Open-file Report - 589
The Relationship Between the Cuatrociénegas Gypsum Dune Field and the Regional Hydrogeology, Coahuila, Mexico

White Sands National Monument and its sister park, the Área de Protección de Flora y Fauna Cuatrociénegas, are linked by their unique gypsum dune fields. Gypsum is a common mineral, but it is extremely rare in the form of sand dunes. While gypsum dune fields unite the parks, at present, there are stark morphological differences between the two dune fields. The White Sands dune field is considered to be a “wet” system due to a very shallow water table that helps anchor the dunes, providing a degree of cohesion between the fine grains that prevents the sand from blowing away. A significant decline in the water table would likely have a profound effect on the overall morphology of the dune field. Where White Sands is considered an active system, dominated by tall dunes that migrate across its dune field, the Cuatrociénegas dune field is dominated by lithified blocks of gypsum. Preserved in these cemented blocks are bedding planes, which indicate that large dunes once dominated the area. At present, there is not a shallow water table found beneath the Cuatrociénegas dune field.

Concerned for the long term preservation of its gypsum dune field, the National Park service sought to determine the cause of the apparent change in morphology of the Cuatrociénegas dune field, and whether a similar fate awaits the White Sands dune field. To understand what lead to the deflation of the Cuatrociénegas dune field a hydrologic investigation of the basin was conducted by the New Mexico Bureau of Geology. This study developed a conceptual model for the Cuatrociénegas dune system to understand the processes that formed and maintained the gypsum dune field in the past. The conceptual model helps our understanding of the effects that anthropogenic activities and natural processes had on the Cuatrociénegas dune field. This insight may help us to predict how similar activities, such as large groundwater diversions, may affect the White Sands dune field.

Geochemical and stable isotope analysis was performed on spring pool samples. From our analysis of recently collected data, as well as reanalysis of previously published data, there is a significant body of evidence that suggests a large regional flow system supports the groundwater flow to the basin. Water is transported to the basin via confined karstic aquifers, rich in evaporites. The ion-rich water that is discharged from springs collects in terminal lakes in the closed basin. PHREEQC modeling found that as the water in the terminal lakes evaporates it becomes saturated with respect to gypsum and begins to precipitate from solution. Now a dry playa, Laguna Grande was a large terminal lake where water would pool near the dune field. Satellite records show the lake had been steadily decreasing in surface area since the 1970s until 2010, when it went dry. Microprobe analysis of gypsum samples collected from the dune field suggests there was likely a shallow water table beneath them in the past.

While it is difficult to determine when the balance was tipped, it is likely that the shift started within the past 100 years, as the water resources in the basin began to be exploited. Possible impacts to the dunes include gypsum mining, draining of the wetlands and the decline of Laguna Grande, high-capacity agricultural water extraction, and long-term drought.

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