New Mexico Geological Society Annual Spring Meeting — Abstracts

Except for surface flow in the upper Gila and Mimbres River valleys, the primary water source in southwestern New Mexico is groundwater stored in Upper Cenozoic (Gila and post-Gila Gp) fill of four intermontane-basin complexes, which are here called basin systems. Secondary groundwater resources occur in fractured volcanic rocks that are interbedded with or at the base of the Gila Gp. Basin systems comprise linked groups of 2 to 6 structural subbasins that are locally separated by intrabasin bedrock highs. They are, in east to west order, the Mimbres (MiBS), Hachita-Moscos (HMBS), Playas (PlBS), and Animas-Lordsburg (ALBS) basin systems. Surface flow occurs in both closed and open topographic settings; and groundwater movement takes place under drained, undrained, and partly drained conditions. The MiBS and HMBS are open basins, with parts of both surface-and groundwater-flow regimes discharging to ephemeral-lake plains (playas) in closed, undrained basins (bolsones) of northwestern Chihuahua, Mexico. The closed but partly drained ALBS (west of the Continental Divide) contributes some underflow to the Gila Valley northwest of Lordsburg. The partly drained PIBS has both closed and open components, with small amounts of suIface and subsurface discharge from the Upper Playas subbasin to the HMBS, but most flow is toward the Playas Lake area of the Lower Playas subbasin.

The primary aquifers are poorly consolidated basin-floor facies dominated by fluvial deposits of larger axial streams. These facies form much of the (informal) upper Gila hydrostratigraphic unit, as well as lower parts of overlying post-Gila units. Saturated thickness of this upper basin-fill sequence rarely exceeds 600ft, and transmissivities of productive aquifer zones commonly range from 10-25x10³ft²/day. Horizontal hydraulic conductivities in these zones are typically in the 5-50ft/day range. Available potable water that is stored in the upper 300-600ft of basin-system fill, much of it very old, is no more than 60x10⁶ ac-ft. The estimated annual aquifer recharge of about 90,000 ac-ft is substantially less than present groundwater use.