. Ice is less dense ($0.917 \text{ g/cm}^{3}$), allowing it to float.

• Surface Phenomena: Capillary action is the result of surface tension, adhesion, and cohesion. Surface tension allows some insects to walk on water.

• Viscosity: Internal resistance to flow. It increases as temperature decreases.

• Thermal Stratification: In lakes, solar energy warms the surface layer (epilimnion), making it less dense than the bottom layer (hypolimnion). The transition zone is the metalimnion or thermocline (defined as a drop of at least $1^{\circ}\text{C/m}$).

Hydrology and Water Supply

• Global Water Distribution: Approximately $97.40\%$ of Earth's water is in oceans. Freshwater makes up $2.60\%$, with the majority trapped in polar ice ($2.01\%$) or deep groundwater.

• Hydrologic Cycle: The continuous movement of water through evaporation, transpiration (together called evapotranspiration), condensation, and precipitation.

• Evaporation Factors: Driven by vapor pressure deficit ($VPD = e<em>{s} - e</em>{a}$). Rate increases with higher temperature and wind velocity and lower relative humidity.

• Groundwater: Contained in aquifers.

  • Unconfined: Top is the water table, open to the atmosphere.

  • Confined: Trapped between impervious rock layers; often under pressure (artesian).

• Runoff: Storm flow that moves over land when rainfall exceeds soil infiltration capacity. Perennial streams are sustained by "base flow" (groundwater seepage).

• Water Footprint: The total water used locally or globally. Global agriculture footprint is approx. $8,363 \text{ km}^{3}\text{/year}$.

Chemical Principles and Solubility

• Atomic Mass and Moles: The sum of protons and neutrons determines atomic mass. A mole is Avogadro's number ($6.02 \times 10^{23}$) of molecules.

• Equilibrium Constant ($K$): Describes the ratio of products to reactants at equilibrium. Determined by Gibbs free energy: $\Delta G^{o} = -RT \ln K$.

• Expressing Concentrations:

  • Molarity (M): Moles of solute per liter of solution.

  • Normality (N): Equivalents per liter; useful for reactions based on charge/electrons.

  • Milligrams per Liter (mg/L): Equivalent to parts per million (ppm) in aqueous solutions.

• Electrostatic Interactions: In natural waters, ions interact, reducing their "effective" concentration. The ratio of reacting to measured concentration is the activity coefficient ($\gamma$), often calculated via the Debye-Hückel equation.

Dissolved and Particulate Matter

• Total Dissolved Solids (TDS): Inorganic and organic matter passing through a $2.0 \text{ }μm$ filter. Typically ranges from $20$ to $1,000 \text{ mg/L}$ in freshwaters.

• Specific Conductance: Measure of water's ability to conduct electricity via ions. Strong positive correlation with TDS ($\text{Factor} \approx 0.6-0.7$).

• Osmotic Pressure: Pressure needed to prevent water flow across a semipermeable membrane ($\pi = CRT$). Essential for osmoregulation in fish.

• Suspended Particles: Classified by size (e.g., clay $< 0.002 \text{ mm}$). Settling velocity is estimated by Stokes' Law: $V<em>{s} = \frac{g(\rho</em>{p} - \rho_{w})D^{2}}{18\mu}$.

• Turbidity: Light scattering by suspended particles. Measured in Nephelometer Turbidity Units (NTU). Secchi disk visibility ($Z<em>{SD}$) is a common field measure; extinction coefficient $K \approx \frac{1.7}{Z</em>{SD}}$.

Gases in Water

• Solubility: Governed by Henry's Law (proportional to partial pressure). Solubility decreases as temperature and salinity increase.

• Dissolved Oxygen (DO): Vital for aerobic respiration ($C<em>{6}H</em>{12}O<em>{6} + 6O</em>{2} \to 6CO<em>{2} + 6H</em>{2}O + \text{energy}$). Fish are stressed below $5 \text{ mg/L}$.

• Gas Bubble Trauma: Occurs when total gas pressure ($TGP$) exceeds barometric pressure ($BP$). Represented by $\Delta P = TGP - BP$.

• Re-aeration: Oxygen transfer across the air-water interface, quantified by the transfer coefficient ($K_{L}a$).

Microorganisms, Productivity, and Nutrients

• Nitrogen Cycle: Forms include $N<em>{2}, NH</em>{3}, NH<em>{4}^{+}, NO</em>{2}^{-}, NO_{3}^{-}$.

  • Nitrification: Aerobic oxidation of ammonia to nitrate; consumes $4.57 \text{ mg/L } O_{2}$ per $1 \text{ mg/L } N$.

  • Denitrification: Anaerobic reduction of nitrate to $N_{2}$ gas.

  • Ammonia Toxicity: Un-ionized ammonia ($NH_{3}$) is highly toxic. Its proportion increases with higher pH and temperature.

• Phosphorus: Usually the primary limiting nutrient for phytoplankton. Found as orthophosphate ($H<em>{2}PO</em>{4}^{-}, HPO_{4}^{2-}$). Sediments act as a major phosphorus sink via adsorption by iron and aluminum oxides.

• Photosynthesis: Net Photosynthesis ($NP$) = Gross Photosynthesis ($GP$) - Respiration ($R$). Measured via the light-dark bottle technique.

• Sulfur: Sulfate is the common form. In anaerobic zones, sulfate-reducing bacteria produce hydrogen sulfide ($H_{2}S$), which is highly toxic and odorous.

Water Quality Regulations

• Clean Water Act (CWA): Establishes stream classifications (e.g., Fish and Wildlife, Public Water Supply) and NPDES permits for point-source discharges.

• Toxicity Evaluation: Measured by the LC50 (concentration killing $50\%$ of test organisms). Safe concentrations are often estimated using an application factor (e.g., $0.05 \times LC50$).

• Primary Drinking Water Standards: Legally enforceable limits (MCLs) for contaminants like arsenic ($0.01 \text{ mg/L}$) and nitrate ($10 \text{ mg/L}$).

• Secondary Standards: Non-enforceable guidelines for aesthetic qualities like taste, odor (sulfate $250 \text{ mg/L}$), and color ($15 \text{ color units}$).