Spatially distributed model to assess watershed contaminant transport and fate

Abstract

Unmanaged contaminant releases from upland areas, transport across the land surface, and delivery to stream networks can have adverse impacts on water quality and stream ecology. Environmental management agencies need high resolution, quantitative tools to assess chemical transport and fate to formulate effective plans to address chemical impacts. To meet this need a spatially distributed, physically-based model was developed to simulate chemical transport and fate at the watershed scale. In addition to runoff and sediment transport, this new model 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. The ability of the model to simulate chemical transport and fate is demonstrated by a site-specific application to the California Gulch watershed in Colorado. Using a database of observations for the period 1984-2004, hydrology, sediment transport, and chemical transport and fate were simulated for a calibration event in June, 2003 and a validation event in September, 2003. The model accurately simulates flow volumes, peak flows, and times to peak. Average relative percent differences for flow volume were -8.6% for the calibration event and +11.3% for the validation event. The model also successfully simulated observed ranges of total suspended solids and total metals concentrations for cadmium, copper, and zinc. Model applicability is further demonstrated for a 1-in-100-year rainfall event. Simulated flows were within the range of other estimated values. The simulated dissolved zinc load was also within the range of values extrapolated from field observations. The model was used to assess the relative impact upstream sources have on downstream areas. The chemical source tracking features of the model were demonstrated for zinc transport. The primary source of zinc export was the lower gulch floodplain. Model results indicate that the lower gulch floodplain zinc inventory increased due to redistribution during the flood and that 76% of the imported zinc originates from nearby areas of the lower gulch watershed and 23% from the upper gulch.