GEOLOGIC MAPPING FOR CHARACTERIZATION OF SPRING HYDROGEOLOGY ON THE SEVILLETA AND SAN ANDRES NATIONAL WILDLIFE REFUGES, NEW MEXICO

Springs are vital water resources on the Sevilleta and San Andres National Wildlife Refuges in New Mexico, but they may be negatively impacted by regional groundwater withdrawal and drought. We recently conducted two collaborative geologic mapping projects at 1:12000 scale to characterize the geologic and hydrologic settings of the springs, delineate the likely groundwater source areas, and provide an assessment of potential effects of regional groundwater withdrawal on spring flow. The results of the studies provide a hydrogeologic basis for management of the refuges’ water resources and more detailed hydrologic studies.

Two springs on the Sevilleta NWR illustrate the diverse geology and hydrology of the refuge and a range of potential susceptibilities to groundwater withdrawal. The springs in San Lorenzo Canyon discharge from contacts between relatively impermeable lavas and tuffs and more permeable coarse sandstones and conglomerates. The water source is the La Jencia Basin to the west, an area that may see groundwater development in the future. Cibola Spring discharges from a normal fault that juxtaposes sandstone and limestone, and shows a rapid response to precipitation, suggesting that it discharges from a karst-type aquifer in the limestone. The likely recharge area is mostly within the refuge, and it is not threatened by development.

The San Andres NWR is located in the San Andres Mountains, a west-tilted fault block of Precambrian crystalline rocks overlain by Paleozoic carbonate rocks. The refuge springs can be grouped into two types, based on geologic setting and water chemistry. The springs in Little San Nicholas Canyon are of the first type. They discharge from fractures in Precambrian rocks and from the Cambrian-Precambrian unconformity, and have specific conductivities of 0.7 – 0.9 mS/cm. Springs in Mayberry Canyon typify the second type. They discharge from Pennsylvanian carbonates and shales and have specific conductivities of 1.4 – 1.6 mS/cm. The mapping and water chemistry data suggest that the two spring types discharge from distinct groundwater systems. Regional groundwater withdrawal does not appear to be a threat to the San Andres springs, but recent drought has caused several springs to decrease flow or dry up.