A SIMPLE MODEL DESCRIBING CHANGES IN UNSATURATED HYDRAULIC CHARACTERISTICS AS MINERALS DISSOLVE AND PRECIPITATE

Previous and current work on the proposed nuclear waste repository at Yucca Mountain in southern Nevada provide an excellent framework to examine a relationship between mineral precipitation/dissolution and the hydrologic properties of geologic materials. This framework includes not only physical information, such as moisture retention data, thermal characteristics and hand specimens, but predictive numerical modeling of changes in geologic material around the proposed repository site. One such numerical model, TOUGHREACT, has been used by researchers at Lawrence Berkeley National Laboratories to predict changes in porosity around a hypothetical waste emplacement drift. Laboratory data and theoretical studies suggest a very small amount of precipitate may have profound impacts on the intrinsic permeability of a host rock. Many researchers have assumed a relationship between grain-size distribution and the saturated and unsaturated hydraulic characteristics of geologic materials and the precipitation and/or dissolution of minerals can affect the intrinsic permeability of geologic material.

The potential impact of precipitation and dissolution of minerals on the hydraulic properties of geologic material is investigated using a series of hypothetical pore structures representing a range of geohydraulic conditions that could reasonably be encountered at Yucca Mountain, Nevada. These pore structures are generated by extrapolating pore structure statistics from moisture retention data using Mualem’s (1976) model of pore structure and unsaturated hydraulic characteristics. Standard van Genuchten (1980) parameterizations (a, n, and m) are obtained for the original moisture retention data/pore structures. The hypothetical pore structures are subsequently modified using a specific precipitation/dissolution paradigm (i.e. all precipitation will occur in the smallest pores) and the modified pore structure is “translated” into predicted moisture retention data and van Genuchten parameters. Using a sequence of precipitation/dissolution events a set of functions describing the changes in the van Genuchten parameters is obtained and the potential impact on flow patterns can be evaluated using a modified version of TOUGHREACT.