Seawater and the Atmospheric Iodine Cycle
Seawater is the most significant natural source of iodine. The concentration of iodine in the ocean is relatively high, and through natural processes, it is released into the atmosphere. Ocean water naturally evaporates, carrying trace amounts of iodine into the air. This atmospheric iodine then returns to land and water sources via rainfall. Coastal areas generally receive more iodine from this cycle than inland regions. This process is the primary reason for the higher incidence of iodine deficiency disorders (IDDs), such as goiter, in inland mountainous areas far from marine sources. The amount of iodine that ends up in local soil and, subsequently, in freshwater and crops, is a direct result of this cycle.
Natural Sources: Groundwater vs. Freshwater
Groundwater
The iodine content in groundwater is highly dependent on geological and geochemical conditions, as well as human activities.
- Geology and Soil: Iodine may leach into groundwater from salt and mineral deposits. Soils and rocks rich in organic matter, sulphides, or iron oxides tend to have higher concentrations of iodine, which can be released into aquifers. Reducing conditions in deeper confined aquifers can also promote the release of iodide from sediments into groundwater. Conversely, areas with thin, organic-poor soils, common in mountainous regions, often have lower iodine levels.
- Human Impact: Intensive human activities, such as groundwater overexploitation, can dramatically alter local iodine levels. In some regions, pumping groundwater can cause the release of iodine-rich pore water from compacted sediments, leading to elevated iodine concentrations. Conversely, in other areas, human consumption and insufficient replenishment can lead to lower availability.
Freshwater
In contrast to the ocean, iodine is rarely found in significant amounts in natural freshwater bodies like lakes and rivers. The concentration of iodine in these sources is typically very low. Freshwater fish, for example, have significantly lower iodine content than their marine counterparts. This highlights the unreliability of freshwater alone as a substantial dietary source of iodine.
Treated Water and Fortified Sources
Tap Water
The iodine in tap water comes from its source—which could be groundwater or surface water—and is influenced by local geology. While trace amounts are usually present, the concentration is highly variable and generally not sufficient to meet daily nutritional needs. In fact, some water treatment processes can affect the form of iodine present. For instance, strong disinfectants like chlorine can oxidize iodide to molecular iodine. Historically, emergency disinfection of water for field use occasionally involved iodine, but this is not standard for larger municipal supplies. For most populations, relying on tap water for iodine is unreliable and often inadequate.
Iodized and Mineral Waters
In recent years, some companies have begun producing bottled mineral water fortified with iodine. The iodine content in these products can vary significantly by brand. Some therapeutic mineral waters, often of marine origin, naturally have high iodine content, but these are typically used under medical supervision. While such waters can contribute to iodine intake, they are not a globally recommended or reliable strategy for preventing iodine deficiency due to inconsistent levels and accessibility. Studies have even shown that the iodine content in fortified mineral waters can decrease over time.
Iodine in Water: A Comparison
To illustrate the diverse iodine content across different water sources, the table below provides a general overview based on average values reported in studies.
| Water Source | Typical Iodine Content (µg/L) | Reliability as Iodine Source | Notes |
|---|---|---|---|
| Seawater | Approx. 58 µg/L | High | Not for direct consumption; primary environmental source. |
| Groundwater | Variable (<1 to >70 µg/L) | Very Low | Depends heavily on local geology and human factors. |
| Freshwater | Very Low | Very Low | Rarely a significant source for humans. |
| Tap Water | Variable (<1 to >70 µg/L) | Very Low | Determined by source and treatment; generally not reliable. |
| Fortified Mineral Water | Variable (e.g., 1–42 µg/L) | Low | Can contribute, but content is often inconsistent or unknown. |
| Therapeutic Mineral Water | High | Conditional | Must be used under medical supervision due to high content. |
A Broader Perspective on Iodine Sources
While water can contain trace amounts of iodine, it is generally not a reliable primary source for human nutrition. The bulk of human iodine requirements should come from a balanced diet, which includes:
- Seafood (fish, shrimp, seaweed)
- Dairy products (milk, cheese)
- Eggs
- Fruits and vegetables (levels vary based on soil content)
The widespread and effective public health measure for preventing IDDs remains the use of iodized salt. For most people, especially in inland regions, dietary sources combined with iodized salt provide the consistent, adequate iodine intake necessary for proper thyroid function and overall health. Relying solely on water could lead to either deficiency or, in some high-iodine areas, excess.
Conclusion
In summary, seawater is the principal natural reservoir of iodine, and its atmospheric cycle is responsible for the trace amounts found in land-based water sources. While groundwater can have variable, sometimes higher, iodine content due to geological factors and human impact, natural freshwater and tap water are not dependable sources for meeting nutritional needs. Fortified and therapeutic mineral waters exist, but their use requires caution and cannot replace a consistent dietary intake. For consistent iodine levels, individuals should focus on iodine-rich foods and, where appropriate, use iodized salt, rather than relying on water sources alone. For further information on the role of iodine in drinking water, the World Health Organization (WHO) provides useful guidance on chemical hazards.
