Conceptualization of groundwater flow in the shallow aquifer along the Apache reach of the San Pedro River, Cochise County, Arizona

Abstract

The Apache Reach of the San Pedro River is situated adjacent to the Apache Powder Superfund Site, in Cochise County, Arizona. An alluvial aquifer, known as the "shallow aquifer", and consisting of flood plain sediments deposited on the St. David Formation by the ancestral San Pedro River, is situated along this reach. The shallow aquifer is underlain by a thick clay unit of the St. David Formation, which provides vertical hydraulic confinement from a deeper regional artesian aquifer. Prior to shallow aquifer deposition, the ancestral San Pedro River and a local paleotributary, identified as Molinos Creek, eroded paleochannels into the top of the St. David Formation. During shallow aquifer sediment deposition, finer-grained, "overbank" sediments were deposited between the paleochannels. The overbank sediments formed contemporary a "laterally confining unit" (LCU) that isolates the western part of the shallow aquifer hydraulically from the remaining part of the shallow aquifer to the east.