INTEGRATING GEOLOGIC, GEOPHYSICAL, AND HYDROGEOLOGIC DATA TO EXPLAIN A HIGH HYDRAULIC CONDUCTIVITY MOUNTAIN FRONT RECHARGE ZONE IN THE UPPER SANTA CRUZ BASIN, SOUTHERN ARIZONA

The aquifer of the upper Santa Cruz basin (USCB), southern Arizona and Northern Sonora, is the only source of water for a transborder population greater than 300,000. New geologic mapping and legacy geophysical data were used to help understand complex hydrogeology in areas that are likely important to groundwater recharge. Geophysical data include aeromagnetic, gravity, and transient electromagnetic (TEM) data and were primarily used to delineate concealed bedrock faults and to define the geometry and depth of basin-fill aquifers.

A water well in Peck Canyon, Rio Rico sub-basin, USCB, has measured hydraulic conductivity values from 200 to 2,000 ft/day and represents one of the highest hydraulic conductivity values observed from wells in the entire USCB. Geologic mapping and gravity gradient maximums indicated that the well is located within a major range front fault zone on the west flank of the Atascosa Mountains. The fault juxtaposes highly fractured Oligocene silicic volcanic rocks in the footwall wall against moderately fractured Late Miocene to early Pleistocene basin-fill gravels in the hanging wall. TEM data indicate a persistent low conductivity zone along the fault that we interpret as saturated gravels with low clay content. This zone of high hydraulic conductivity may act both as an effective recharge zone for the Rio Rico sub-basin of the USCB as well as a conduit for groundwater to move down gradient to the northern parts of the USCB. The Rio Rico sub-basin may serve as a subsurface reservoir that stores water from both surface mountain front recharge zones and subsurface inflow. Groundwater movement to the north along the fault may help explain the stable groundwater levels that are observed in the northern portion of the Rio Rico sub-basin and in the more northerly Tubac sub-basin. These relations are substantiated in Peck Canyon where surface water flowing over Oligocene volcanic rocks disappears at the range front fault zone and flows downward into the highly permeable basin-fill gravels.

The TEM data also indicate saturated volcanic rocks at depth in the footwall of the fault along the mountain front recharge zone and suggest that the volcanic rocks form a volcanic aquifer system on the western margin of the USCB that may be an import source of groundwater.