GEOS2060 Week 4 Water and Soil Physics

Shape of a water molecule and its H-O-H angle

Bent (V-shaped); H-O-H angle of 104.5°

Why is water a polar (polarised) molecule?

The O end carries a partial negative charge (δ−) and the H atoms a partial positive charge (δ+), because of the bent shape and unequal sharing of electrons

What is a hydrogen bond in water?

An attraction between the δ+ hydrogen of one water molecule and the δ− oxygen of a neighbouring molecule

Structure water forms through hydrogen bonding

Irregular hexagonal or pentagonal structures

What explains water's physical properties (viscosity, surface tension, phase changes)?

Hydrogen bonds (from water's polarity)

Temperature ranges of water's phases (per lecture)

Solid < 0°C; liquid 0-100°C; gas > 100°C

Thermodynamics

The branch of physics that deals with heat, work, temperature and the statistical behaviours of systems

Three main phases of water

Solid (ice), liquid (water), gas (vapour)

Melting

Solid to liquid

Freezing

Liquid to solid

Vaporisation

Liquid to gas

Condensation

Gas to liquid

Sublimation

Solid to gas

Deposition

Gas to solid

How do water's thermodynamic properties support ecosystems?

Through phase transitions, water regulates environmental temperatures and supports diverse ecosystems

Gas form of water: where is it found?

Water vapour in the atmosphere and shallow subsurface; also contained in liquid water

Solid form of water: where is it found?

Snow or ice sitting on the surface, and permafrost in the shallow subsurface

Types of liquid water in the Critical Zone

Free water, capillary water, and physically bound water (hygroscopic and pellicular)

Free water

Liquid water not bound by other substances, maintaining its liquidity under normal conditions

Where is free water found?

Soil pores and underground geological layers (groundwater), lakes, underground aquifers

Why is free water important?

Essential for plant uptake, drinking water supply and agricultural irrigation

How is free water replenished?

By infiltration of precipitation and surface runoff

Main risk to free water

Vulnerable to contamination, affecting water quality and ecosystems

Capillary water

Water held in the tiny pores of soil by capillary forces (from surface tension) but not bound to specific soil particles

Physically bound water: two types

Hygroscopic (adsorbed) water and pellicular (loosely bound) water

Hygroscopic (adsorbed) water

Physically bound water adsorbed directly onto soil particle surfaces (the tightly held layer around the particles in the lecture diagram)

Pellicular water

Loosely bound water held in a film outside the hygroscopic water

Density (definition and formula)

The degree of compactness of a substance; ρ = m/V in kg/m³ (m = mass in kg, V = volume in m³)

Temperature of maximum density of water

4°C (about 1.000 g/cm³)

Density of ice compared with water

Ice is less dense (about 0.917 g/cm³) so it floats; density drops abruptly on freezing at 0°C

Why is water density an important property?

It depends on temperature, and density differences can cause flow

Viscosity

The state of being thick, sticky and semi-fluid in consistency, due to internal friction from weak chemical bonds between molecules

Low viscosity vs high viscosity example

Low viscosity: water. High viscosity: honey

Dynamic (absolute) viscosity

A fluid's internal resistance to flow when an external force is applied; symbol μ; units Pa·s

Kinematic viscosity

A fluid's resistance to flow under the influence of gravity; ν = μ/ρ; units m²/s

Dynamic viscosity of water at 25°C (STP in lecture)

8.91 × 10⁻⁴ Pa·s

How does water viscosity change with temperature?

Decreases as temperature increases (non-linear curve; about 1.8 × 10⁻³ Pa·s at 0°C down to about 0.3 × 10⁻³ Pa·s at 100°C)

Surface tension

The contractive tendency of a liquid's surface that allows it to resist an external force; symbol σ; units J/m²

Surface tension of water at 25°C

Approx. 71.97 × 10⁻³ J/m²; highly temperature dependent

Everyday examples of surface tension in the lecture

Water droplets beading up, and a water strider standing on water

Capillary rise

A liquid ascends or descends in a small-diameter (capillary) tube due to capillary action

Capillary rise equation

h_c = 2σ / (ρ g r), in m

Variables in the capillary rise equation

σ = surface tension (J/m²); ρ = density (kg/m³); g = gravity (m/s²); r = radius of pores (m)

How does pore radius affect capillary rise?

Smaller pore radius gives a higher capillary rise (h_c is inversely proportional to r)

What causes capillary action?

Surface tension causing the capillary effect, due to intermolecular forces between water and solid molecules

Capillary action is a function of...

Fluid properties, solid properties, and pore size (diameter)

Capillary rise in clay vs sand vs gravel

Highest in clay (smallest pores), then sand, lowest in gravel (largest pores)

Soil water content above the water table: clay vs loam vs sand

Clay stays wet to greatest height above the water table, then loam; sand drops off sharply just above the water table

Why does capillary rise matter for plants?

It lifts water up through soil so plants can access it

Ideal soil composition by volume

45% mineral, 25% water, 25% air, 5% organic matter

Phases of soil and sediment

A three-phase system: solid, liquid, gas (can be quantified by volume or by mass)

Total volume equation for soil

Vt = Vs + Vw + Va

Total mass equation for soil

mt = mw + ms

Pore (void) volume Vv equals

Vv = Vw + Va (water volume plus air volume)

Porosity (definition and formula)

Fraction of soil or sediment volume that is non-solid (voids); n = Vv/Vt = (Vw + Va)/(Vs + Vw + Va); expressed as a fraction or %

Factors influencing porosity

Grain size, texture, structure, organic matter content, state of compaction

Soil structure

The shape and size of aggregates; it defines the pore space distribution

Ped

A soil structure unit formed by soil minerals with organic matter; micropores lie inside peds and macropores lie between peds

Sorting and porosity

Well sorted (uniform grains) gives high porosity; poorly sorted gives low porosity because small grains fill gaps between large ones

Three main features to consider when describing soil pores

1. Total volume of pores; 2. Size distribution of pores; 3. Continuity of pores

Why is total porosity alone not enough?

It does not describe pore size or continuity, which control drainage, aeration and water retention

Which has greater total porosity: free-draining sand or poorly drained clay?

Clay (sandy soils have smaller total pore volume), but sand has roughly twice as many macropores, which explains its better drainage

Micropores

Smaller pores (< 30 μm); hold water by capillary action

Macropores

Larger pores (> 30 μm); provide drainage and aeration

Small pores: pros and cons

Hold water well (positive) but restrict aeration (negative)

Large pores: pros and cons

Hold water poorly (negative) but permit free air flow (positive)

Rule of thumb for adequate drainage and aeration

Desirable: macropore diameters greater than 80-100 μm. Problematic: macropore diameters less than 50 μm

Continuity of pores

The length of macropores (and larger micropores) and the depth to which they extend down the profile

Example of a discontinuous pore network

A compacted layer (e.g. plough pan) interrupting the macropore network

Pore types in vesicular basalt, limestone (karst), fractured rock

Vesicular basalt: separate gas-bubble cavities; limestone (karst): solution channels; fractured rock: pores are cracks and fractures

Sugar beet harvester core sample (Wildenschild)

Soil driven over by machinery 14 years earlier still showed far fewer, less continuous macropores than undisturbed soil, which had abundant macropores 0.6 mm and larger

Texture, structure and porosity: how are they interconnected?

Texture = the 'building' materials; structure = how particles are arranged into peds; porosity = the gaps between peds where the 'action' takes place

Specific surface area (SSA)

Surface area of a grain divided by its mass; A_s = A_g/m_g in m²/kg

SSA for spherical grains

A_s = 6 / (ρ_s × d), in m²/kg (ρ_s = grain density in kg/m³; d = mean grain diameter in m)

Relationship between SSA and grain size

Inverse: smaller particles have higher SSA because they have more surface area relative to volume

Particle shape and internal porosity effect on SSA

Non-spherical particles and those with internal pores can have higher SSA even if their overall size is larger

Uniformity and SSA

For a given mass, uniform fine grains have higher SSA than larger or irregularly shaped grains

Material composition and SSA

Different materials can have different SSA at the same grain size due to differences in density and internal structure

Significance of high SSA (four points)

Adsorption (adsorbs gases, liquids, solutes); soil fertility (nutrient retention and availability); reactivity (chemical and physical interactions); catalysis (more active sites)

Cube-splitting example: total surface area of 1 mm³ cut into 1, 2, 8 pieces and clay-sized particles

1 piece = 6 mm²; 2 pieces = 8 mm²; 8 pieces = 12 mm²; clay-sized (about 1000 million pieces) = 6000 mm²

Particle size classes (USDA)

Very coarse sand 2.00-1.00 mm; coarse sand 1.00-0.50; medium sand 0.50-0.25; fine sand 0.25-0.10; very fine sand 0.10-0.05; silt 0.05-0.002; clay < 0.002 mm

Surface area per gram by size class (Foth 1978)

Very coarse sand 11 cm²; coarse sand 23; medium sand 45; fine sand 91; very fine sand 227; silt 454; clay about 8,000,000 cm² (montmorillonite)

Number of particles per gram: very coarse sand vs clay

Very coarse sand about 90; clay about 90,260,853,000

Texture vs reactivity graph

As particle diameter decreases (sand to silt to clay), surface area, reactivity and ability to hold nutrients and water increase

Surface area correlates with...

Retention of water, cation exchange capacity (CEC), and adsorption and release of various chemicals

Density (general)

Mass divided by volume; ρ = m_t/V_t in kg/m³ or g/cm³

The two densities used for sediments and soils

Particle density and bulk density

Particle density

Average density of individual soil particles, excluding pores; ρ_s = m_s/V_s

Typical particle densities

Inorganic materials approx. 2,650 kg/m³; organic material approx. 1,500 kg/m³

Bulk density

Density of dry soil including solids AND pore space (no water); ρ_b = m_s/V_t = m_s/(V_s + V_v)

Typical bulk density range

800-1,600 kg/m³

Why must bulk density be lower than particle density?

Bulk density includes the volume of pore space (gaps between particles)

Porosity from densities

n = 1 − ρ_b/ρ_s

Worked example: porosity when bulk density = 900 kg/m³ and particle density = 2,600 kg/m³

n = 1 − (900/2600) = 0.65 or 65%

Calculation: porosity when ρ_b = 1,300 kg/m³ and ρ_s = 2,650 kg/m³

n = 1 − 1300/2650 = 0.51 (51%)

Calculation: bulk density when n = 0.40 and ρ_s = 2,650 kg/m³

ρ_b = ρ_s(1 − n) = 2650 × 0.60 = 1,590 kg/m³

Calculation: porosity when Vs = 0.5 m³, Vw = 0.3 m³, Va = 0.2 m³

n = (Vw + Va)/Vt = 0.5/1.0 = 0.50 (50%)

Calculation: SSA of spherical grains with d = 0.5 mm and ρ_s = 2,650 kg/m³

A_s = 6/(2650 × 0.0005) = approx. 4.5 m²/kg

Which is relatively constant: particle density or bulk density?

Particle density is relatively constant; bulk density varies between soil types

Organic soils (peat) vs mineral soils: bulk density

Peat has much lower bulk density than mineral soil