The Interplay of Regolith Evolution and Watershed Hydrodynamics on Shale Weathering Fluxes

Abstract:

The initial transformation of protolith into regolith has been associated with the dissolution of highly soluble minerals such as carbonates as well as oxidation of Fe(II)-minerals. The weathering of these minerals is then thought to control the formation of regolith and the emergence of secondary porosity. These reaction fronts at depth have been related to the groundwater table position and are believed to parallel the surface topography. These observations lead to the following questions: over long timescales does the water table position control nested weathering fronts, or conversely, do these reaction fronts dictate the water table position? At shorter times scales, how do processes related to the evolution of regolith influence groundwater solute fluxes?

Underlain entirely by shale, Susquehanna Shale Hills Critical Zone Observatory (SSHCZO) provides an ideal platform for investigating nested reaction fronts and their control on groundwater geochemistry. At SSHCZO, several ridgetop and valley floor boreholes have been drilled. Borehole optical televiewer and outcrop data define variations in geologic structure. Eighteen shallower wells further refine patterns in water table position and groundwater chemistry. Residence times of groundwater are constrained by measurement of atmospheric tracers (i.e., SF₆, CFCs & ³H).

Sulfur and ferrous iron concentrations from the drill cores document that the deepest reaction front is pyrite oxidation, which is roughly coincident with the groundwater table. The drill core carbonate content from the valley floor boreholes and one ridgetop hole are indicative of a carbonate reaction concurrent with the oxidation front. However, low carbonate contents observed over four other ridgetop holes indicate a complex geometry of the carbonate dissolution front, perhaps controlled by lithological heterogeneity. Groundwater levels and geologic observations suggest the interplay between stratigraphy and topography control the water table position, and thus the location of nested reaction fronts. Elements released to groundwater due to weathering (i.e., Mg, Ca & Si) show very little variability in concentration seasonally despite their strong variations in residence time, suggesting relatively fast chemical reactions that control the fluxes from the system.