TERRAIN FACTORS IN CAPITAN AQUIFER RECHARGE, NORTHEASTERN GUADALUPE ESCARPMENT, NEW MEXICO

The Guadalupe Escarpment southwest of Carlsbad, New Mexico, the most direct recharge area for the Capitan Aquifer, is under accelerating oil and gas development. The Capitan Aquifer provides critical water supplies to communities of Carlsbad and White City. Key infiltration areas on the escarpment must be identified for special groundwater quality protection measures during petroleum drilling and production. The landscape has 400-600 feet of local relief, with 22% of surface area in uplands, 61% in slopes and 17% in canyon floors. Infiltration on uplands and slopes is generally through soil-filled fracture openings. Slope areas include many headwater stream channels in V-shaped valleys, with abundant signs of ephemeral groundwater seepage. On the floor of Dark Canyon, the most major drainage traversing the area, bedrock pavement sections have some potential for stormflow infiltration via abrasion-formed basins along fracture lines, and along eastward-dipping strata with some karst conduit development. Depth of boulder fill along some reaches of the canyon exceeds ten meters, providing a large-volume medium for streamflow transfer to the subsurface during and after floods.

It is likely that water moving slowly downdip through backreef units will only feed the reef aquifer if it does not again contact the surface. Two simplified end-member models can be postulated for escarpment area recharge to the Capitan Aquifer. In the first model, infiltrating water descends vertically trough carbonate units and then runs downdip on the major sandstone beds. The second model treats the entire mass of the backreef as a material with strongly anisotropic permeability, much enhanced parallel to bedding, with infiltrating water proceeding in a downdip direction immediately on reaching bedrock. The likely real condition resides between these end members, but the two models agree in identifying the following portions of the escarpment landscape as recharge areas that should be given special attention in terms of water quality protection: (1) The floors and downdip side slopes of Dark Canyon and other major canyons, and (2) The outer escarpment, extending a mile west from the escarpment front. In the latter area, zones of rectangular, fracture-associated drainage warrant special attention as potential quick recharge sites.