Chapter 25: Airborne Geophysics versus Groundwater—An Example
The San Pedro River Basin is an ecologically significant region, in part because the river hosts a major North American migratory bird flyway. The importance of surface flow in this river was acknowledged when Congress created the San Pedro National Riparian Conservation Area in 1988. Concern about regional water resources increased almost immediately with calls for the elimination of nearby Fort Huachuca Army base. The Upper San Pedro aquifer of southeastern Arizona is bracketed by crystalline and sedimentary rocks of the Huachuca Mountains on the west and the volcanic and sedimentary rocks of the Mule Mountains and Tombstone Hills on the east (Figure 1). Recharge of the Upper San Pedro Valley aquifer between them is believed to come primarily from the Huachuca Mountains–in fact, most of the aquifer lies to the west of the river on the Fort Huachuca side. Fort Huachuca and the adjacent city of Sierra Vista lie between the Huachuca Mountains and the river, and have been tapping this aquifer for over a century. The historical flow in the river has apparently been affected by water withdrawal by large copper mines in neighboring Mexico beginning in the late 1940s (Brown et al., 1966), but to what extent is unknown. Estimates of groundwater withdrawal versus recharge on the American side of the border, despite conservation efforts by the U. S. Army and local jurisdictions, still suggest a deficit (Pool and Coes, 1999).
Figures & Tables
Near-surface geophysics uses the investigational methods of geophysics to study the nature of the very outermost part of the earth’s crust. Man interacts with this part of the earth’s crust: he walks on it; he drills and excavates into it; he constructs structures on and in it; he utilizes its water and mineral resources; and his wastes are stored on and in it and seep into it. The very outermost part of the Earth’s crust is extremely dynamic-in both technical (physical properties) and nontechnical (political, social, legal) terms-which leads to both technical and nontechnical challenges that are much different than the challenges faced by “traditional” applications of geophysics for regional geologic mapping and for oil and gas exploration (see Chapter 2).