Hydrogeologic characterization of a groundwater system using sequential aquifer tests and flowmeter logs
Stephen G. Mclin
Four sequential aquifer tests were conducted in new municipal water supply wells located in Guaje Canyon on the northeastern flank of Pajarito Plateau. The plateau borders the western perimeter of the Española Basin and defines the western margin of the Rio Grande rift system. Each aquifer test consisted of a pumping and recovery phase, and adjacent wells were used to record both drawdown and recovery data. Collectively, these tests reveal horizontal propagation of drawdown in the regional aquifer beyond about 2500-4800 ft from individual pumping wells. Each well is approximately 2000 ft deep, and has about 1300 ft of continuous louvered screen at the bottom. However, dynamic flowmeter logs from each well show an effective aquifer thickness (or high water-yielding interval) that is only about 435 ft in the east and thins to about 325 ft in the west over a lateral distance of about 8300 ft. These logs demonstrate that the aquifer is highly stratified and laterally discontinuous. In the east, this high-yielding interval is located above the Miocene basalt flows that separate poorly sorted, fluvial Santa Fe Group deposits from lower yielding fine-grained sandstone and siltstones. In the west, this high-yielding interval is interbedded with the basalts. Aquifer transmissivity varies from about 3440 ft2/day in the east to about 700 ft2/day in the west. This westward decline in transmissivity is strongly correlated to the observed westward thinning of the high-yielding interval so hydraulic conductivity remains nearly constant. The storage coefficient averaged about 0.00062 for the four aquifer tests and is characteristic of early-time confined behavior. Complex barrier, or no-flow, boundary effects are also apparent after several hundred minutes of pumping and mask the theoretical transition from a confined to phreatic aquifer response. Mesa-top surface expressions of faulting were mapped in the Puye Quadrangle by Dethier (2003); here, several of his fault traces are projected into the alluvial canyon drainage system where the well field is located. These projections coincide with predicted locations of buried, no-flow boundaries obtained from aquifer test analyses. These results imply that aquifer units not only thin to the west but are also displaced vertically downward by normal faulting as one moves to the east and toward the floor of the Rio Grande rift system.
- Mclin, Stephen G., 2007, Hydrogeologic characterization of a groundwater system using sequential aquifer tests and flowmeter logs, in: Geology of the Jemez Region II, Kues, Barry S.; Kelley, Shari A.; Lueth, Virgil W., New Mexico Geological Society, Guidebook, 58th Field Conference, pp. 485-491.