What is salinity?
Salinity is the total amount of minerals (also known as salts) that are dissolved in water. Salts include sodium chloride (regular table salt) and other minerals such as potassium, calcium or magnesium. Salinity is measured in terms of Total Dissolved Solids (TDS) in milligrams/Liter (mg/L).
Based on salinity, water can be classified into three groups:
- Fresh Water — less than 1,000 mg/L;
- Brackish Water — between 1,000 and 25,000 mg/L; and
- Seawater — greater than 25,000 mg/L.
Caption plum:Impacts of salinity on plumbing fixtures
What are the effects of salinity?
According to the World Health Organization, water with a TDS over 1,200 mg/L is generally designated as unacceptable for human consumption because it may cause adverse health effects, such as diarrhea. At levels in the range 900-1,200 mg/L, it can affect the taste and color of water. However, the EPA does not treat dissolved salts as contaminants and does not set maximum limits on their levels. The main effects of high salinity levels are deposits on pipes and fixtures and difficulty in growing some crops and plants.
Water hardness and salinity are not the same. Hardness is the total concentration of calcium and magnesium in water, whereas salinity includes other dissolved solids. Water hardness is measured in grains per gallon (GPG). The U.S. Department of Interior has defined hard water as water having more than 7 GPG of calcium and magnesium. In Arizona, hardness levels can reach 20 GPG, well over the limit for water to be considered very hard. Below is scale of relative hardness values.
|MEASUREMENT||GRAINS PER GALLON||MILLIGRAMS PER LITER|
|Soft||under 1 gpg||under 17.1 mg/L|
|Slightly Hard||1.0 - 3.5 gpg||17.1 - 60 mg/L|
|Moderately Hard||3.5 - 7.0 gpg||60 - 120 mg/L|
|Hard||7.0 - 10.5 gpg||120 - 180 mg/L|
|Very Hard||over 10.5 gpg||over 180 mg/L|
What are the sources of salinity?
Salinity comes from many sources including:
- Natural minerals in rocks found in lakes, rivers, streams and aquifers;
- Water from natural salt springs that enters into rivers, lakes and streams;
- Agricultural fertilizers that drain from fields into rivers, lakes, streams and aquifers;
- Water treatment chemicals such as chlorine that make water safe for human consumption;
- Home water treatment systems, like water softeners, that treat water for hardness;
- Cleaning chemicals; and
Salinity in your water system
Salinity occurs naturally in surface water from the Salt River and Colorado River systems as well as some groundwater areas in central Arizona. In addition, human use adds salinity into the wastewater system so that salinity levels in reclaimed water are typically higher than in other sources. The table below shows typical salinity concentrations found in waters in central Arizona.
|Water Source||Total Dissolved Solids|
|Salt River||580 mg/L|
|Verde River||270 mg/L|
|Central Arizona Project (CAP)||650 mg/L|
|Groundwater||200 - 5,000 mg/L|
|Reclaimed Water||Typically 300 - 500 mg/L higher than source water|
Why are water providers concerned about salinity?
Salinity restricts how we can use our water. The following are examples of how high salinity levels may impact the use of water and reclaimed water.
- Farmers may have to grow salt tolerant crops such as cotton and crops may need more water.
- High salinity water leaves mineral deposits on municipal and household pipes and fixtures, reducing the expected lifespan of this equipment.
- Using high salinity water to recharge groundwater systems may increase the salinity of groundwater even further.
- Irrigation of parks, golf courses and other open space with high salinity water may reduce the growth rates in turf and other plants.
- Build-up of salts in cooling towers can increase water usage and may lead to equipment damage.
- High salinity water may increase manufacturing costs for some industries.
Where can I find out more?