DEVELOPMENT OF A METAL EXPOSURE AND TRANSFORMATION ASSESSMENT MODEL FOR USE IN WATERSHED MANAGEMENT, RESTORATION AND TMDL ANALYSES

This paper describes the coupling of an chemical equilibrium simulation model, based upon MINTEQA2, with the generalized Water Analysis Simulation Program (WASP), which includes algorithms for predicting the dynamic impacts of transport and kinetics. The Metal Exposure and Transformation Assessment (META4) Simulation Program is a generalized metals transport, speciation, and kinetics model developed for application to a variety of receiving waters experiencing metals contamination, including ponds, streams, rivers, lakes and estuaries. The META4 model uses the basic transport scheme of WASP, allowing its application to a variety of waterbodies in one, two or three-dimensional mode, as well as the simulation of both water column and benthic layers. Algorithms for the simulation of crucial metal transformation processes, such as aqueous speciation, sorption/desorption, chemical precipitation/dissolution and kinetics were added to this basic structure, resulting in the META4 model. Model development and an application will be discussed.The model has been applied and tested as part of studies for the Clear Creek Superfund Site (Colorado) in the Clear Creek watershed to describe the transport and transformations of copper, cadmium and zinc. These metals have been identified as major contributors to aquatic resource impairment in this mining-impacted watershed. As an example of the use of the model to evaluate watershed management and load reductions, modeling in North Clear Creek has been used to evaluate a series of potential remedial activities. The dominant point loads affecting the water and sediment quality in North Clear Creek include Chase Gulch, Gregory Incline, Gregory Gulch and the National Tunnel. Non-point sources include seepage from waste piles, storm runoff, and dissolution of metals from soils and sediments, bank exchange, sediment release/resuspension, and alluvial system exchange. Chemical sinks within the North Clear Creek environment include coprecipitation/sorption losses to the bed region, alluvial system transfers, and sedimentation of particulate metals. Each of these source types have been addressed in the subsequent modeling. Modeling studies were directed at point and non-point load reductions needed to meet existing or projected goals for water quality management in the basin.Following successful calibration, the model was utilized to evaluate the significance of point loads, sediment-water interactions and non-point source inputs of dissolved metals (i.e., groundwater intrusion into North Clear Creek). The modeling would also be used to evaluate the impacts of several remediation scenarios on improving water quality of North Clear Creek including point source control, groundwater management and sediment remediation. The approach for evaluating the remediation scenarios involved a step-wise reduction of loading in order to better understand metal dynamics in the basin. The modeling results indicated that the point source control in the upper basin would only result in a significant improvement in water quality for a limited distance downstream. Due to the remaining non-point loads and the contaminated sediments in the downstream segments, water quality was not improved at the mouth of North Clear Creek. The return to initial levels in the lower stream segments was directly related to the interactions of the water column with the benthic regions, and, the associated release of metals from the contaminated sediments. With additional point source control, but without sediment remediation, the improvement in water quality for zinc was about 60% for the majority of the system but only 28% at the confluence with Clear Creek. For cadmium, the improvement was under 40% at the mouth.Greatest improvement was realized when major point sources were treated along with some groundwater treatment and remediation of contaminated sediments. The results indicated that there would be a 75-80% improvement in water quality for cadmium, copper and zinc due to the potential source controls in various stream segments in the basin. Results of the simulations underscore the importance of an integrated approach to managing load reductions in this watershed, particularly those related to contaminated sediments in the lower segments of North Clear Creek. While control of the four major point sources should result in significant improvement in water quality, the full benefit of the loading reduction would not be realized until contaminated sediment influx is controlled. Modeling results indicated the benefit of a complex management tool, such as META4, in addressing complex issues for resource restoration which cannot be evaluated with more simplified methods.