Hydroperiod and hydraulic loading for treatment potential in urban tidal wetlands

Abstract. Conventional methods of estimating water quality improvement due to wetland treatment are not well suited to the dynamic water level and wetted area fluctuations observed in coastal settings. We present a new method to quantify hydroperiod and hydraulic loading at different elevations in a coastal wetland profile in which the principal inflows and outflows are due to tides. We apply our method to an urban coastal setting (part of the New York-New Jersey Harbor Estuary) where a major water quality problem persists due to fecal coliform contamination from combined sewer overflow (CSO) discharges. Based on three types of simplified hydrograph, we show how such an approach and conceptual model of a terraced tidal wetland with constant mean slope can be used to assess hydrologic constraints for wetland vegetation species and the potential treatment effectiveness for adjacent impaired coastal waters. Resulting hydroperiods and hydraulic loading values decrease approximately exponentially with elevation along the wetland profile with considerable variation in overall slope depending on the hydrograph pattern. Application of a first-order contamination reduction model using our calculated hydraulic loadings indicates that such tidal treatment wetlands could reduce average fecal coliform concentrations in the range of 27% to 94% depending on the pattern of water level fluctuation, wetland surface elevation and vegetation density. Our analysis shows the performance potential for tidal wetlands to treat adjacent coastal waters. Restoration of existing salt marshes, and construction of new tidal wetlands would therefore be a promising part of an ecohydrologic strategy to improve water quality in contaminated urban coastal settings like the New York-New Jersey Harbor Estuary.