Incorporation of geomorphology in transient-storage models for solute transport in streams: Effect on TMDL quantification

Solutes moving in natural streams are temporarily retained in surface water dead zones and the hyporheic zone (subsurface surrounding the stream). This transport is commonly modeled using the one-dimensional advection-dispersion equation in the stream with additional terms that account for the transverse exchange between the main channel and these surface and subsurface storage zones. The exchange parameters are typically assumed reach-averages, with reach lengths varying from tens of meters to a few kilometers. We attempted to increase the physicality of the model parameters by linking them to stream topography. We conducted for this purpose stream tracer studies in a 200-m reach of an urban stream in Philadelphia. The discharge of the stream was about 40 L/s. When the model parameters were assumed to be reach averages, the fitting of the model to the data gave parameter values that are comparable to existing studies. The cross sectional area of the stream was measured at about seven meter intervals and was input to the model while the remaining parameters were assumed reach averages. Fitting of the model to the data in this case required that the parameters take "extreme" values, not reported in previous studies. This indicates that partial geomorphic information about a stream setting might not be useful. Guidelines for using these results in evaluating TMDL are discussed.