Modeling cyanide transport and fate in surface waters

Cyanide contamination of surface waters can arise as a concern from various causes such as industrial usage and by-product generation, disposal, and accidental spills. Regardless of the source of contamination, inorganic cyanide in groundwater exists primarily in the form of Fe (+II and +III) complexes, or hexacyanoferrates. Upon entry into surface water, however, these complexes can dissociate to form highly toxic free cyanide due to photolysis, which results from exposure to sunlight. The objectives of this research were to develop a comprehensive numerical model to simulate transport and fate of iron-complexed cyanide and free cyanide in surface waters, to determine the major physical, chemical and biological processes governing free cyanide fate in surface water and to evaluate the relative significance of each process contributing to free cyanide fate.; A more complex cyanide transport and fate model (CTFM), which incorporated advection, dispersion, as well as the three previously identified cyanide fate-determining mechanisms, was developed to describe concentration distribution and reactions of FeCN and HCN in a receiving water system affected by point and distributed input. Results obtained from applying the Lagrangian parcel method (LPM) to solve the mass balances were compared to analytical solutions and proved the LPM to be suitable for solving the CTFM governing equations. The model was then calibrated to observations from cyanide-contaminated receiving water systems to estimate reaction rates and cyanide input loads under different conditions. The model calibration results indicated that CTFM can be applied to simulate longitudinal concentration distributions of cyanide in natural streams and may be useful for developing load allocation strategies. Sensitivity and uncertainty analyses, involving numerous parameters, were performed on CTFM as configured for surface water systems. The sensitivity analysis results showed receiving water discharge was the most influential parameter with respect to both FeCN and HCN concentrations. The uncertainty analysis showed that standard error for predicting the FeCN concentration and HCN concentration at a point in the system was about 8 and 9 percent of the mean FeCN and HCN concentration, respectively, based on the simulations using the selected parameter distributions.; Demonstration of the utility of the CTFM for water quality management decision support was undertaken for a second industrial drainage system that exhibited greater complexity than the one used for development. The model permitted estimation of the location and magnitude of previously unidentified cyanide loads and an evaluation of the effectiveness of various cyanide load management strategies. It was evident from the management simulation results that the CTFM would be a useful tool for decision-makers concerned with cyanide management in surface waters. (Abstract shortened by UMI.)...