Direct evidence of lateral hyporheic flows revealed through single well tracer dilution tests

Characterising the spatial and temporal distribution of the magnitude and direction of subsurface water fluxes within the zone of groundwater - river interaction is essential for reliable estimation of the speciation and pattern of nutrient fluxes between the two contrasting water bodies. Most conventional experimental approaches (e.g. using vertical gradients derived from piezometer heads, or one dimensional temperature profiles) ignore the role of lateral flows within this environment and yet such fluxes clearly contribute to the conceptualisation of a hyporheic zone. Single well dilution (drift) tests have been widely used in hydrogeological studies as a means of determining local lateral flux estimates. However, the adoption of such approaches in groundwater - surface water investigations has been generally ignored. Here, we present the findings from two field campaigns in which over 100 piezometers, installed within the bed and banks of a 250m reach of the River Leith in Cumbria, UK, were monitored for the dilution of an injected salt tracer. The results reveal significant lateral flows, in particular in the shallow (20cm depth) and deep (100cm depth) piezometers. In contrast, lateral fluxes within the intermediate zone (50cm depth) appear generally much smaller and are of a similar level to those derived from tests conducted in local riparian piezometers. The observed lateral fluxes in the river bed units are of a similar order to vertical fluxes determined from vertical hydraulic gradients and slug test hydraulic conductivities. Our findings corroborate with observed natural tracer profiles from multi-level samplers at the site and suggest that the zone of mixing between surface water and groundwater is constrained to the upper few tens of centimetres at the site. Furthermore, the high spatial density of our tracer testing highlights the spatial variability of fluxes within the reach and, in particular, indicates transverse and longitudinal trends along the reach.