Simulation analysis of the unconfined aquifer, Raft River geothermal area, Idaho-Utah

Abstract

This study covers about 1,000 mi 2 (2,600 km 2 ) of the southern Raft River drainage basin in south-central Idaho and northwest Utah. The main area of interest, approximately 200 mi2 (520 km 2 ) of semiarid agricultural and rangeland in the southern Raft River Valley that includes the known Geothermal Resource Area near Bridge, Idaho, was modelled numerically to evaluate the hydrodynamics of the unconfined aquifer. Computed and estimated transmissivity values range from 1,200 feet squared per day (110 meters squared per day) to 73,500 feet squared per day (6,830 meters squared per day). Water budgets, including ground-water recharge and discharge for approximate equilibrium conditions, have been computed by several previous investigators; their estimates of available ground-water recharge range from about 46,000 acre-feet per year (57 cubic hectometers per year) to 100,000 acre-feet per year (123 cubic hectometers per year). Simulation modeling of equilibrium conditions represented by 1952 water levels suggests: (1) recharge to the water-table aquifer is about 63,000 acre-feet per year (77 cubic hectometers per year); (2) a significant volume of ground water is discharged through evapotranspiration by phreatophytes growing on the valley bottomlands; (3) the major source of recharge may be from upward leakage of water from a deeper, confined reservoir; and (4) the aquifer transmissivity probably does not exceed about 12,000 feet squared per day (3,100 meters squared per day). Additional analysis carried out by simulating transient conditions from 1952 to 1965 strongly suggests that aquifer transmissivity does not exceed about 7,700 feet squared per day (700 meters squared per day). The model was calibrated using slightly modified published pumpage data; it satisfactorily reproduced the historic water-level decline over the period 1952-65.