REVISITING A CLASSIFICATION SCHEME FOR SOUTHWESTERN ALLUVIAL BASIN AQUIFERS

Intermontane basins in the southwestern United States are target areas for disposal of a variety of wastes. Understanding fluid movement within and between these basins is needed to assess potential contaminant movement. Common terms that are often misused to describe groundwater flow in intermontane basins include "closed" and "open" basins. These terms should be used to refer to surface drainage, whereas the terms "undrained, partly drained, and drained basins" refer to intrabasin or interbasin groundwater discharge. To distinguish between basin types it is necessary to define interior types of playas that act as discharge areas. Playas include phreatic, or wet playas; and vadose, or dry playas. Phreatic playas are groundwater discharge areas that are moist near the playa surface. Vadose playas are often dry because depth to groundwater is too deep for capillary water to reach land surface. With respect to the presence or absence of these types of playas, basins can be classified as (1) topographically closed and undrained basins; (2) topographically closed and partly drained basins; (3) topographically open or closed, drained basins; and (4) topographically open, through-flowing basins. Topographically closed and undrained basins have interior groundwater discharge in the vicinity of a phreatic playa. The phreatic playa also serves as the focal point of surface drainage. Topographically closed and partly drained basins have some groundwater discharge to the phreatic playa where surface runoff also collects, but some groundwater discharges through permeable rock to another basin. A drained basin may be either topographically open or closed and surface discharge is either to an interior vadose playa or to an adjacent basin; however all of its natural groundwater discharge is by subsurface interbasin flow through permeable rock. In topographically open and through-flowing basins, surface drainage and groundwater discharge are focused at a perennial stream that carries baseflow and runoff out of the basin. We have identified several basins in the southwest by this classification scheme. Methods of investigation included fluid potential mapping, isotopic tracers, geochemical tracers, and temperature.