Rare earth element geochemistry of groundwater in the Palouse Basin, northern Idaho–eastern Washington

Abstract

Concentrations of rare earth elements (REE) were determined in groundwaters from the Palouse Basin, located in northern Idaho and eastern Washington. Samples were taken from wells and springs tapping waters in four different rock units: loess, Wanapum Basalt, Grande Ronde Basalt, and Moscow Mountain Granite. In general, unfiltered aliquots of the waters had higher concentrations of REE than those filtered through a 0.45 μm filter, sometimes by several orders of magnitude, indicating the presence of REE as suspended particles or large colloids. Overall REE concentrations in the waters decrease, and the unfiltered and filtered aliquots become more similar in concentration, with increasing depth within the stratigraphic section (i.e. loess → Wanapum → Grande Ronde). There are some distinctions in NASC (North American Shale Composite)-normalized REE patterns among the four groups of waters, but there is considerable variation among the waters within each group. When normalized to their host rocks, most filtered and many unfiltered aliquots of groundwaters from all four units show depletion of the LREE relative to the HREE. The results are consistent with a model in which meteoric water enters the basalt aquifers via the loess, picking up a heavy particulate and some dissolved REE load, followed by infiltration downward through the units. While moving downwards, the waters are subjected to natural filtration processes in which the particulate REE are largely removed.