CHARACTERIZING GROUNDWATER IN THE GRANDE RONDE FORMATION OF THE COLUMBIA RIVER BASALT GROUP OF CENTRAL AND EASTERN WASHINGTON

The Columbia Plateau Regional Aquifer System (CPRAS) is an important source of potable and irrigation water in central and eastern Washington. The CPRAS is mostly composed of the Columbia River Basalt Group (CRBG), overlain by surficial sedimentary aquifers. The CRBG is a series of Miocene age flood basalts that are separated into four formations, from oldest to youngest; the Imnaha, Grande Ronde (the largest by volume and aerial extent), Wanapum, and Saddle Mountains. Each formation consists of multiple stacked basalt flows, with permeability concentrated in the “interflow zones” between the individual lava flows and adjoining vesicular flow margins. Though the CRBG were laterally extensive during emplacement, they have been faulted and folded through time, truncating and rerouting groundwater flow paths and compartmentalizing the CPRAS.

This study of 1,468 data points from basalt wells in the Columbia Basin builds upon previous water quality studies of the CRBG. Specifically, we expand our dataset to include wells from the basin margin, oil and gas wells, and over 200 data points from 126 water wells completed in the Grande Ronde Formation. This is a contrast to earlier studies focused primarily on the central basin, Saddle Mountains and Wanapum formations, and surface water-groundwater connectivity. We find the Grand Ronde Formation of the CRBG below 500 m depth has a distinctly different chemistry from that of the overlying formations. Specifically, we find enrichments in sodium (196 ± 460 vs. 53 ± 68 mg/L), silica (37 ± 17 vs. 23 ± 11 mg/L), chloride (234 ± 718 vs. 28 ± 55 mg/L) and fluoride (10 ± 11 vs. 3 ± 9 mg/L) with a corresponding reduction in calcium (14 ± 39 mg/L vs. 26 ± 30 mg/L) and bicarbonate (158 ± 55 mg/L vs. 196 ± 379 mg/L). These deeper waters broadly correlate to temperatures >25°C and reflect decreasing carbonate solubility and increasing silicate solubility. Underlying marine sediments and basement rocks, and lacustrine sedimentary interbeds, are possible contributing sources of chloride and fluoride. The trends in our data support previous work implying a relationship between groundwater quality, temperature, and depth in the CRBG. Continued investigations will better establish linkages between deep groundwater chemistry and geologic structures (faults, folds, and CRBG feeder dikes).