Nitrate concentration changes at the groundwater-surface water interface of a small Cumbrian river

For an experimental field site at the River Leith, United Kingdom, the spatial and temporal distribution of nitrate was observed along the upwelling flow path from groundwater to surface water. The study was carried out during baseflow conditions for two Successive years. For two contrasting stream reaches, the physical and chemical characteristics of streambed sediment cores were analysed together with observations of hydraulic head, dissolved oxygen, redox and nitrogen speciation using an array of nested streambed piezometers. Pressure head gradients in the streambed piezometers showed that upwelling flows dominated the exchange between groundwater and Surface water throughout the observation period. Infiltration of surface water into the streambed was not evident at depths below 10 cm. Pore water collected from sediment cores and streambed piezometers showed spatially variable redox conditions and nitrogen speciation within up to 100 cut depth in the streambed. In particular, nitrate concentrations along upwelling flow paths appeared to follow two opposite trends, with both decreasing and increasing nitrate concentrations being observed at different points in the experimental reach. The observed changes of nitrate concentrations in the upwelling groundwater are restricted to the loose superficial sediments that overlay the sandstone bedrock and do not appear to coincide with Surface water-groundwater mixing in the streambed. The magnitude of variation in nitrate concentration along the upwelling flow path to the streambed appears to be governed by the sediment Structure and characteristics in the two contrasting field sites. The results suggest that changes in redox status and pore water nitrate concentrations in the hyporheic may occur at depths greater than surface water infiltration into the streambed and may call for new conceptual understanding of hyporheic nutrient transformations. Copyright (C) 2009 John Wiley & Sons, Ltd.