Urban groundwater dissolved silica concentrations are elevated due to vertical composition of historic land-filling.

Human influences on global silicon (Si) cycling include land-use change, deforestation, and wastewater discharge. Here we quantified the effect of urban expansion and historic land fill on dissolved silica (DSi) concentrations in urban groundwater in a northern temperate city. We hypothesized that historical land use, fill material, and urban infrastructure buried below cities create a unique anthropogenic geology which acts as a DSi source. We found that concentrations of DSi in urban groundwater are significantly higher than those from non-urban environments. We also found that historic land-use variables out-perform traditional topographic variables predicting urban DSi concentrations. We show that higher groundwater DSi concentrations result in increased subterranean groundwater discharge (SGD) fluxes, thereby altering coastal receiving water DSi availability. Further, we demonstrate that accounting for urban SGD DSi fluxes globally, could increase DSi SGD export by 20%. Together these results call for a re-evaluation of anthropogenic impacts on the global Si cycle.