Effects of Discharge in Residence Time Distributions in a Small Headwaters Wetland in the Ipswich River Watershed

The Ipswich River Watershed, a 401 km2 watershed located in northeastern Massachusetts, has been observed to be undergoing increasing urbanization with resulting increases in nutrient loading, in particular, nitrogen. Nitrogen uptake occurs in a 1st-order process which is dependent on the concentration of nitrogen as well as the amount of time the water containing nitrogen remains within the wetland, which is described as the residence time distribution (RTD). To better understand how discharge affects the RTD of the wetland, a number of tracer studies were conducted between May 2011 and August 2011. Additionally, fluxes into and out of the wetland were calculated for this same period to estimate groundwater flow into or out of the wetland in order to understand interactions of groundwater with the wetland. The RTDs calculated from four tracer studies suggest that lower discharges result in longer detention times and higher discharges result in shorter detention times, though the results are not conclusive. Estimates of water budget fluxes suggest that the direction and magnitude of groundwater flow may change depending on whether the wetland is at base flow or flood flow