Groundwater in the Mancos Shale, Pagosa Springs, Colorado

By Darrel Dunn, Ph.D., PG, Hydrogeologist

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The Cretaceous Mancos Shale is at the land surface in a large area extending northwest and southeast of Pagosa Springs in southwestern Colorado.  The area is on the northeastern flank of the Archuleta Anticlinorium.  The anticlinorium is on the northeastern side of the San Juan Basin.  The Dakota Sandstone, which underlies the Mancos Shale is exposed along the crest of the anticlinorium.  The eroded Mancos Shale is over 1000 feet thick thick in the eastern part of the area and thins southwestward toward the exposed Dakota Sandstone.

Water wells in the Mancos Shale are as much as 200 feet deep and yield from 1 to 30 gallons per minute (gpm) .  The water is produced from fractures and discontinuous sandy layers.  The quality of the water from the Mancos Shale is generally poor.  Total dissolved solids (TDS) range from 1000 to 2500 milligrams per liter (mg/l), with high concentration of sodium, calcium, iron, and sulfate.  Dissolved hydrogen sulfide is common.

Where the Mancos Shale is thin or absent, wells are completed in the Dakota Sandstone.  The water comes from fractures.  Yields range up to 30 gpm.  The quality of the water is variable but some TDS  is less than 1000 mg/l.