Week 3 - Water Flashcards
(7 cards)
What is the atomic structure of a water molecule, and how does its polarity affect interactions between molecules?
A water molecule (H₂O) consists of two hydrogen atoms covalently bonded to one oxygen atom.
The covalent bonds involve sharing one electron from each atom, creating stable electron pairs.
Water molecule is bipolar: it has a partial positive charge (δ⁺) near the hydrogen atoms and a partial negative charge (δ⁻) near the oxygen atom.
This polarity enables hydrogen bonding between water molecules:
The δ+ hydrogen of one molecule is attracted to the δ- oxygen of another.
Hydrogen bonds are weak and can be broken with energy (e.g., heat), leading to the formation of water vapour (gas phase).
What are the key physical properties of water, and how do they relate to water’s molecular structure?
Low Viscosity:
Due to weak hydrogen bonds between molecules that can break easily with force, allowing water to flow freely.
Excellent Solvent:
Water is polar, so it can surround charged particles (ions), separating and keeping them in solution.
This makes water an effective solvent for ionic and polar substances like salt ions.
High Specific Heat Capacity:
A large amount of energy is needed to break hydrogen bonds before water’s temperature increases. Which is why it heats up slowly.
Helps moderate temperature in environments; useful for climate regulation and biological stability.
What is residence time in the hydrological cycle, and how is water storage in a reservoir calculated?
Describes how long water stays in one part of the environment (a reservoir) before moving elsewhere, e.g oceans, atmosphere, rivers, lakes, glaciers.
Equation:
ResidenceTime = VolumeofReservoir / Fluxinto and out of reservoir, measured per year typically
Water Storage (Change in Volume Over Time):
Tracks how the volume of water in a reservoir (e.g. ocean) changes over time.
Equation:
dV/dT = P + R - E
dV/dT: rate of change in volume of reservoir
P: precipitation
R: runoff (e.g river flowing into ocean)
E: evaporation
Steady-State System:
When volume stays constant over time, then:
dV/dT = 0, as
P+R=E
Water entering = water leaving
What is advection in geochemistry, and how do conservative and non-conservative transport differ?
The transport of a substance by the bulk movement of fluid (e.g., water).
Common in weather systems, ocean currents, and pollutant dispersion.
Types of Transport:
Conservative Transport:
Substance moves with the flow without any chemical alterations, i.e no reactions, just physical movement.
Non-Conservative Transport:
Substance is chemically altered (e.g., dissolved, precipitated, reacts) along the flow path.
What is the difference between advective flux
𝐽 and total mass flux 𝐹 in geochemical transport?
Advective Flux (𝐽):
Amount of substance transported per unit area per unit time
Units: mol/m²/s
Describes the local intensity of transport
Equation:
𝐽 = V × 𝐶
where:
V = fluid velocity (d/t) (m/s)
C = concentration of substance in fluid (mol/m3)
Total Mass Flux (𝐹)
Total amount of substance transported across the full cross-section per unit time
Units: mol/s
Describes the total transport rate
Equation:
𝐹 =𝐽 ×𝐴
where:
J = advective flux
A = cross-sectional area of flow (m2)
or
𝐹 = 𝑄 × 𝐶
where:
Q = volumetric flow rate (m3/s)
C = concentration (mol/m3)
What is saturated hydraulic conductivity, and how does Darcy’s Law describe water flow through saturated porous media?
Saturated Hydraulic Conductivity (Ksat):
Measures how quickly water flows through a fully saturated porous material (no air in pores).
Influenced by:
Porosity: Amount of empty space in material
Permeability: How well pores are connected
Tortuosity: Twisted paths water must take around solid grains
Fluid density & viscosity
Darcy’s Law (for saturated flow):
v = −Ksat × dh/dl
Where:
v = Flow velocity (m/s)
Ksat = Saturated hydraulic conductivity (m/s)
dh/dl = Hydraulic pressure gradient (change in head per unit length)
Negative sign indicates water flows from high to low pressure
Discharge (𝑄):
Represents the total volume of water flowing through a material per unit time (e.g. m³/s)
Unlike velocity, which tells how fast water moves, discharge Q tells how much water in total is moving through the system.
Equation:
Q = −Ksat × A × dh/dl
Where:
Q = Discharge (m³/s)
A = Cross-sectional area of flow (m²), e.g. width × depth of a river or aquifer
What is the catchment water balance equation, and what do its terms represent?
Catchment Water Balance:
Describes how the amount of water stored in a catchment changes over time.
Equation:
dS/dT =P − Q − ET
where:
dS/dT = Rate of change in water storage in the catchment (e.g. m³/day)
P = Precipitation (input)
Q = River discharge or runoff (output)
ET = Evapotranspiration (output via evaporation and plant uptake
If:
P>Q+ET, storage increases (water builds up)
If :
P<Q+ET, storage decreases (water is lost)
This equation is essential for understanding water availability and managing water resources in a catchment area.
