Impact of Pristine Groundwater Upwelling on Nitrate Impairment of Streams

Abstract:

Nitrate is one of the most common contaminants in the world's aquifers, streams, and nearshore waters. Theoretical and experimental studies have shown that in-stream processing of nitrate by hyporheic exchange is a major control on the magnitude of nitrogen export from watersheds and it can be affected by groundwater-stream interactions via gaining or losing conditions. However, implication of this observation for stream function and water quality is not yet clear. In this study, we set out to answer the question: can groundwater-stream interactions affect nitrate removal within streams and exacerbate surface water quality impairments, even in cases where the groundwater itself is not a direct source of nitrate? We couple a model for local hyporheic exchange and regional groundwater upwelling to a nitrate fate and transport model to investigate the influence of gaining stream conditions on nitrate processing in the hyporheic zone over a range of Damkohler numbers (Da = transport time over respiration time) and for three different water quality scenarios (agricultural-impacted stream, urban runoff impacted stream, and sewage impacted stream). We find that upwelling groundwater perturbs nitrate processing within the hyporheic zone by increasing the Da number at which significant denitrification occurs and, in two of three cases, causing sediments to “switch” from being a net sink of nitrate (by denitrification) to a net source of nitrate (by nitrification). These results provide some insights on how upwelling groundwater can influence nutrient processing in the hyporheic zone of streams, and demonstrate that changes in regional hydrology (e.g., brought on by climate change and land-use change) can significantly impact important ecosystem services and receiving water quality.