Geochemistry of the Rare Earth Elements in Natural Terrestrial Waters:A Review of What Is Currently Known

The range of observed Chemical compositions of natural terrestrial waters varies greatly especially when compared to the essentially constant global composition of the oceans.The concentrations of the REEs in natural terrestrial waters also exhibit more variation than what Was reported in seawater. In terrestrial waters, pH values span the range from acid up to alkaline. In addition, terrestrial waters can range from very dilute waters through to highly concentrated brines. The REE concentrations and their behavior. in natural terrestrial waters reflect these compositional ranges. Chemical weathering of rocks represents the source of the REEs to natural terrestrial waters and, consequently, the REE signature of rocks can impart their REE signature to associated waters. In addition, because of the typical low solubilities of the REEs, both surface and solution complexation can be important in fractionating REEs in aqueous solution. Both of these processes are important in all natural terrestrial waters, howev-er, their for relative importance varies as a function of the overall solution composition. In alkaline waters, for example, solution complexation of the REEs with carbonate ions appears to control their aqueous distributions whereas in acid waters, the REE signature of the labile fraction of the REEs is readily leached from the rocks. In circumneutral pH waters, both processes appear to be important and their relative significance has not yet been determined.