White Sands National Monument (WHSA), located in southern New Mexico, includes a portion of the largest gypsum dune field in the world. This unique landscape not only offers beautiful views and outdoor activities for tourists, but also serves as a valuable resource for researchers. Scientists from all fields conduct research on a variety of topics, ranging from geology to evolutionary biology. One very interesting feature at WHSA is a very shallow water table. Within the dune field in WSHA, depth to water in interdunal areas ranges from one to three feet below the surface. This type of dune field is called a wet dune system, and the shallow groundwater directly affects dune processes. Groundwater effectively stabilizes sand that has accumulated for thousands of years. At WHSA, up to thirty feet of gypsum sand is stored below interdunal areas. A significant decrease in groundwater levels in the dune aquifer system would likely make much of this accumulated sand more available for transport by wind, significantly changing dune dynamics as well as local ecosystems and habitats.
The dune field is located in the Tularosa Basin, which is a closed basin that encompasses an area of about 5,400 square miles. Regional groundwater flow in the basin is primarily from the north and east (Sacramento Mountains) to the south and west. There is evidence that some groundwater discharges by evaporation at Lake Lucero, which is a playa in the study area. Communities in the Tularosa Basin have a very limited potable water supply to maintain municipal and agricultural uses in the area. It is estimated that less than 0.2% of saturated deposits in the Tularosa Basin contain fresh water (TDS<1,000 mg/L). Local communities, such as Alamogordo and Tularosa, are actively looking for new water sources, and will likely begin desalinating brackish groundwater in the near future. Increases in groundwater pumping and changes in precipitation trends due to climate change may have a severe impact on regional groundwater levels throughout the basin. Therefore, the National Park Service (NPS) is concerned about changes in local water levels at WHSA as a result of increased groundwater pumping in the Tularosa Basin and possible effects of climate change.
In 2009, researchers at the New Mexico Bureau of Geology and Mineral Resources (NMBGMR) began a hydrologic study at WHSA with the primary goals of: 1) Identifying water sources that contribute to the shallow hydrologic system in the dune field, and 2) Assessing the interactions between the shallow aquifer in the dune field and the larger regional system. Moneys obtained over several years from the NPS were used to fund the following activities:
- Monitoring of water levels in several monitoring wells at WHSA
- Geochemical analyses of groundwater samples
- Monitoring of hydrologic parameters in the unsaturated zone within a dune
- Aquifer tests
- Geophysical surveys
- Hydrologic flow modeling
The results of this study show clear evidence that the dune field is a recharge area. Groundwater levels within the dune field respond to local precipitation events very quickly. Hydraulic gradients in the unsaturated zone within the dunes are fairly constant in a downward direction. It appears that pore spaces within the dunes usually store the maximum amount of water that can be held against the pull of gravity (field capacity). Therefore, when it rains, water stored in the dunes is quickly flushed into the shallow aquifer. Groundwater chemistry also indicates that local precipitation recharges the shallow aquifer within the dune field. A small volume of groundwater within the dune aquifer that is generally located directly below the dunes appears to be derived from local precipitation. This water has the lowest total dissolved solids values (TDS), high relative calcium and sulfate concentrations, which is a result of the dissolution of gypsum, and is the youngest groundwater observed in the study area. Groundwater within the interdunal areas is observed to have much higher TDS values, higher relative sodium and chloride concentrations and is significantly older than the fresher water found beneath the dunes. This older water is a regional component that likely flows from basin fill sediments below the gypsum sand. Electrical resistivity data shows that this regional component is the dominant water source that is present in the shallow system.
An aquifer test was conducted to assess interactions between the shallow dune aquifer and the groundwater system in the basin fill sediments directly below the accumulation of gypsum sand. A pumping well and observation well were installed approximately sixty feet apart from each other with screened intervals from 145 to 195 feet below the surface. The screened intervals are well below the gypsum sand/ basin fill interface, which is about thirty feet below the surface at this location. The pumping well was located about twelve feet from a shallow observation well installed in the gypsum dune aquifer. Deposits below a depth of about 25 feet consist of interbedded to interlaminated clay and silty fine gypsum sand. Clay beds in the interval below twenty-five feet commonly contain an abundance of coarse, secondary selenite crystals. Beneath 105 feet, the stratigraphic succession contains a relatively large proportion of clay and thin, siliciclastic silty sand interbeds. Pumping from the deeper aquifer at about four gallons per minute for approximately three days resulted in over one hundred feet of drawdown in the pumping well and over thirty feet of drawdown in the observation well about sixty feet away from the pumping well. This significant drawdown was due to the low permeability that characterizes the basin fill sediments in this area. Despite the large drawdown observed in the deeper aquifer, no drawdown was observed in the shallow aquifer during or after the test. Therefore, interactions between the shallow and deep groundwater systems did not occur under the test conditions.
From the results of this study, we have constructed a conceptual hydrogeologic model of the shallow groundwater system in the dune field within WHSA. Geologic, hydrologic, and geochemical data indicate that the shallow groundwater system in the gypsum dune field is a single aquifer that is separate from the shallow systems to the east and west and the deeper hydrologic system directly below. Toward the eastern and western edges of the dune field the shallow aquifer behaves as a perched aquifer with a downward gradient that results in groundwater flowing from the shallow dune aquifer to adjacent shallow groundwater systems on both sides. The shallow dune aquifer is recharged by local precipitation, while very little infiltration occurs outside the dune field. Local recharge is observed in the dune aquifer and is easily identified by geochemical and isotope data. Groundwater discharges by evaporation throughout the study area.
Aquifer test data also suggest that the dune aquifer appears to be relatively isolated from the underlying groundwater system due to the thick sequence of clay rich playa deposits of low permeability that lies below the accumulated gypsum sand. However, regional groundwater is the dominant component in the shallow dune groundwater system. This regional groundwater component has a distinct geochemical signature and is greater than 10,000 years old. It is likely that this regional groundwater enters the system primarily from the east and probably from below.
We used mathematical modeling techniques on varying spatial and temporal scales to characterize the relative importance of the sources of water (local versus regional) to the dune aquifer, and to quantify the timescales on which changes may affect the water table in the dune field. A 1-dimensional (1D), dune-scale, heat-and-fluid-flow model simulates the seasonal temperature fluctuations to estimate the vertical groundwater flow in the dune field, providing an estimate of inflow from the regional system to the dune-field aquifer. The 1D model estimated the regional aquifer contributes about 70 - 120 cm/year, which sustains the high local evaporation rate. We have also constructed a 3-dimensional (3D), finite difference, hydrologic model of the Tularosa Basin. This model quantifies basin-wide hydrologic characteristics, sources, and sinks of groundwater in the basin and near dune field. Computed and observed groundwater residence times and water-table elevations are the primary means of model calibration. The 3D model of the Tularosa Basin showed that increased pumping will result in water-level decline of up to 1.5 m for the regional groundwater system near the WHSA. This decline will be the result of a change in the balance between groundwater recharge and extraction on the eastern side of the Tularosa Basin, causing indirect drawdown near WHSA, as an altered, steady-state, regional water balance is reached. The results also illustrate the sensitivity of the groundwater levels in the Tularosa Basin to the evapotranspiration (ET) rate. The basin-wide groundwater drawdown simulated response may represent conservative estimates to true expected drawdown because if the limitations of MODFLOW to calculate variable-density groundwater flow. Results from the two models indicate that the regional groundwater system does contribute flow to the dune aquifer, and that regional groundwater elevations are sensitive to increased groundwater extraction and ET.
Summary of Conclusions
- Groundwater levels and other hydrologic data, including aquifer test data, indicate that the shallow dune aquifer is perched above and is relatively isolated from the underlying regional groundwater system due to the thick sequence of clay rich playa deposits of low permeability that lies below the accumulated gypsum sand.
- There is clear evidence that local precipitation quickly recharges the shallow dune aquifer, primarily through the dunes as opposed to the interdunal areas.
- This local recharge component has the lowest total dissolved solids values (TDS), high relative calcium and sulfate concentrations, which is a result of the dissolution of gypsum, and is the youngest groundwater observed in the study area.
- Regional groundwater, which likely enters the shallow system from the east, is the dominant component in the shallow dune groundwater system. This regional groundwater component has a distinct geochemical signature and is greater than 10,000 years old
- Groundwater discharges from interdunal areas as evaporation.
- A 1D Heat flow model estimated the regional aquifer contributes about 70 - 120 cm/year to the shallow dune aquifer, which sustains the high local evaporation rate.
- A 3D hydrologic flow model of the Tularosa Basin showed that increased pumping in Alamogordo will result in water-level decline of up to 1.5 m for the regional groundwater system near the WHSA due to a change in the
Funded by the National Park Service with assistance from the Bureau of Geology, the USGS and the National Cave and Karst Research Institute.
For more information, please contact:
Dr. Talon Newton, Hydrogeologist
Newton, B. Talon; Allen, Bruce, 2014, Hydrologic investigation at White Sands National Monument, New Mexico Bureau Geology Mineral Resources, Open-file Report, v. 0559, pp. 1-51.
Supported Student Work
Bourret, S.M., 2015, Stabilization of the With Sands Gypsum Dune Field, New Mexico, by Groundwater Seepage: A Hydrological Modeling Study: Unpublished Masters thesis, New Mexico Institute of Mining and Technology, 136 pp.