DIFFERENCES IN DUNE MORPHOLOGY RELATED TO CHANGES IN GROUNDWATER GEOCHEMISTRY, WHITE SANDS NATIONAL MONUMENT, NEW MEXICO, TX

The White Sands of New Mexico is a dune field composed of transverse, barchan and parabolic dunes composed of gypsum sand. This study focuses on the transition from barchan dunes that migrate rapidly (12 m/year) through an unvegetated landscape, to isolated parabolic dunes migrating and 1 to 2 m/yr though vegetated, stabilized dunes and sand sheets. One theory is that the vegetated parabolic dunes can only form where enough sand has been deposited above the permanent saline water table to allow up to allow the formation of a fresh-water lens. Conversely, where there is not enough vegetation, due to the permanent and highly saline groundwater table, the sand blows away as barchan dunes, deflating the dune field. In a 5-km-long swath through the dunes, 6 sites were established, two barchan, two parabolic and two in the transition zone. Groundwater as well as soil samples are collected to measure the effects of seasonal variations on water salinity and of sand movement in the dune field. The water table is less than 1.5 m at all sites. At the barchan sites the water is almost three-times more saline than at the parabolic sites. TDS (total dissolved solids) decreases from (7g/L) in the barchan to (2g/L) in the parabolic dunes. In the barchan area, the amount of DO (dissolved oxygen) is also greater. It is presumed that the vegetation and the associated ecology use up the fresh soil water, creating a higher saline soil water. Within the parabolic area, there seems to be at least one layer with a mean grain size of 5 microns. This may have formed through precipitation from the ground water or through alteration of the eolian sand. This research will aid in the understanding of the reasons why some dunes are mobile and others are stabile, and how dunes might change with climate or changes in groundwater hydrology. The dramatic changes in groundwater salinity suggest dune fields can be shaped by feedback between eolian dynamics and groundwater chemistry.