Assessing Contaminant Transport Vulnerability in Complex Topography Using a Distributed Hydrologic Model

Some facilities, such as Los Alamos National Laboratory, have contaminants at widely scattered locations Modeling of vadose zone hydrology is required to address a variety within a landscape of complex topography. This type of of applied problems in general and risk assessments associated with spatial distribution of contamination poses a particularly contaminants in particular. Risk assessments increasingly must focus on multisite, multipathway analyses as opposed to single-site, single challenging problem for risk assessment. While many of pathway analyses. Such assessments can be particularly challenging the contaminated sites within an area of concern may when contaminants are widely dispersed in complex topography. Here have relatively low levels of contamination, the redistriwe highlight how a set of contaminated sites situated within complex bution and concentration of these contaminants through topography can be effectively prioritized relative to vulnerability of environmental processes such as surface and subsurface contaminant transport from surface and subsurface flows. We used flow must be considered (Wilcox et al., 1996a, 1996b, 1997; a distributed hydrologic model, SPLASH, to assess the lateral flows Wilcox and Breshears, 1997; Newman et al., 1998; Johanof surface and subsurface water following the simulation of a 100-year sen et al., 2003). Because remediation associated with large precipitation event, which could correspond to an intense thunderareas of dispersed contaminants can be costly, assessstorm. Our case study was conducted in the North Ancho watershed ments that can prioritize remediation resources and efof Los Alamos National Laboratory, in northern New Mexico, USA, an area with widely dispersed contaminants and diverse topography. forts on those sites with some combination of the highest Simulated surface flows generally exceeded subsurface flows by more concentrations and vulnerability to transport are greatly than four orders of magnitude, indicating the relative importance of needed. Site contaminant inventories usually can be obpotential redistribution of contaminants by surface flows for this type tained with straightforward, albeit sometimes costly, samof precipitation event. For the 18 potential contaminant release sites pling, whereas assessing the vulnerability of a site to coninvestigated, the maximum surface flow varied by more than an order taminant transport is more challenging. of magnitude across the sites. Half of the sites had surface flows 25% A major determinant of contaminant transport is the of the maximum surface flow for a site, allowing for prioritization of amount of hydrological flow at a given site. Vulnerability those sites with the greatest vulnerability. Our results highlight how to contaminant transport can be associated with surrisks of contaminant transport can be effectively assessed in complex face and subsurface flows of water laterally through a topography using distributed hydrologic modeling. site. These flows themselves are influenced not only by the hydrologic characteristics of a site but also by the characteristics of nearby sites that may contribute or M of vadose zone hydrology is required to receive water flows. Data or predictions from a given address a variety of applied problems in general location may or may not scale up to be relevant to larger and risk assessments associated with contaminants in scales of concern (Johansen et al., 2003; Wilcox et al., particular. Risk assessments related to the vulnerability 2003a); hence the assessment of many locations may be of contaminant transport often focus on an individual needed. Assessing the relative magnitudes of surface site. Numerous models exist for such analyses, such as the and subsurface lateral flows can be particularly challengRESRAD model, which is commonly used within the ing in complex topography, where accounting for hyDepartment of Energy (DOE) (Cheng et al., 1991; Cheng drology of surrounding sites may be crucial. Distributed and Yu, 1993; Wang et al., 1993; Yu et al., 1993a, 1993b; hydrologic models provide a means for addressing these Wilcox and Breshears 1997). Such a modeling approach issues. Indeed, distributed hydrologic models have been is useful for rapid and conservative assessments of risk applied to similar problems, such as prioritization of from contaminants at a site. However, much of the conpost-fire remediation efforts within burned areas in comtamination within DOE facilities is in low concentraplex topography (Beeson et al., 2001; Wilson et al., 2001). tions over widespread areas (Riley and Zachara, 1992). In this paper, we highlight the utility of a distributed hydrologic model for ranking vulnerability of different S.N. Martens, Department of Land, Air, and Water Resources, Unisites to contaminant transport by estimating relative versity of California at Davis, Davis, CA 95616 and Sierra Science, magnitudes of surface and subsurface lateral flows across Three Rivers, CA 93271 (Present address); D.D. Breshears, Earth a complex landscape. More specifically, we used a distriand Environmental Sciences Division, Mail Stop J495, Los Alamos National Laboratory, Los Alamos, NM 87545 and School of Natural buted hydrologic model, SPLASH (Simulator for ProResources, Institute for the Study of Planet Earth, and Department cesses on Landscapes: Surface/Subsurface Hydrology), of Ecology & Evolutionary Biology, University of Arizona, Tucson, to assess the lateral flows of surface and subsurface AZ 85721-0043 (Present address). *Corresponding author (daveb@ag. water following the simulation of a 100-yr precipitation arizona.edu). event. This event was designed (McLin, 1992) to simuPublished in Vadose Zone Journal 4:811–818 (2005). late a high-intensity rainfall event, which could correSpecial Section: Los Alamos National Laboratory spond to an intense thunderstorm. Our case study was doi:10.2136/vzj2004-0037 conducted in the North Ancho watershed of Los Alamos © Soil Science Society of America 677 S. Segoe Rd., Madison, WI 53711 USA National Laboratory, in northern New Mexico, USA, 811 Published online August 16, 2005