INVESTIGATING THE ROUGNHNESS AND ADVANCE RATE OF WEATHERING INTERFACE

The most well studied examples of geochemically evolving systems are found at earth's surface: depth profiles of weathered rock. This project will focus on the development of deeper understanding of the relationship of fractures and weathering in rocks, with particular emphasis on one lithology of great interest, shales. The approach will be to use simple numerical analyses applied to field studies while making both geochemical and geophysical observations. The focus will be on observations on one well-studied shale-underlain watershed.Project objectivesIn this proposed work we will explore how to use both geophysical and geochemical measurements together by i) focusing on a field setting developed exclusively on shale; ii) making dense geophysical, geochemical, and porosity measurements; iii) using rock physics models to relate fracturing, geochemistry, porosity, and geophysical observations; iv) developing geochemical models that incorporate fragments (dominated by diffusive transport) subtended by fractures (dominated by advective transport); and v) comparing models and observations and extending models to other watersheds as appropriate.Short description of the projectWe will investigate a weathering shale that has been the focus of intense geochemical, hydrological, and geophysical research. If time allows, we will extend our approach to another similarly well studied watershed on shale in northern CA, or to other intensively investigated watersheds on other lithologies. Our team consists of a geochemist (Brantley), a field geophysicist (Nyblade), a rock physics expert (Mavko), and a numerical modeler (Lebedeva). We will link the observations from classical geophysical, geochemical, and mineralogical methods to develop models of reactive transport and rock physics to explore the weathering of shale. A key benefit of our work will be the development of new methods and models to translate between geochemical and geophysical measurements for weathering systems. This outcome will be of particular importance because it will promote better understanding of reaction, fracturing, and porosity of shale.Potential impact of the projectTo understand geological systems that are important to our national well-being -- such as nuclear waste repositories or hydraulically fractured rocks -- requires the ability to make quantitative projections of the evolution of rock-water systems forward in time. A fundamental goal of much DOE-funded geochemical research is therefore to develop numerical models of reactive transport to simulate the evolution of natural water-rock systems. The geochemical community has provided such models and they are rapidly being improved with field-model cross-testing. This project will allow field-model testing for well-constrained weathering systems and will provide new conceptual understanding of how these systems function. In particular, we will inter-relate fracturing and weathering in shales. As a further benefit of the project, one postdoctoral scholar and one graduate student will be trained to understand these field, laboratory, and modelling approaches.