The marshland upwelling system (MUS) was developed as an alternative domestic wastewater treatment system for coastal residences to address the special problems typifying coastal wetland environments, such as sparsely populated areas, high water tables, anaerobic soil, poor hydraulic soil conditions and saline groundwater.  An understanding of the nitrogen cycling processes occurring within the MUS is vital to the efficient, long-term operation and success of the system.  Field studies of the MUS, at both intermediate and saltwater marshes, show the majority of the nitrogen enters the marsh subsurface as NH₄⁺, but aeration of the holding tank could lead to partial conversion of NH₄⁺ to NO₃^- before the wastewater enters the subsurface.  Synthetic wastewater was used in a column lab study performed to better understand the specific nitrogen transformations occurring within the MUS.  Treatments included two salinities, 2 and 20 ppt, and two nitrogen loads, 100 mg L^-1 NH₄⁺-N and 80 mg L^-1 NH₄⁺-N /20 mg L^-1 NO₃^--N.  The ability of plants to provide additional treatment of the wastewater was also studied.  Columns held under 2 ppt were planted with a brackish marsh plant, while the 20 ppt treatments were planted with a saltmarsh plant.  Each treatment combination was also performed without plant addition to determine the effects of plants on wastewater treatment.  Plant growth in wastewater treated columns was compared to control columns, which received no synthetic wastewater, to determine if the wastewater significantly increased plant growth.