Water-rock interactions play a pivotal role in the formation and response of the critical zone to natural and anthropogenic perturbations. The extent of interactions is dictated by an array of factors including the total amount of reacting minerals, water flow that flushes out dissolved reaction products, and spatial patterns that regulate water distribution. With the ubiquitous occurrence of spatial heterogeneities and the unique topographic characteristics at different scales, it is important to understand and quantify how they control chemical weathering in the critical zone. In this talk I will share our work along this line across spatial scales from columns (centimeters) to the watershed scale (kilometers).
At the column scale of 10 - 20 centimeters, we use flow-through experiments and reactive transport modeling to understand how the spatial patterns of magnesite govern dissolution rates under a variety of flow velocity and permeability contrast conditions. Columns were packed with the same total mass of magnesite (10%) distributed in quartz sand in different spatial patterns. The spatial patterns vary from the uniformly-distributed mixture of magnesite and quartz in the “Mixed” column to the “zoned” columns where magnesite grains are distributed in 3, 2, or 1 zones, oriented either in the direction parallel (flow-parallel) or transverse (flow-transverse) to the main flow direction. We found that under sufficiently low flow condition, the reactions reach equilibrium and the spatial patterns do not matter. Under high flow regimes (>0.4 m/d), dissolution rates can be more than an order of magnitude lower than those from the corresponding Mixed column when magnesite is distributed in a low permeability zone in the flow-parallel direction. Dissolution rates are highest when the spatial pattern maximizes the water flow through the reactive magnesite zones. The effective surface area Ae quantitatively measures the actively-dissolving magnesite and varies by more than 4 orders of magnitude, although the BET surface area in all columns is the same. This corresponds to the same range of observed variation in dissolution rates under the tested flow-through conditions.
Modeling of the regolith formation in Marcellus Shale formation over geological time scale (104 yrs) and over a length scale of 1.2 meters indicates that the measured surface area has be lowered by more than 2 orders of magnitude in order to reproduce the observed pore water chemistry and soil profile, indicating limited water and rock contact revealed by the column-scale experiments.
At the watershed scale (km), we use the Susquehanna-Shale Hills Critical Zone Observatory (SSHCZO) as a natural laboratory to explore water-rock interactions using a newly developed model, RT-FLUX-PIHM, which couples a subsurface reactive transport module (RT) with the watershed hydrological and land-surface model (FLUX-PIHM). With constraints from observations of soil profile and pore water and stream water chemistry, this tool enables mechanistic and integrative understanding on the topological, hydrological, and climatic controls of chemical weathering at the water shed scale.
Li Li (2014): When water meets rock: controls of chemical weathering across scales (Invited). CUAHSI 2014 Biennial Colloquium, July 28-30, 2014, Shepherdstown, WV.
This Paper/Book acknowledges NSF CZO grant support.