Table 1 below displays in-stream irrigation season (March 2nd-October 31st) electrical conductivity (EC) and sodium adsorption ratio (SAR) standards for the Powder River, Little Powder River, Tongue River, and Rosebud Creek. 30-day average EC and SAR values cannot exceed the values listed, and any single sample cannot exceed the maximum EC and SAR values listed. Table 2 presents standards approved for the non-irrigation season (November 1st-March 1st).

Table 1. Irrigation season EC and SAR standards for streams within the Powder River Basin.
Stream	30-day Average EC (mmhos/cm)	Maximum EC (mmhos/cm)	30-day Average SAR	Maximum SAR
Powder River	2000	2500	5.0	7.5
Little Powder River	2000	2500	5.0	7.5
Tongue River	1000	1500	3.0	4.5
Rosebud Creek	1000	1500	3.0	4.5

 

Table 2. Non-irrigation season EC and SAR standards for streams within the Powder River Basin.
Stream	30-day Average EC (mmhos/cm)	Maximum EC (mmhos/cm)	30-day Average SAR	Maximum SAR
Powder River	2500	2500	6.5	9.75
Little Powder River	2500	2500	6.5	9.75
Tongue River	1500	2500	5.0	7.5
Rosebud Creek	1500	2500	5.0	7.5

Source: http://www.deq.state.mt.us/dir/Legal/Chapters/CH30-06.pdf

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