Stable isotope tracers of shallow groundwater recharge dynamics and mixing within an Icelandic sandur, Skeiarársandur

Stable isotope (δ¹⁸O, δD) tracers are used to investigate the temporal and spatial distribution of groundwater recharge sources, flow patterns and mixing processes within a proglacial outwash plain aquifer at Skeiarársandur, southeast Iceland. There are two principal and isotopically-distinct groundwater recharge sources: glacier/ice melt and local precipitation, and a third minor recharge source of lake water. Shallow groundwater typically becomes isotopically heavier with decreasing depth and increasing distance from the glacier margin. This trend varies spatially and temporally due to the superimposition of a number of factors. The proportions of different recharge sources and evolution of the isotope composition of shallow groundwater are controlled by: (1) proximity to spatially restricted recharge sources (icemelt, surface water bodies); (2) groundwater depth; (3) vertical flow patterns controlling the extent of mixing; (4) recharge dynamics in response to local precipitation events; (5) connection to localised flow systems; and (6) seasonal mixing processes through cryogenic turnover.