Ground-Water Flow Direction, Water Quality, Recharge Sources, and Age, Great Sand Dunes National Monument, South-Central Colorado, 2000-2001
By Michael G. Rupert and L. Niel Plummer
Available from the U.S. Geological Survey, Branch of Information Services, Box 25286, Denver Federal Center, Denver, CO 80225, USGS Scientific Investigations Report 2004-5027, 28 p., 9 figs.
The citation for this report, in USGS format, is as follows:
Great Sand Dunes National Monument is located in south-central Colorado along the eastern edge of the San Luis Valley. The Great Sand Dunes National Monument contains the tallest sand dunes in North America; some rise up to750 feet. Important ecological features of the Great Sand Dunes National Monument are palustrine wetlands associated with interdunal ponds and depressions along the western edge of the dune field. The existence and natural maintenance of the dune field and the interdunal ponds are dependent on maintaining ground-water levels at historic elevations. To address these concerns, the U.S. Geological Survey conducted a study, in collaboration with the National Park Service, of ground-water flow direction, water quality, recharge sources, and age at the Great Sand Dunes National Monument.
A shallow unconfined aquifer and a deeper confined aquifer are the two principal aquifers at the Great Sand Dunes National Monument. Ground water in the unconfined aquifer is recharged from Medano and Sand Creeks near the Sangre de Cristo Mountain front, flows underneath the main dune field, and discharges to Big and Little Spring Creeks. The percentage of calcium in ground water in the unconfined aquifer decreases and the percentage of sodium increases because of ionic exchange with clay minerals as the ground water flows underneath the dune field. It takes more than 60 years for the ground water to flow from Medano and Sand Creeks to Big and Little Spring Creeks. During this time, ground water in the upper part of the unconfined aquifer is recharged by numerous precipitation events. Evaporation of precipitation during recharge prior to reaching the water table causes enrichment in deuterium (2H) and oxygen-18 (18O) relative to waters that are not evaporated. This recharge from precipitation events causes the apparent ages determined using chlorofluorocarbons and tritium to become younger, because relatively young precipitation water is mixing with older waters derived from Medano and Sand Creeks.
Major ion chemistry of water from sites completed in the confined aquifer is different than water from sites completed in the unconfined aquifer, but insufficient data exist to quantify if the two aquifers are hydrologically disconnected. Radiocarbon dating of ground water in the confined aquifer indicates it is about 30,000 years old (plus or minus 3,000 years). The peak of the last major ice advance (Wisconsin) during the ice age occurred about 20,000 years before present; ground water from the confined aquifer is much older than that. Water quality and water levels of the interdunal ponds are not affected by waters from the confined aquifer. Instead, the interdunal ponds are affected directly by fluctuations in the water table of the unconfined aquifer. Any lowering of the water table of the unconfined aquifer would result in an immediate decrease in water levels of the interdunal ponds. The water quality of the interdunal ponds probably results from several factors, including the water quality of the unconfined aquifer, evaporation of the pond water, and biologic activity within the ponds.
Purpose and Scope
Study Area Description
Methods of Investigation
Well Construction and Nomenclature
Measurement of Ground-Water Levels, Water-Quality Sampling, and Analyses
Age-Dating and Isotopic Characterization Techniques
Ground-Water Age, Piston Flow, and Mixing
Hydrogen and Oxygen Stable Isotopes
Water-Quality Sampling Results
Hydrogen and Oxygen Stable Isotopes
Ambient Air Samples
Ground-Water Flow Direction, Water Quality, Recharge, and Age in the Unconfined Aquifer
Ground-Water Recharge Near the Mountain Front
Ground-Water Flow Direction in Unconfined Aquifer
Downgradient Changes of Major Ion Chemistry
Ground-Water Recharge by Precipitation on the Dune Field
Evaporation of Recharge Waters
Big and Little Spring Creeks
Ground-Water Age and Water Quality in the Confined Aquifer
Ground-Water Age in the Confined Aquifer
Major Ion Chemistry and Connection Between Unconfined and Confined Aquifers
Summary and Conclusions
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