Abstract

Both fractured rock and unconsolidated deposits are aquifers in San Juan County. The major water-yielding rock units have been grouped into six aquifers. From oldest to youngest (in order of decreasing depth at any given location), these aquifers are the Redwall (the Leadville Limestone and permeable intervals in the overlying Molas, Pinkerton Trail, and Honaker Trail Formations), P (permeable beds in the undifferentiated Cutler Group in northeastern San Juan County and the Cedar Mesa Sandstone of the Cutler Group elsewhere), C (the DeChelly Sandstone of the Cutler Group), N (the Wingate Sandstone, Kayenta Formation, Navajo Sandstone, Carmel Formation, and Entrada Sandstone), M (the Bluff Sandstone, Salt Wash, Recapture, and Westwater Canyon Members of the Morrison Formation), and D (the Burro Canyon Formation and Dakota Sandstone) aquifers. In general, the shallowest aquifers are best because they commonly contain higher quality water and are more easily accessible than deeper aquifers. The Redwall aquifer is probably the most widespread, continuous aquifer in San Juan County. The N aquifer is the main source of domestic and livestock water. Unconsolidated deposits provide water to springs and wells at many locations but are a major source of water only in southern Spanish Valley and the San Juan River valley. Aquifers receive recharge through infiltration of precipitation and stream flow, mostly in areas with elevations greater than 8,000 ft (2,440 m). Regional movement of ground water is generally from higher elevation recharge areas in the mountains and high plateaus to lower elevation discharge areas such as deeply incised canyons along major rivers, especially the Colorado.