Urban Dissolved Silica: Quantifying the Role of Groundwater and Runoff in Wastewater Influent

Human impacts on silicon (Si) cycling are just being explored. In particular, we know little about the role of urban environments in altering the flux of Si from land to sea. Here we describe the annual load of dissolved Si (DSi) in the influent of the second largest wastewater treatment plant (by volume) in the United States (Deer Island Wastewater Facility, Boston, MA). We partition the ∼69 500 kmol DSi year(-1) influent load between three sources: runoff (12%), groundwater infiltration (39%), and sewage (49%). Based on these results, we hypothesized that instead of being delivered to local rivers, DSi in groundwater and runoff is redirected to the combined stormwater-sewage overflow system. To test this hypothesis we compared long-term (2007-2012) observations of DSi flux from the three urban rivers surrounding Boston to modeled DSi fluxes based on land use and land cover. As predicted, the modeled fluxes were higher than the measured fluxes indicating that the sewage infrastructure of Boston diverts watershed DSi to the treatment plant. This research increases our understanding of human changes to the Si cycle, demonstrates the potential usefulness of DSi as a groundwater infiltration tracer within sewage treatment systems, and highlights the underappreciated interannual variability of riverine DSi fluxes.