Dynamics Of Riparian Saltwater Intrusion In Urban Streams

Accumulation of road salt in stream and groundwater approaching or exceeding salinity tolerances of aquatic life and recommendations for healthy drinking water is well documented across cold regions of the US and elsewhere. However, mechanisms by which salt enters groundwater, from where it contributes to long-term contamination of surface water, are less understood. Here we utilize groundwater flow models based on field observations at two study sites, located on headwaters streams in Boone, North Carolina, USA, to investigate the efficacy of near-stream reverse gradients created by elevated stream stage during storm and runoff events as one forcing mechanism for salt plume intrusion into the near-stream aquifer. These models allow us to build upon previous work in comparing the combined effects of stream channel size and morphology, permeable layer thickness, and storm flashiness on the efficacy of salt plume intrusion. We find storm-induced reverse gradients are an effective mechanism at introducing salt to the floodplain aquifer, in agreement with previous work. Additionally, we find salt accumulation in the aquifer follows a logarithmic trend; that smaller, flashier streams are more effective at aquifer salinization; and that thinner aquifers promote more efficient salt export by keeping baseflow salt concentrations high.