Implications of groundwater-surface water connectivity for 
nitrogen transformations in the hyporheic zone

Rivers have (rather controversially) been described as 'simply 
outcrops of groundwater'. Many of the rivers in the UK are 
supplied mainly from groundwater sources, especially during the 
summer months when rainfall is characteristically low. The 
hyporheic zone is a critical interface between surface and 
subsurface waters in groundwater catchments. Here, the mixing of 
groundwater and surface water and the resulting biological and 
chemical reactions, may exert a lot of control on the water 
quality of the river and also its ecology: so much so that the 
hyporheic zone has been ascribed pollutant attenuating properties 
by some. Groundwater abstraction, effluent disposal and diffuse 
nutrient pressures - especially nitrogen - may all compromise the 
capacity of the hyporheic zone to influence the water quality of 
a river. Although quite a few researchers have recognised that 
the hyporheic zone has some special control on the river habitat, 
most have looked at it only from the perspective of the 
relationship between river water and the upper few centimetres of 
the sediments of the riverbed. They have ignored the fact that as 
well as downward flux from the river into the sediments of the 
riverbed there will also be upward flows from groundwater through 
the hyporheic zone and into the river. This research considers 
what happens to the chemistry of groundwater as it moves through 
the hyporheic zone and investigates the claim that the hyporheic 
zone can attenuate groundwater contaminants such as nitrate.