Structure of Groundwater Flow in the Espanola Basin Near Rio Grande and Buckman Wellfield

Groundwater within the Espanola basin occurs within a complex hydrogeological system. Groundwater within the basin extends beneath Los Alamos National Laboratory (LANL) and interacts with surface water (e.g., Rio Grande, Pojoaque River). Based on water-level data, the general direction of the groundwater flow within the portion of the aquifer beneath LANL is generally from west to east; the general flow direction on the east side of Rio Grande is predominantly from east to west. West of Rio Grande, the large-scale flow direction is controlled by areas of aquifer recharge to the west (the flanks of Sierra de los Valles and the Pajarito fault zone) and discharge to the east (the Rio Grande and the White Rock Canyon Springs). East of Rio Grande, the aquifer is recharge predominantly along the Sangre de Cristo mountains. The Buckman wellfield is located just east of the Rio Grande in the central portion of the Espanola basin. Understanding of the interaction of groundwater beneath LANL with the Rio Grande and groundwater within the Buckman wellfield is provided by multiple lines of evidence, including (1) basin geology and hydrostratigraphy, (2) hydrogeologic data (pre- and post- development water levels, pumping drawdowns, spring-discharge rates, etc.), (3) ground-surface subsidence, (4) groundwater geochemistry, and (5) naturally occurring stable isotopes. Analyses of these data suggest that the deep groundwater pumped at the Buckman wellfield is in relatively poor hydraulic connection with the Rio Grande and with groundwater in the aquifer beneath LANL. These conclusions can be explained by the pronounced westward-dipping stratification of the Santa Fe Group sediments near the Buckman wellfield which causes the regional aquifer to be highly anisotropic and under confined (artesian during pre-development) conditions. The aquifer properties appear to limit hydrologic connection between groundwater in the regional aquifer beneath LANL and deep groundwater produced in the Buckman wellfield.