Sandstone and Limestone Aquifers Along the Northwest Flank of the Little Rocky Mountains, Fort Belknap Indian Reservation

The study covers about 55 square miles in the southwestern Fort Belknap Indian Reservation, north-central Montana. It focuses on five aquifers: Lodgepole Limestone, Mission Canyon Limestone, Kootenai Formation, Colorado Group, and Eagle Sandstone. The goal is to assess water levels, hydraulic characteristics, water quality, and supply potential for domestic and municipal use. Thirty-one test holes were drilled; 25 completed as monitoring wells.

Findings

Eagle Sandstone (Virgelle Sandstone Member) 

• 17 wells drilled; water levels fluctuate seasonally (0.6–9 ft). 
• Water flows predominantly west/northwest, away from the mountains. 
• Specific capacity ranges from 0.02 to 0.59 gal/min/ft; median is 0.27 gal/min/ft. 
• Transmissivity values: 56–91 ft²/day. 
• Water is mostly calcium/magnesium bicarbonate type; dissolved solids range from 381–1,930 mg/L. 
• Most wells suitable for livestock watering; some exceed EPA secondary contaminant levels for sulfate, dissolved solids, iron, and manganese.

First Cat Creek & Third Cat Creek Sandstones 

• One well each; water levels in First Cat Creek declined steadily, Third Cat Creek fluctuated seasonally. 
• Specific capacities: 0.16 (First Cat Creek), 0.08 (Third Cat Creek) gal/min/ft. 
• Transmissivity: 21 ft²/day (First Cat Creek), 15 ft²/day (Third Cat Creek). 
• Water from First Cat Creek: sodium bicarbonate type, 1,160 mg/L dissolved solids (exceeds EPA secondary limits). 
• Water from Third Cat Creek: calcium magnesium bicarbonate type, 428 mg/L dissolved solids (suitable for domestic/stock use).

Mission Canyon & Lodgepole Limestones 

• Five wells (Mission Canyon), one well (Lodgepole); depths 130–475 ft. 
• Water levels fluctuate seasonally (3–21 ft). 
• Specific capacity (Mission Canyon): 0.35–4.6 gal/min/ft; median 2.0 gal/min/ft. 
• Transmissivity: up to 1,100 ft²/day (Mission Canyon). 
• Water is calcium bicarbonate type; dissolved solids 263–335 mg/L (Mission Canyon), 392 mg/L (Lodgepole). 
• Water quality suitable for domestic, municipal, or stock supply.

Eagle Sandstone

• Wells likely produce ≥5 gal/min, adequate for domestic/stock supply; quality may be limiting due to dissolved solids, iron, manganese. 

First Cat Creek

•  Adequate yield for domestic/stock use; quality issues with sulfate/dissolved solids. 

Third Cat Creek

• Yield may be insufficient for domestic use; quality generally suitable. 

Mission Canyon Limestone

• Can supply large quantities (≥66 gal/min), but yield depends on intersecting fracture systems or caverns; quality generally good. 

Lodgepole Limestone

• Yield depends on intersecting fractures; quality suitable.

Summary

• Water levels in all aquifers show seasonal fluctuations, generally highest in spring/summer and lowest in fall/winter. 
• Water quality varies by aquifer; some exceed EPA secondary standards, especially for dissolved solids and iron. 
• The Mission Canyon Limestone offers the best potential for large yields and good water quality, but production is highly variable depending on local geology.