Mark Velleux

TREX: Spatially distributed model to assess watershed contaminant transport and fate

Contaminant releases from upland areas can have adverse water quality and stream ecology impacts. TREX (Two-dimensional, Runoff, Erosion, and Export) is a spatially distributed, physically-based model to simulate chemical transport and fate at the watershed scale. TREX combines surface hydrology and sediment transport features from the CASC2D watershed model with chemical transport features from the WASP/IPX series of water quality models. In addition to surface runoff and sediment transport, TREX simulates: (1) chemical erosion, advection, and deposition; (2) chemical partitioning and phase distribution; and (3) chemical infiltration and redistribution. Floodplain interactions for water, sediment, and chemicals are also simulated. To demonstrate the potential for using TREX to simulate chemical transport at the watershed scale, a screening-level application was developed for the California Gulch watershed mine-waste site in Colorado. Runoff, sediment transport, and metals (Cu, Cd, Zn) transport were simulated for a calibration event and a validation event. The model reproduced measured peak flows, and times to peak at the watershed outlet and three internal locations. Simulated flow volumes were within approximately 10% of measured conditions. Model results were also generally within measured ranges of total suspended solid and metal concentrations. TREX is an appropriate tool for investigating multimedia environmental problems that involve water, soils, and chemical interactions in a spatially distributed manner within a watershed.

Exposure assessment framework for antimicrobial copper use in urbanized areas.

Copper is used as an antimicrobial agent in building materials such as algae-resistant roofing shingles and treated wood products for decks, fences, and utility poles used in urbanized areas. Releases from these materials may pose risks to aquatic and terrestrial organisms. Copper exposures in surface water, sediment, and soil were estimated for a hypothetical urban setting using the TREX watershed model. Drainage and soil characteristics were based on an existing watershed. Urban landscape characteristics were developed from data regarding housing densities and copper use in building materials. This setting provides a spatially distributed, upper-bound assessment scenario. Release rates from algae-resistant shingles and treated wood were defined based on surface area and rainfall. Simulations for the urban landscapes were performed for a 10-year period. Simulation results were used to evaluate exceedences of benchmark concentrations for water, sediment, and soil. For algae-resistant shingles, exposures did not exceed benchmarks in any media. For treated wood, exposures did not exceed sediment and soil benchmarks, and surface water benchmarks were exceeded on 2 days in 10 years. Based on this analysis, copper use as an antimicrobial agent in algae resistant shingles and treated wood is not expected to pose significant adverse environmental risks on an individual use basis.

Development of a Distributed Watershed Contaminant Transport, Transformation, and Fate (CTT&F) Sub-model

CTT&F is a physically based, spatially distributed watershed contaminant transport, transformation, and fate sub-model designed for use within existing hydrological modeling systems. To describe the fate of contaminants through landscape media as well as spatial variations of contaminant distributions, physical transport and transformation processes in CTT&F are simulated for each cell in the model and routed to the watershed outlet. CTT&F simulates contaminant erosion from soil and transport across the land surface by overland flow. The model also simulates contaminant erosion from stream bed sediment and transport through channels in addition to transport of contaminants inputs by overland flow. CTT&F can simulate solid (granular) contaminant transport and transformation, including partitioning between freely dissolved, dissolved organic carbon (DOC) bound, and particle-sorbed phases. To demonstrate model capabilities, CTT&F was coupled with an existing distributed hydrologic model and was tested and validated to simulate RDX and TNT transport using two experimental plots. These experiments examined dissolution of solid contaminants into the dissolved phase and their subsequent transport to the plot outlet. Model results were in close agreement with measured data. Such a model provides information for decision makers to make rational decisions relevant to the fate of toxic compounds.

Distributed Modeling of Extreme Floods on Large Watersheds

Estimates of extreme floods and probabilities are needed for hydrologic engineering and dam safety risk analysis. Physically-based, distributed watershed models are used as an avenue to estimate extreme floods, and as a basis to extrapolate frequency curves. The main elements of this research include improving and using a two-dimensional, physically-based rainfall-runoff model (CASC2D) to estimate extreme floods and probabilities for dam safety on a large (12,000 km 2 ) watershed, the Arkansas River above Pueblo, Colorado. New tools have been developed so the model can be applied at this scale. CASC2D can be successfully used to model extreme floods based on observations of extreme rainfall (from both rain gage networks and weather radar) for large watersheds. Future research will focus on the storm transposition concept and linkages with radar.

Upland Erosion Modeling

Significant advances in upland erosion modeling have been achieved in the past decade. The TREX (Two-dimensional Runoff, Erosion, and Export) watershed model has been developed at Colorado State University for the simulation of surface runoff from spatially and temporally distributed rainstorms on watersheds. The model has been applied in several countries with different climatic conditions. TREX can calculate surface infiltration, surface runoff, sediment transport, and the partition of metals in dissolved, adsorbed, and particulate form. The focus of this chapter is on the calculation of surface flows and total suspended solids at the watershed scale. The chapter is comprised of three parts: (a) a description of the main processes and governing equations, (b) a description of the model components and algorithms, and (c) an application example on a large watershed. The application example for Naesung Stream in South Korea provides powerful visual evidence of upland erosion processes at the watershed scale during large rainstorms (300 mm of rainfall). Model calibration was successful and overall model performance is acceptable. Hydrologic simulation results were in good to very good agreement with measured flow volume, peak flow, and time to peak at the watershed outlet as well as several stations within the watershed. Sediment transport simulation results were also in reasonable agreement with the measured suspended solids concentration.

River Restoration: A View from Wisconsin

River cleanup and restoration efforts pose significant legal and technical challenges for state and other regulatory agencies. Efforts are often conducted under different authorities, particularly CERCLA (Superfund) and NRDA rules. Wisconsin is home to a number of cleanup and restoration sites. The experience of project managers who worked for the Wis consin Department of Natural Resources on the Fox River and other cleanup and restoration site projects is presented. A number of common issues affect restoration projects. At many sites, human use has significantly affected the landscape and the presence of dams will have altered the function and habitat of river systems. Dam removal is a tool of choice for restoration but the appropriateness of removal can be influenced by: the presence of contaminated sediments, public acceptance, public safety and floodplain management. Consequently, restoration is typically more difficult than just dam removal. The presence of contaminated sediments is a big hurdle to project implementation. To move forward with cleanup and restoration where contaminated sediments exist, it may be necessary to work under CERCLA, NRDA, and other processes. Regardless of the processes used, project managers should use a diverse range of methods that collectively document the need and best approach for action at the site. Most importantly, regulatory agencies should combine CERCLA and NRDA processes for synergy and leverage but be aware of pitfalls where these processes can conflict. The combined results of CERCLA and NRDA efforts can create leverage for settlement by demonstrating that slower, less complete cleanup actions can escalate damages and associated costs. This can in turn provide an economic incentive for more extensive cleanup in order to reduce overall liabilities in the form of resources damages and promote development of remedial actions that facilitate overall restoration efforts.