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

Abstract

Low to moderate temperature (<150°C) geot hemal f 1 ui d s are bei ng developed by EG & G Idaho, Inc. and the U. S. Department o f Energy in the southern Raft River Valley of Idaho. i n depth from 4911 feet t o 6543 feet (1490 to 1980 meters) and two intermediate d e p t h (3858 feet or 1170 meters) injection wells have been drilled w i t h i n the Raft River KGRA. Several shallower (1423-500 feet o r 430-150 meters) wells have also been constructed to monitor the environmental effects of geothermal developnent on the shallower aquifer Five deep geothermal wells ranging systems. Sampling of water from wells within the KGRA has been conducted since the onset of the project i n 1974. have conducted analyses on waters from the KGRA. calculations conducted on the data produced from these laboratories indicated that data from three laboratories were reliable w h i l e two were not. regression analyses on sets of paired data from various laboratories. Five analytical laboratories Charge-balance error A method o f equating a l l data was established by using linear The chemical data collected from the deep goethermal wells indicates that a two reservoir system exists w i t h i n the Raft River KGRA. Each reservoir i s associated w i t h a major structural feature. These features are known as the Bridge Fault System (BFS) and the Narrows Structure (NS). The BFS i s a series of normal faults. These trend in a northeasterly direction and d i p steeply towards the valley floor. NS i s a r i g h t lateral strike-slip fault trending roughly east-west. The i i The majority of the geothermal activity occurs near the intersection of these two features. The fluids associated w i t h the BFS have much lower concentrations of dissolved constituents than the fluids from the NS. have a t o t a l dissolved soilds (TDS) level o f about 1300 mg/l (milligrams per l i t e r) while the fluids from the NS exhibit a TDS of about 4300 mg/l. The f l u i d s from both systems are alkali chloride type geothermal waters w i t h the major cation being Na' and the major anion being C1-. reservoir analyses, the BFS appears t o be able t o transmit f l u i d more rapidly than the NS. fluids i n the NS thus resulting i n a more concentrated fluid. The BFS fluids Based on This f a c i l i t a t e s a longer residence time for the Geothermal fluids move vertically up fractures from depth and then mix laterally w i t h water from shallower aquifers. geothermal fluids i s best exhibited i n well USGS-1. of geothermal f l u i d has resulted along the major fractures. does not appear t o extend t o the south o r east more than a mile from the central geothermal area. The most pronounced movement i s along the BFS. This i s believed t o be due t o the fact that t h i s fracture system i s more transmissive than the NS.