NITRATE IN THE SUMMIT VALLEY OF SOUTHWEST MONTANA

The Summit Valley is a 60-square mile alluvial-filled intermontane basin located in southwest Montana. The valley is bounded on the south and east by the Continental Divide, is home to the city of Butte, and is drained by Blacktail and Silver-Bow Creeks. Alluvium, derived from the granitic rocks that frame the valley, and fractured bedrock along the valley margin yield sufficient water for domestic purposes.

Reconnaissance ground-water sampling in the Summit Valley revealed elevated nitrate concentrations (greater than 2.0 mg/L) in the alluvial and bedrock aquifers. A review of the valley’s recent and historic ground-water analyses shows that 64 percent (96 of 149) of the samples had nitrate concentrations greater than an assumed background concentration of 2.0 mg/L, and 15 percent (22 of 149) of the samples exceed the 10.0 mg/L health standard. Elevated nitrate concentrations were detected below sewered urban/residential areas, and unsewered residential areas, as well as in shallow (less than 50 feet deep) wells and deep (greater than 200 feet deep) wells. Sampling of Blacktail and Silver Bow Creeks, during base flow conditions showed that concentrations of nitrate exceeded 1.0 mg/L over a 5-mile stretch through the most densely populated part of the valley. The results indicate that the elevated nitrate in the ground water impacts the surface water in the valley during low-flow conditions.

To assist with source identification, ground-water samples were collected from fourteen wells in different aquifers and land use settings for analysis of stable isotopes of nitrogen and oxygen. The results showed that the nitrate in all the samples had a similar isotopic signature. The δ¹⁵N values ranged between 4 and 12 per mill, while the δ¹⁸O (of the NO₃) values ranged between –4 and +2 per mill. The measured δ¹⁵N and δ¹⁸O values are suggestive of animal or septic waste rather than a synthetic fertilizer source for the nitrate.