RARE EARTH ELEMENTS IN WATERSHED MASS BALANCE CALCULATIONS OF PRIMARY MINERAL WEATHERING RATES AND CLAY GENESIS RATES AT THE COWEETA HYDROLOGIC LABORATORY, WESTERN NORTH CAROLINA, USA

Watershed geochemical mass balance methods are considered the most accurate means of quantifying weathering reactions in nature and elemental transfers at the Earth’s surface. However, mass balance calculations often suffer from the number of unknowns (weathering rates or mass transfer coefficients) exceeding the number of solute mass-balance equations, yielding a system of linear equations that cannot be solved mathematically.

Mass balance methods have been used to calculate rates of primary mineral weathering and secondary mineral formation in three watersheds located at the Coweeta Hydrologic Laboratory, western North Carolina. The mass balance calculations of this study include rare earth elements (REE), which is fundamentally different from any study performed at Coweeta or elsewhere. The primary advantage of using additional elements such as the rare earths is that additional mass-balance equations can be constructed to permit determination of a larger number of unknowns than previously possible. The REE and Ca in Coweeta stream waters are strongly influenced by the weathering of allanite. Garnet is present at Coweeta in relatively low modal abundances (<2%), with complete dissolution being limited by the formation of secondary protective surface coatings. Low modal abundances combined with restricted dissolution should result in relatively low garnet weathering rates. The garnet weathering rates of this study are up to approximately 90% slower than previous calculations. Calculated rates of feldspar and biotite weathering compare favorably with previous results.

The stream water REE chemistry is a one-time sample analysis that has been converted to an approximate long-term flux using stream SiO2 chemistry. Furthermore, no data have been collected on REE concentrations in precipitation. In spite of these potential limitations, the mass balance calculations of this study are believed to be the best to date for the watersheds investigated. Furthermore, it suggests potential success for future mass balance calculations that include trace elements.