Experimental and theoretical study of subsurface flow and transport in a riparian wetland

In recent years, increasing attention has been focused on riparian zones because of their important role in controlling non-point source pollution from agricultural applications. The studied system is a small riparian wetland (approximately 200 m long and 80 m wide) that is located on a golf course near Cheraw, South Carolina. The wetland receives nutrient input from the nearby fairways in the form of runoff during and after storm events. The study presented here illustrates an attempt at numerical simulation of ground water flow in the wetland, and incorporation of the flow field in a denitrification transport model.; Soil borings, surveying, flooding experiments and slug tests were performed to obtain information about the hydrogeology of the wetland. The wetland receives influx of water during storm events at the inlet through surface runoff, and further downgradient from a highly permeable sand layer below the wetland. The developed wetland flow model successfully simulated observed head and flow patterns of a tracer experiment that was performed to mimic storm events by amending the wetland with bromide and nitrate. Model results indicated that more than 90% of total inflow comes through wetland boundaries. Ground water flow velocities in the flow model averaged 310 times slower than what was observed on the field. This significant difference was attributed to horizontally oriented root channels and macropores. Furthermore, a dual-domain model was used to simulate tracer and nitrate transport in the wetland. Rapid macroporous flow in the wetland formed the motivation for the application of this model. Transport model results prompted an adjustment of the calibrated hydraulic conductivities (K), leading further to the introduction of a mobile fraction K. The application of the dual-domain model with the increased K resulted in good agreement of tracer arrival times. It was concluded that initially measured K with slug tests was not realistic, and were not high enough to reproduce the preferential flow through macropores. Nitrate transport was simulated using a dual-domain, first-order reaction model. The model adapted a heterogeneous nitrate decay rate that was elevated in the mobile domain and lower in the bulk soil. The attenuation efficiency and the denitrification rate resulting from the model were comparable to previous denitrification enzyme activity measurements. Further analysis of breakthrough curves revealed that a denitrification lag occurred in the wetland. It was concluded that the nitrate transport model could be further improved by introducing a factor to the reaction term in the model.; For the first time, the dual-domain model was applied to riparian wetland soils with success on simulating tracer and nitrate transport. This study demonstrated also how insight was gained into the complex hydrogeology of the wetland by using numerical modeling. Numerical models, as presented in this study, can be integrated in non-point source pollution management practices to assess outcomes of different exposures.