What is salinity?
Salinity is the total amount of minerals (also known as salts) that are dissolved in water. Salts can be made of sodium chloride (regular table salt) or other minerals such as potassium, calcium or magnesium.
Salinity is measured by Total Dissolved Solids (TDS) in milligrams/Liter (mg/L)
Fresh Water <1,000 mg/L
Brackish Water >1,000 - 25,000 mg/L
Seawater >25,000 mg/L
Is salinity damaging to human health?
Water with a TDS over 1,200 mg/L is generally designated at unacceptable for human consumption by the World Health Organization (1996) because it may cause adverse health effects, such as diarrhea. It is typically rejected as a drinking water source in ranges between 900-1,200 mg/L because it can affect the taste and color of water. TDS does not have a set Maximum Contaminant Limit (MCL) and is therefore not regulated by the United States Environmental Protection Agency (EPA). A Secondary MCL (SMCL) of 500 mg/L is set for TDS, but a SMCL is not an enforceable limit. See the EPA guidelines for TDS.
How is "hard water" different from salinity?
Hard water is the total concentration of calcium and magnesium in water. 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.
Below is a 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
Does a water softener make water less saline?
No, water softeners exchanges sodium (or potassium) ions for calcium or magnesium ions, but the total measurement of TDS does not change. Therefore, water softeners only make hard water softer.
How do water softeners work?
The typical water softener has two parts: a mineral tank and a brine tank. The mineral tank is filled with small polystyrene beads, also known as resin or zeolite.
The beads carry a negative charge. Calcium and magnesium in water both carry positive charges. This means that these minerals will cling to the beads as the hard water passes through the mineral tank.
The brine tank is filled with a brine solution created by adding common salt (sodium) to water. Like calcium and magnesium, sodium ions have positive charges, although not as strong as the charge on the calcium and magnesium.
In normal operation, hard water moves into the mineral tank and the calcium and magnesium ions move to the beads, replacing sodium ions on the beads. The sodium ions go into the water. Once the beads are saturated with calcium and magnesium, the unit enters a regenerating cycle.
First, a backwash phase reverses water flow through the mineral tank to flush any dirt out of the tank. Then in the recharge phase, the concentrated sodium-rich salt solution from the brine tank is flushed through the mineral tank.
When the very strong brine solution is flushed through the mineral tank that has beads already saturated with calcium and magnesium, the sheer volume of the sodium ions is enough to drive the calcium and magnesium ions off the beads. The sodium collects on the beads, replacing the calcium and magnesium, which go down the drain. Once this phase is over, the mineral tank is flushed of excess brine and the brine tank is refilled.
What are the sources of salinity?
Salinity comes from many sources including:
1. Natural minerals in rocks that as water flows over the rock into rivers, lakes, streams or through aquifers.
2. Water from natural salt springs that enters into rivers, lakes and streams.
3. Added fertilizers on agricultural fields that drain into rivers, lakes, streams and aquifers.
4. Treatment chemicals such as chlorine that make water safe for human consumption.
5. Home water treatment systems, like water softeners, that treat water for hardness.
6. The use of cleaning chemicals.
Where is salinity found in our water systems?
Salinity is naturally occurring in our 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 and is therefore in our reclaimed water system.
Below is a summary of typical (average) salinity concentrations.
WATER SOURCE TDS IN MILLIGRAMS PER LITER
Salt River 580 mg/L
Groundwater 200 - 5,000 mg/L
Reclaimed Water Typically 300 - 500 mg/L higher than source water
If human health is not affected, why are water providers concerned about salinity?
Salinity reduces how we can use our water resources. The following are several examples where high salinity water may impact the use of water and reclaimed water.
Many farmers must grow salt tolerant crops such as cotton. In addition, in order to keep the salts away from the plants root zone, as much as 20% more water is needed to keep the plants alive.
Many golf courses use reclaimed water for irrigation purposes. If this water has a salinity content of 1,000 mg/L, this high salinity levels make it difficult to grow turf and in some cases extra water is needed to flush the salts away from the grass roots.
High salinity can deposit minerals on municipal and household pipes and fixtures; therefore, reducing the expect lifespan of these items.
High TDS water that is used to artificially recharge groundwater system may increase salinity of groundwater.
Irrigation of parks and other open space with high TDS waters may reduce the growth rates in turf and other low water use plants. May also increase the water use to flush salts from the root zone.
Build-up of salts on cooling towers limits number of cycles that it can be reused, causing an increase in water usage and may potentially damage cooling tower systems.