Distance‐based mixing models of δ18 and δ18 in a marsh‐lined estuary with multiple, distinct sources (Murderkill Estuary, Delaware, USA)

The Murderkill Estuary (Delaware, USA) receives NO3- principally from its upland watershed and from a wastewater treatment facility. Due to disparate NO3- sources, one-dimensional salinity-based mixing models were inadequate for describing distributions of NO3-, delta(15) N-NO3, and delta O-18(NO3). Distance-based mixing models with multiple, spatially-specified inputs were, therefore, applied to describe conservative mixing of these constituents and determine the extent to which biogeochemical reactions lead to non-conservative behavior of NO3-. These models closely matched Si observations in both winter and summer, consistent with high wastewater silicate loads and light limitation, and serve to validate modeling parameters for both seasons. A close fit of distance-based models to estuarine NO3- observations suggested a lack of uptake and fractionation in early winter. In the summer, modeled predictions of NO3-, and delta(15) N-NO3, and delta O-18(NO3) diverged from estuarine observations, particularly in the oligohaline and polyhaline regions, consistent with in situ nitrogen cycling or additional sources and sinks. Effluent from an adjacent marsh in the lower estuary contained NO 2 3 with low delta(15) N-NO3 and delta(15) N-NO3, low DO and high NH4+ concentrations in late summer. This data and previous studies of adjacent Delaware Bay suggest that reactions in marshes and Bay waters likely drove the nonconservative behavior of NO3- and its stable isotopes. Potential uncertainty in watershed discharge, however, limited explicit quantitation of NO3- loss in the estuary. Nonetheless, distance-based models are useful tools for the study of NO3-, delta(15) N-NO3, and delta O-18(NO3) distributions and cycling patterns in complex marsh-lined estuaries with multiple NO3- inputs.