Age-dating, isotopes, and quality of ground water at the Great Sand Dunes National Monument, Colorado
Age-dating, isotopes, and quality of ground water at the Great Sand Dunes national Monument, Colorado
Study Area: The Great Sand Dunes National Monument
The Great Sand Dunes National Monument is located in south-central Colorado along the eastern edge of the San Luis Valley. The predominant feature in the Monument is the 39 square-mile sand dune field which rises up to 750 feet above the valley floor. The dune field is a young geologic feature; U.S. Geological scientists estimate that the dunes are between 2,000 and 12,000 years old. The composition of the sand indicates that the majority of it originated in the San Juan Mountains to the west. The sand was transported to the Valley floodplains by the Rio Grande and its tributaries and carried by prevailing southwesterly winds to the Sangre De Cristo Mountain front. The shape of the Sangre De Cristo Mountains acts as a sand trap.
Two creeks flow along the perimeter of the dune field and influence its shape. Sand Creek flows along the northwest edge and Medano Creek flows along the east and southeast edges. These creeks erode sand from the downwind parts of the dune field and transport and deposit it to the upwind side. This recycling of sand is a crucial process in maintaining stability and long-term viability of the dune field. All the water in Medano and Sand Creeks, except what is lost to evapotranspiration, infiltrates into local aquifers. An important ecological feature of the Monument is the palustrine wetlands located within the dune field, which are comprised of intermittent ponds anddepressions that nearly intersect the water table and support plants which prosper in waterlogged conditions (hydrophytes). These interdunal ponds provide important habitat for a variety of species that depend on a wetland habitat in an otherwise desert environment.
There is much concern that ground-water levels within the Great Sand Dunes National Monument may be changing. The existence and natural maintenance of the dune field, and the occurrence of interdunal ponds, are dependant on maintaining ground-water levels at historic elevations. Water flow in Medano and Sand Creeks stabilizes the overall dune field by capturing and transporting blown sand back into the upwind side of the dune field, which would otherwise escape into the Sangre De Cristo Mountains. A drop in local ground-water levels would likely increase infiltration into the creek beds by decreasing the flow in the creeks and decreasing the distance the creeks flow before disappearing into the sand. This would reduce the capability of the creeks to transport sand back to the upwind side of the dune field, jeopardizing the long-term viability of the dune field. The interdunal ponds are of special concern because they have diminished markedly in recent years. Analysis of ten sets of aerial photographs from 1936 through 1995 demonstrated that the number of ponds with water present at the surface dropped from 36 in 1936 to zero in 1966, and then gradually increased to nine in 1995 (Chatman and others, 1997). The relatively sudden disappearance of ponds appears to indicate a significant change in Monument hydrology, the cause of which has yet to be determined. Quantification of ground-water ages and sources of recharge can assist in understanding the complexities of the ground-water flow system at the Monument.