Field Sodium Adsorption Ratio (SAR) Analyzer
January 2004
(Superceded February 2005)

Sodium adsorption ratio (SAR), calculated from the concentrations of sodium (Na), calcium (Ca), and magnesium (Mg), is an important water-quality indicator, especially for agricultural purposes. Because of the importance of real-time SAR information for irrigators in the Tongue River watershed, this project is developing two methods for providing real-time SAR data. The first method relies on estimating SAR from continuous data on specific (electrical) conductance. However, this method can only provide an estimate of SAR, and the reliability of the estimated SAR data may change with changes in streamflow or changes in sources or loads of salts to watershed streams. Fluctuations in Na, Ca, or Mg concentrations could significantly change the SAR without necessarily changing the specific conductance. Therefore, we are exploring a second approach to providing real-time SAR data. This approach involves real-time monitoring of Na, Ca, and Mg concentrations. If successful, this approach would provide valuable confirmation of SAR values estimated from specific conductance as well as timely information to Tongue River water users.

The new real-time field SAR analyzer will be placed directly in the stream. The analyzer will automatically analyze water samples for Na, Ca, and Mg, and then calculate the SAR. The field SAR analyzer will be a battery-powered instrument that uses a syringe pump and multi-port valve to draw sample water into the analyzer, add reagents, and propel the sample and reagents to the detector (fig. 1). Two detection methods will be evaluated: 1) ion-selective electrodes, and 2) photometric and fluorometric methods. The field SAR analyzer is self-calibrating, analyzing an onboard standard and blank several times a day to verify that the instrument is working properly. An onboard computer with non-volatile memory performs instrument control, data storage, and transfers data to the telemetry system at an existing USGS gaging station (table 1).

Water samples periodically collected by field personnel will be analyzed for Na, Ca, and Mg in the laboratory. Results of these analyses will provide independent confirmation that the real-time field SAR analyzer is working properly. In contrast to traditional techniques where water samples are sent to a laboratory and results are available weeks later, the use of field SAR analyzers can dramatically increase the number of samples analyzed (several per day versus 20 per year), reduce sampling costs, and provide important real-time surface-water-quality information to stakeholders along the Tongue River.

Table 1. General design characteristics of the field SAR analyzer.

For more information, contact:
Thomas Chapin
U.S. Geological Survey
P.O. Box 25046, Mail Stop 973
Denver Federal Center
Denver, CO 80225
303-236-5795

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