Simulation analysis of the unconfined aquifer, Raft River Geothermal Area, Idaho-Utah

Abstract

This study covers about 1,000 mi2 (2,600 km2) of the southern Raft River drainage basin in muth-central Idaho and northwest Utah. The main area of interest, approximately 200 m i 2 (520 kmz) 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 groundwater recharge and discharge for approximate equilibrium conditions, have been computed by several previous investigators; their estimates of available groundwater 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 year1 (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 simnlating 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 195!2-65.

Hydrochemistry of selected parameters at the Raft River KGRA, Cassia County, Idaho

“…T h i s conclusion was based on calculations derived from the formation of an a n a l y t i c model o f the shallow ground water system. Steady-state and t r a n s i e n t simulation analyses were used by Nichols (1979) t o evaluate and modify earlier concepts about the hydrodynamics of t h e unconfined system i n the southern portion of the Raft River Basin. He concluded t h a t ground water flow and stream l o s s i n the v i c i n i t y of t h e Raft River Narrows were not the principal means o f recharge t o the aquifer.…”

“…of Idaho by Morilla (1976) investigating the p o s s i b i l i t y of using the shallow ground water system i n the cooling cycle of a power plant. second, Nichols (1979), was an aquifer simulation model o f the shallow system based on previous collected data.…”