While often perceived as entirely salt-free, freshwater does contain dissolved salts and minerals. The perception of it being “fresh” comes from its very low concentration of these substances, making it distinct from the salty taste of ocean water. These small amounts of dissolved solids are naturally present in all water found on Earth.
What is Freshwater?
Freshwater is characterized by its low concentration of dissolved salts, less than 1,000 parts per million (ppm) or 1 gram per liter. This distinguishes it from brackish water, which has intermediate salinity, and saltwater, like oceans, which average around 35,000 ppm. The term “fresh” refers to this limited salt content, not a complete absence.
Common sources of freshwater include rivers, lakes, ponds, streams, glaciers, and groundwater. These bodies of water are continually replenished by precipitation, such as rain and snow. The degree of saltiness is the defining factor, allowing these diverse aquatic environments to support life adapted to low-salinity conditions.
Where Does Salt in Freshwater Come From?
Dissolved salts and minerals naturally enter freshwater systems through several environmental processes. Rainwater, slightly acidic from absorbed carbon dioxide, acts as a solvent. As this water flows over land and through the ground, it weathers rocks and soil, dissolving minerals like calcium, magnesium, sodium, and potassium. These dissolved ions are then carried into rivers, lakes, and groundwater.
Atmospheric deposition also contributes to the salt content of freshwater. Dust, aerosols, and sea spray carried by winds can deposit small amounts of salts, particularly near coastal areas. Decomposition of organic matter within aquatic environments can also release dissolved solids. Human activities, such as agricultural runoff, industrial discharges, and road salts, can significantly increase salinity beyond natural levels. This human-induced salinization is often called secondary salinization, contrasting with natural primary processes.
Why Low Salinity Matters
The low salinity of freshwater is important for the survival of most aquatic organisms and for widespread human use. Freshwater organisms, including fish, insects, and plants, have evolved to thrive in low-salt environments. Higher salt concentrations disrupt their cellular processes through osmosis, causing dehydration and cell damage. Increased salinity can lead to reduced biodiversity and negatively impact the growth and survival of sensitive species.
For human societies, low-salinity freshwater is an important resource. It is the primary source for drinking water, as high salt content makes water unpalatable and can pose health risks. Agriculture heavily relies on freshwater for irrigation, as saline water can stunt plant growth, inhibit nutrient uptake, and damage soil structure, leading to reduced crop yields. Industrial processes, such as manufacturing and cooling, also depend on low-salt water to prevent equipment corrosion and ensure efficient operations. Treating highly saline water through desalination is an energy-intensive and costly process, making naturally fresh sources economically advantageous.