CITIES ON THE RIVER-SCAPE: MODELING FRESHWATER SALINIZATION FROM URBAN CENTERS AND URBAN WATERSHEDS ALONG THE MERRIMACK RIVER

River salinity throughout New Hampshire reflects the use of roadway deicers for the previous 60 years leading to potential aquatic habitat degradation. We present a regional river network transport model using the Framework for Aquatic Modeling in the Earth System (FrAMES) used to understand anthropogenic loading of salts to river systems, and to predict potential affects under varying climate drivers. The regional scale of the model places this urban water quality concern in the context of the broader river network. Data from an extensive sensor network measuring salt concentrations in catchments spanning the spectrum of development from pristine to near complete imperviousness are assimilated using the model to constrain four lumped loading sources including deicer inputs to impervious areas and a diffuse anthropogenic loading (e.g. inputs from waste, fertilization, and irrigation). Predictions, validated against sensor measurements in major downstream rivers, develop a continuous spatial mapping of potentially degraded aquatic habitat. Our findings suggest that 32% of stream reaches in the Merrimack River watershed of New Hampshire and Massachusetts are potentially salt impaired for at least a month of the summer under current conditions using a conservative threshold at 50% of the existing guidance concentrations for chloride. Dilution capacity from pristine runoff maintains low salt concentrations along the profile of the Merrimack through major urban centers. However, downstream of confluences of heavily urbanized watersheds such as the Nashua and Concord the Merrimack itself shows elevated summer-time salt concentrations. The watershed-scale salinization model is unique in that the loading mechanisms are immediately sensitive to climate and development, whereas reported alternatives use estimated fluxes benchmarked to current conditions. Future applications of the model will test hypotheses regarding alternative deicing practices, and potential synergistic impacts of increasing development and a changing climate.