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Tongue River EC and SAR Water Quality Standards

In 2003, electrical conductivity (EC) and sodium adsorption ratio (SAR) water quality standards were set for the Tongue River. Those standards are shown in the tables below.

Table 1. Electrical Conductivity Water Quality Standards for the Tongue River

Table 2. Sodium Adsorption Ratio Water Quality Standards for the Tongue River

What is EC and SAR?

Electrical conductivity (EC) is a measure of the amount of dissolved solids (salts) in water and is generally expressed as microSiemens per centimeter (µS/cm). Electrical conductivity is also a measure of the ability of water to conduct electricity, and is temperature dependent. The more cations (Na⁺ Ca²⁺, Mg²⁺, etc.) and anions (HCO₃^-, SO₄^2-, Cl^-, etc.) that are in the water, the higher the EC. 

As the EC in soil water increases, a threshold is reached where further increases in EC cause decreases in plant growth. The EC of soil water is directly affected by the EC of irrigation water. The EC of the soil water may be higher than the EC of the irrigation water because plants and evaporation remove water from the soil but do not remove salts. Unless salts are removed or leached from the soil by excess water, the concentration of salts in the soil will build up as irrigation water is added over time.

Sodium adsorption ratio (SAR) is a measure of the abundance of sodium relative to the abundance of calcium and magnesium in water. It is a unitless measure. A high SAR means high amounts of sodium compared to calcium and magnesium (and vice versa). SAR is directly related to the amount of sodium that is absorbed by soils. A high SAR in irrigation water has the potential to impair soil structure and thus, the permeability of the soil leading to a lack of soil moisture.

How the Monthly EC and SAR Standards were Derived/Set

The EC and SAR standards were set to protect the most sensitive beneficial use, which for the Tongue River is the agricultural water quality use. The time period between March 1 and October 31 was chosen for the irrigation season standards. As shown in the tables above, there are also standards for the non-irrigation season, which is set for November 1 through March 1.

The electrical conductivity monthly average standard of 1,000 µS/cm for the irrigation season (March 2 – October 31) was set to protect the most salinity-sensitive crops being irrigated in the Tongue River watershed. This information was based on surveys of irrigators using Tongue River water in 2001 and established that strawberries, common beans, and carrots were the most sensitive crops being produced.

The electrical conductivity monthly average standard of 1,500 µS/cm for the non-irrigation season (November 1 – March 1) was set to protect riparian vegetation. Montana’s narrative water quality standards prohibit concentrations of substances that are harmful to plant life (ARM 17.30.637 (1)(d)).

The irrigation season monthly average sodium adsorption ratio standard was set to protect soil structure in the Tongue River watershed, based on numerous scientific studies and taking rainfall effects into account. Monthly average SAR limits of 5.0 on the Tongue River where sensitive riparian species were inventoried were chosen to protect these species during the non-irrigation season.

Tongue River Project Contacts

See the Tongue River Watershed Project Contacts page for a full description of each project team member's role and for a list of additional contacts. 

Page Released: December 6, 2022

Last Updated: May 24, 2023