Hydraulic and Geochemical Framework of the Idaho National Engineering and Environmental Laboratory Vadose Zone

Abstract

southwestern portion of the INEEL where, because of potential contaminants in this area, some of the most Questions of major importance for subsurface contaminant transintensive subsurface characterization efforts have been port at the Idaho National Engineering and Environmental Laboratory (INEEL) include (i) travel times to the aquifer, both average or done (Fig. 1). A major source of these contaminants is typical values and the range of values to be expected, and (ii) modes the INEEL facility known as the Subsurface Disposal of contaminant transport, especially sorption processes. The hydraulic Area (SDA) for radioactive and other hazardous waste. and geochemical framework within which these questions are ad-As a result, the SDA and its nearby surroundings are dressed is dominated by extreme heterogeneity in a vadose zone and the portion of the INEEL that has been most intensively aquifer consisting of interbedded basalts and sediments. Hydraulically, studied with respect to hydraulic and geochemical issues. major issues include diverse possible types of flow pathways, extreme This paper reflects this emphasis, though where possible anisotropy, preferential flow, combined vertical and horizontal flow, we have formulated generalizations to apply to most or and temporary saturation or perching. Geochemically, major issues all of the INEEL. This review is designed to complement include contaminant mobility as influenced by redox conditions, the that of Smith (2004). In terms of local-scale geologic concentration of organic and inorganic complexing solutes and other local variables, the interaction with infiltrating waters and with the framework, it picks up from Smith's presentation of teccontaminant source environment, and the aqueous speciation of con-tonics, volcanism, and basalt characteristics; it emphataminants such as actinides. Another major issue is the possibility of sizes relatively small-scale features and their role in colloid transport, which inverts some of the traditional concepts of transport processes. We cover basic issues of transportmobility, as sorbed contaminants on mobile colloids may be transrelevant characteristics as known mainly from investigaported with ease compared with contaminants that are not sorbed.tions since the 1950s by relatively standard techniques. With respect to the goal of minimizing aquifer concentrations of con-Much material in this paper derives from a 1999 taminants, some characteristics of the vadose zone are essentially USGS panel review of DOE-contractor investigations completely favorable. Examples include the great thickness (200 m) related to existing and potential contaminant transport of the vadose zone, and the presence of substantial quantities of fine (Rousseau et al., 2004). It follows the series of USGS sediments that can retard contaminant transport both hydraulically and chemically. Most characteristics, however, have both favorable Reports on the INEEL subsurface by Barraclough and and unfavorable aspects. For example, preferential flow, as promoted ...

Perched-Water Evaluation for the Deep Vadose Zone Beneath the B, BX, and BY Tank Farms Area of the Hanford Site

“…The area has received increased attention, especially in the arid western United States where thick unsaturated zones and highly heterogeneous sediment and fractured rock has led to the development of significant perched water zones (e.g., Nimmo et al 2004;Wu et al 2004;Robinson et al 2011). A perched water zone in the subsurface is defined as a saturated zone that is above or not directly connected to the regional water table (Freeze and Cherry 1979).…”

“…Horizontally, there might be greatly increased flow, resulting in water and contaminant transport at a rate much faster than predicted based on bulk hydraulic properties and the traditional Richards' equation. Under these conditions, only a small fraction of the subsurface interacts with the contaminants, which limits adsorption and other attenuation processes (Nimmo et al 2004). Another complication might be that, due to the nature of perched water, recharge to the water table at many sites may be spatially nonuniform (Robinson et al 2011).…”

“…Perched-water zones at much greater depths affect flow and contaminant transport at a large number of arid U.S. Department of Energy (DOE) sites, such as Yucca Mountain (Wu et al 1999(Wu et al , 2004, the Idaho National Engineering and Environmental Laboratory (INEEL; Nimmo et al 2002Nimmo et al , 2004Duke et al 2004;Wang et al 2010), the Pantex Plant in Amarillo, Texas (Adam et al 2004), and the Los Alamos National Laboratory (LANL; Birdsell et al 2004;Robinson et al 2005Robinson et al , 2011.…”

“…The various perched-water zones at the INEEL were created mostly as the result of large fluxes of waste water (Nimmo et al 2004). In most cases, the perched-water zones are temporary, persisting until horizontal and vertical flow spreads them out enough to leave the porous materials unsaturated.…”

The hydraulic conductivity structure of gravel-dominated vadose zones within alluvial floodplains

“…How-ever, investigations examining the heterogeneity of the vadose zone are limited relative to the saturated zone by the fact that pumping and tracer tests, which depend on groundwater movement, are impossible. Those studies must utilize other methods, including particle size analysis (Nimmo et al, 2004;Dann et al, 2009), air permeability (Dixon and Nichols, 2005;Chief et al, 2008) and permeameter testing (Xiang et al, 1997;Reynolds, 2010;Miller et al, 2011). When the vadose zone is dominated by coarse gravel such efforts are made more difficult by the problems associated with sampling in gravel: large particles resist penetration by and clog samplers, excavated pits are prone to collapse, and high hydraulic conductivity rates require large volumes of water for in situ permeameter testing.…”