Nutrient dynamics in relation to surface–subsurface hydrological exchange in a groundwater fed chalk stream

Mini-piezometers were used to assess surface–subsurface hydrological exchange and biogeochemical processing in different patches on the river bed (coarse gravel, fine gravel/sand, silt/sand) at two sites on the River Lambourn (Berkshire, UK). Positive vertical hydraulic gradients (VHG) dominated the riverbed, indicating potentially upwelling subsurface water. Hydraulic conductivity was highly variable in the shallow sediments, but was generally low at greater depths, suggesting that positive VHG may not translate to the rapid movement of subsurface water. Well defined areas of downwelling were not identified, although negative VHG did occur at the low/intermediate depths (∼10–20 cm). Furthermore, steep temperature gradients within the top 30 cm suggested that connectivity with the surface water was restricted to a shallow layer within the sediments. The three patch types differed in biogeochemical activity, largely as a function of their sediment size distribution, organic content and surface–subsurface exchange dynamics. Nitrate reduction was associated with hypoxic (30 cm) that contained oxygen >90 μM. A high background concentration of nitrous oxide (∼180 nM) occurred within the river bed, regardless of patch type and depth into the bed. In the nitrate reduction zone, N 2O concentration was highly variable, with production and reduction of N 2O. Ammonium concentration was higher within the river bed than in surface water and decreased with depth, particularly in the organically rich silt/sand and coarse gravels, where organic matter may get trapped. From a hydrological/biogeochemical perspective, the Lambourn differs from many other rivers. Despite being hypernutrified (surface and subsurface nitrate >400 μM), the bed of the Lambourn is predominantly aerobic and does not serve as a significant nitrate sink. In addition, despite the permeable geology of the catchment, biogeochemical processing of nutrients may be restricted to the thin, but biologically productive, layer in the shallow river bed sediments.