**Abstract**

The lanthanide elements, or rare earth elements (REEs), are an active research area, with increasing attention directed toward soil and water evaluation and protection. Rare earth element concentrations in surface and groundwaters may be appropriately evaluated by partitioning the REEs into (i) a dissolved fraction (REE3+, hydrolysis, and simple anion complexation products) and (ii) REEs associated with inorganic and organic colloidal fractions. Given the total REE concentration and the organic, inorganic, and clastic composition, each fraction of REE concentration and the speciation within the fraction may be thermodynamically simulated to estimate (i) transport potential, (ii) biological availability, and (iii) system reactivity toward changes in pH, oxidation-reduction, chemical composition, mineralogy (facies) changes, or anthropogenic alteration. Chemical thermodynamic simulations using freely available USEPA software are presented to illustrate REE alterations attributed to pH changes, inorganic and organic adsorption, mineral precipitation, and oxidation-reduction. The purpose is to position researchers to better anticipate REE reactivity and transport potential in aquatic and soil resources.

**Keywords:** lanthanides, Minteq, aquatic resources, environmental degradation, thermodynamic simulation
