You differentiate seawater, brackish water, and groundwater primarily by their Total Dissolved Solids (TDS) levels:

• Seawater: Very salty, typically 10,000–35,000 ppm TDS

• Brackish water: Slightly salty, typically 1,000–10,000 ppm TDS

• Freshwater: Fresh to mildly salty depending on location (<1,000 ppm TDS)

TDS (Total Dissolved Solids) indicates how much mineral/salt content is in water. Measured using a TDS meter.

WHO's TDS Palatability Scale (mg/L):

• < 300: Excellent

• 300 - 600: Good

• 600 - 900: Fair

• 900 - 1200: Poor

• > 1200: Unacceptable