Ch5: Sources and Usage of Water

Total water on Earth is overwhelmingly saline: only $3\%$ is fresh, potable water.
- Common misconception (held by many students) that there is “plenty” of freshwater because we visually observe many surface sources.
Breakdown of that $3\%$ :
- $68\%$ locked in glaciers, ice caps, permanent snow ➔ not readily accessible.
- $30\%$ stored as groundwater (aquifers) – must be pumped to the surface.
- $0.3\%$ present in lakes, rivers, wetlands – the most accessible fraction.
Analogy (two-liter soda bottle):
- Entire bottle = all Earth’s water.
- Only $60\,\text{mL}$ (≈ $3\%$ ) would be freshwater.
- Just four drops represent all water in lakes & rivers.
Significance: highlights extreme scarcity; underpins urgency for conservation and innovation in water management.

Daily withdrawal (all sectors, U.S. only): $322\,000\,000\,000\,\text{gal day}^{-1}$ (≈ $1.22\times10^{12}\,\text{L day}^{-1}$ ).
Dominant use sectors (per instructor’s referenced graph):
- Thermoelectric power generation.
- Irrigation (agriculture).
Minor fraction sourced from seawater (desalination), majority remains freshwater.
Ethical/practical connection: feeding and powering society currently demand the bulk of withdrawals; prompts research into efficiency gains, reuse, and alternative technologies.

Agriculture consumes the largest share (irrigation).
UC Davis’ world-renowned agriculture department works on drought-tolerant crops that require progressively less water.
- Example research directions: genetic modification, selective breeding for water-use efficiency (WUE).
Encouragement to students: consider majors or undergraduate research in this field; science can be a positive force, not merely “playing God.”

Scientists measuring aquifer levels report declines year-after-year.
Predictions (hope they do not materialize): future wars may be fought over water rather than oil.
Moral imperative: develop water-wise technologies and policies to avert conflict.

Analogous to carbon footprint; quantifies total water embedded in goods & services.
Critical because humans can endure heat but cannot survive > $48$ – $72\,\text{h}$ without water.
Classroom discussion: compare crops vs. meat.
- For meat you must add:
1. Water the animal drinks.
2. Water to grow its feed (plants).
Representative values (per “Table 1” referenced):
- $1\,\text{kg}$ beef → $15\,400\,\text{L}$ water.
- Crop products markedly lower (exact numbers not verbalized but implied).
Take-home: dietary choices materially affect global water demand; science may yield solutions (e.g.
synthetic meat, improved feed conversion, mindful diets).

Map shows large portions of Africa, Asia, parts of South America where < $75\%$ of the population has reliable access to clean drinking water.
Social impacts highlighted:
- Many girls/women walk 5–6 h daily to fetch water ➔ lost educational opportunities.
- Links to health, hygiene, disease burden.

Evaluate each daily activity:
1. Role of water (consumptive vs. process use).
2. Source (tap, bottled, recycled, etc.).
3. Quantity.
4. Fate (where does it go?).
5. Degree of contamination introduced.
Discussion points raised by students:
- Laundry & dishwashing add detergents → water becomes chemically laden, challenging to reuse in agriculture.
- Potential for greywater systems: collect lightly contaminated water, reuse for toilet flushing or landscape irrigation.
- Technological needs: additional plumbing loops, underground storage tanks, on-site treatment/purification.

Mean per-capita use: $\approx100\,\text{gal day}^{-1}$ .
About $\tfrac{3}{4}$ of that volume ultimately goes down the drain.
- Raises questions of capture, treatment, and reuse rather than single-pass disposal.

Key U.S. example: the Ogallala Aquifer (Great Plains) – one of the world’s largest.
Ideal assumption: groundwater is pure, naturally filtered.
Reality: four major contamination vectors
1. Abandoned/poorly managed mines → acid mine drainage, heavy metals.
2. Runoff from fertilized fields → nitrates, phosphates, leading to eutrophication when surface water is affected.
3. Improperly constructed landfills & septic systems → leachate of organics, pathogens.
4. Household chemicals poured down drains/onto ground → solvents, surfactants, pharmaceuticals.
Chemical contamination often travels with infiltrating rainwater → eventually reaches aquifers.

Multiple stages possible: physical (filtration, sedimentation), chemical (coagulation, disinfection), biological (activated sludge, biofilms).
Greywater vs. blackwater treatment distinctions.
Instructor’s mantra: “Chemistry has been part of the problem, but must also be part of the solution.”
Students encouraged to apply core chemical principles to innovate in treatment technologies.

Water scarcity is a global, cross-disciplinary issue intertwining science, engineering, policy, and social justice.
Students at UC Davis (or any institution) possess agency to contribute through coursework, research, and personal choices.
Mindful stewardship today mitigates human suffering and geopolitical tension tomorrow.