THE OTHER PART OF WATER-ROCK INTERACTIONS: MINERALS AND MINERAL SURFACES IN ENVIRONMENTAL GEOCHEMICAL PROCESSES

Fate and transport models are routinely used in regulatory decisions. Typically, groundwater is analyzed and the fluid chemical compositions are used in analysis and decision-making. However, mineralogical compositions and their spatial distributions are seldom characterized in detail, although they are important initial conditions for reactive transport modeling. For characterizing geochemical processes at a site, it is essential to know: (1) what mineral species are present in the aquifer; (2) what are their chemical compositions; (3) what are their crystallinities and reactivities under surficial conditions; (4) what are their abundances; (5) what are their spatial distributions; and (6) what are their surface areas.

To demonstrate the significance of these questions, two lines of arguments will be presented. First, an uranium mill tailings site in western USA is used as a case study and a coupled multi-component reactive mass transport model is used for simulation of contaminant fate and transport. Numerous groundwater geochemistry data are available at this site, but mineralogical data are sketchy. Even given the well-defined pore fluid chemistry, variations of input secondary mineral species, mineral surface properties, and mineral abundances in the aquifer resulted in significantly different modeling outcomes. Second, the complexity and difficulty in characterizing minerals and mineral surfaces in environmental systems is illustrated by the coatings on sediments and fracture surfaces (not from the same site). In many geological systems, mineral coatings, not bulk minerals, represent the direct interfaces with contaminants and groundwater. Poorly crystalline and nanometer-sized clayey and iron oxide coating materials from a number of geological settings are analyzed with X-ray Diffraction, Scanning Electron Microscopy, Transmission Electron Microscopy, acid digestion, and sequential extraction.