MASS BALANCE MODELING OF MINERAL WEATHERING RATES IN THE HAUVER BRANCH WATERSHED, CATOCTIN MOUNTAIN, MARYLAND: THE ROLE OF BIOMASS, SOLVING MORE EQUATIONS IN MORE UNKNOWNS USING SOLID-PHASE DATA, AND CARBON DYNAMICS

Owen Bricker was a pioneer of solute flux-based geochemical mass balance modeling of small watersheds starting in the late 1960s. Such methods are regarded as the most accurate means for quantifying elemental transfers at the Earth’s surface and are used to calibrate global hydrogeochemical models. Watershed mass balance methods allow for the calculation of individual mineral weathering rates, and account for the influence of biomass on stream solute chemistry. However, the stoichiometry of elemental exchange with the biomass has proven elusive. Mass balance calculations of weathering rates also often suffer from the number of unknowns exceeding the number of equations.

Present-day weathering rates of individual primary minerals of metabasaltic bedrock are calculated for the Hauver Branch watershed located in the Catoctin Mountain Research Site (CMRS) of northern Maryland. The method of calculation of these rates differs from previous studies in that a biomass term is included. The stoichiometry of the deciduous biomass term was determined using multi-year seasonal stream water chemistry for the nearby CMRS Bear Branch watershed which hosts similar forest vegetation but drains relatively unreactive quartzite bedrock. Detailed mineralogic information is utilized to add more equations to the mass balance matrix. With more equations than unknowns, the mass balance calculations may be performed multiple times with all iterations being combined to yield a single statistically robust solution.

The Hauver Branch watershed mineral weathering rates calculated as part of this study generally compare favorably with previous studies. The influence of biomass on stream water chemistry is small, with slight degradation of the biomass during the 1982-1992 period of stream sampling. Neither a transformational weathering product of chlorite nor epidote is required to be included in the mass balance calculations. CO₂ consumption calculations indicate that the inorganic carbon concentration measured at the watershed outlet approximately equals that predicted by the calculated mineral weathering rates. Consequently, no other acid other than carbonic is needed for chemical weathering. Of the CO₂ consumed, calcite is responsible for approximately 25%, and silicate weathering exceeds that of the global average.