Rainwater is known for having very low levels of Total Dissolved Solids (TDS).
According to water quality categorizations, rainwater falls into the low TDS water category. This means it typically contains between 0 and 20 MgL-1 TDS. (Note: MgL-1 is equivalent to mg/L, or milligrams per liter).
Understanding Total Dissolved Solids (TDS)
Total Dissolved Solids refers to the total concentration of all dissolved substances in water, including minerals, salts, metals, and organic matter. It's a measure of the amount of non-water particles present in a water sample that are small enough to pass through a very fine filter (typically 2 micron).
Common dissolved substances can include:
- Minerals like calcium, magnesium, sodium, and potassium.
- Salts (e.g., chlorides, sulfates).
- Trace amounts of metals and organic compounds.
Why is Rainwater Low in TDS?
Rainwater starts as evaporated water vapor, which leaves most impurities behind. As the vapor condenses into clouds and falls as rain, it is essentially distilled water.
While rainwater can pick up small amounts of dissolved gases (like carbon dioxide, making it slightly acidic) and airborne particles as it falls through the atmosphere, these contribute very little compared to the dissolved minerals and salts found in groundwater or surface water.
TDS Levels and Water Quality
TDS is one parameter used to indicate overall water quality, although it doesn't tell the whole story (it doesn't measure contaminants like bacteria or pesticides).
- Low TDS Water: Often perceived as having a bland taste because it lacks dissolved minerals. Rainwater fits here (0-20 MgL-1).
- Moderate TDS Water: Generally considered palatable. Tap water typically falls in this range (often 50-500 mg/L, but varies greatly).
- High TDS Water: Can have a distinct taste (often described as mineraly, salty, or bitter). The provided reference notes that most consumer complaints about taste are caused by high TDS. Water with very high TDS may also cause scaling in pipes and appliances.
Summary Table
| Water Type | Typical TDS Range (MgL-1 or mg/L) | Note |
|---|---|---|
| Rainwater | 0-20 | Naturally distilled, very low TDS |
| Distilled Water | ~0 | Pure H₂O, no dissolved substances |
| Tap Water | Varies widely (e.g., 50-500+) | Contains dissolved minerals/salts |
| Seawater | ~35,000 | Very high in dissolved salts |
Based on the reference provided: Rainwater is categorized as low TDS water which contains (0-20) MgL-1 TDS.
