Modeling of vadose zone hydrology is required to address a variety of applied problems in general and risk assessments associated with contaminants in particular. Risk assessments increasingly must focus on multisite, multipathway analyses as opposed to single-site, single pathway analyses. Such assessments can be particularly challenging when contaminants are widely dispersed in complex topography. Here we highlight how a set of contaminated sites situated within complex topography can be effectively prioritized relative to vulnerability of contaminant transport from surface and subsurface flows. We used a distributed hydrologic model, SPLASH, to assess the lateral flows of surface and subsurface water following the simulation of a 100-year precipitation event, which could correspond to an intense thunderstorm. Our case study was conducted in the North Ancho watershed of Los Alamos National Laboratory, in northern New Mexico, USA, an area with widely dispersed contaminants and diverse topography. Simulated surface flows generally exceeded subsurface flows by more than four orders of magnitude, indicating the relative importance of potential redistribution of contaminants by surface flows for this type of precipitation event. For the 18 potential contaminant release sites investigated, the maximum surface flow varied by more than an order of magnitude across the sites. Half of the sites had surface flows <25% of the maximum surface flow for a site, allowing for prioritization of those sites with the greatest vulnerability. Our results highlight how risks of contaminant transport can be effectively assessed in complex topography using distributed hydrologic modeling.

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