RARE EARTH ELEMENTS AS NATURAL TRACERS OF GROUNDWATER FLOW IN A FRACTURED BASALT AQUIFER: EASTERN SNAKE RIVER PLAIN AQUIFER, IDAHO, U.S.A

Effective management of limited groundwater and surface water resources require an understanding of the complex interaction between surface water and groundwater flow regimes. The Eastern Snake River Plain hosts a hydrogeological regime that is characterized by extensive interactions between surface water (Snake River) and a fractured basalt aquifer (Eastern Snake River Plain Aquifer, ESRPA). Because of aquifer heterogeneity and uncertainties in surface and groundwater interactions, delineation of regional groundwater flow paths and the extent of mixing of surface and groundwater are difficult to quantify, especially near the Snake River. Natural abundances of dissolved rare earth elements (REE) have been used as tracers in other aquifer systems to investigate source water mixing and water rock interactions. Dissolved REE are ideally suited for this purpose because they exhibit subtle systematic changes with increasing atomic number. Specifically, we have used dissolved REE as a natural tracer in the ESRPA to understand the role of surface and groundwater interactions on the geochemistry and flow in the aquifer.

Fifty-two ground and surface water samples collected along two postulated ESRPA groundwater flow paths were characterized for major, trace, and REE using Inductively Coupled Plasma – Mass Spectrometry and Ion Chromatography. Multivariate statistical analyses of the geochemical data suggested that a) surface water interactions in the ESRPA heavily influence the aquifer’s geochemistry, b) groundwater along the two flow paths are significantly different, and c) groundwater has been modified by transpiration. In areas dominated by groundwater irrigation, REE patterns often resemble the REE pattern of the basalt and/or possibly the clay and carbonate components of the aquifer. In areas dominated by surface water irrigation, REE patterns are fractionated and exhibit light REE depletions. Our results show that REE can be used to assess the extent of surface water recharge and water rock interactions and may prove useful to better quantify regional groundwater flow patterns.