Stranded on an Island: Contaminated Water vs. Seawater — Choose Contaminated

The Lesser of Two Deadly Evils

If forced to choose between bacterially contaminated fresh water and seawater — drink the contaminated fresh water. Here's why, and how to survive it.

Why Seawater Kills Faster

Salinity: ~3.5% salt (35 g/L). Kidneys can only produce urine ~2% salt. To excrete 1 L seawater, you need 1.5 L body water. Net loss: 0.5 L per liter drunk. Accelerates dehydration.
No Simple Fix: Boiling concentrates salt. Filtration (cloth, sand) doesn't remove dissolved ions. Desalination requires distillation (energy-intensive) or reverse osmosis (high pressure) — impossible with island improvised gear.
Physiology: Hypernatremia → cellular shrinkage → confusion, seizures, coma, death. Timeline: days.

Why Contaminated Fresh Water Is Treatable

Contaminants: Mostly pathogens (bacteria, viruses, protozoa, cysts) — particulates, not dissolved solids.
Boiling: 1 min rolling boil (3 min >2,000 m) kills virtually all pathogens. Zero tech.
Filtration: Cloth (removes copepods/guinea worm), sand/charcoal filter (improvised), ceramic filter, Lifestraw — remove bacteria/protozoa. Viruses need chemical/UV/boiling.
Chemical: Bleach (2 drops/L, 30 min), iodine tablets, chlorine dioxide.
SODIS: Clear PET bottle, 6 hrs sun (UV-A + heat) — WHO endorsed.
Distillation: Solar still (hole, container, plastic sheet) — produces pure water from any source, even urine/seawater. Slow (0.5–1 L/day).

Decision Protocol

Find fresh water (streams, rain catchment, dew, solar still, digging at vegetation line).
If only contaminated fresh water: boil (best), filter + chemical, or SODIS.
If ONLY seawater: solar still (only viable desalination). Do NOT drink seawater directly.
Conserve sweat: rest by day, move by night. Minimize protein (metabolic water cost).

Exceptions

Chemical contamination (industrial, agricultural, radiological) — boiling/filtration won't help. Seawater still worse. Solar still may concentrate volatiles. Avoid visibly polluted sources.