Long-term variability of proglacial groundwater-fed hydrological systems in an area of glacial retreat, Skeiðarársandur, Iceland

Proglacial groundwater-fed features, such as seeps, substantially impact proglacial geomorphology, hydrology, and ecology. However, there is a paucity of research on the impacts of climate change and glacial retreat on the extent of these important features. This paper aims to investigate the impact of glacial retreat on proglacial groundwater levels and on the extent of groundwater-fed seeps. Research has taken place in western Skeiðarársandur, the large proglacial outwash plain of Skeiðarárjökull, a retreating temperate glacier in SE Iceland. Changes in the extent of proglacial groundwater seeps were mapped using historical aerial photographs from 1978, 1997, and 2012. Proglacial groundwater levels were monitored in shallow boreholes between 2000 and 2012. The western margin of Skeiðarárjökull has retreated approximately 1 km from its position in 1978. However, this retreat was punctuated by short periods of readvance. The geomorphology and groundwater systems at the site were substantially impacted by the 1991 glacial surge and the November 1996 jökulhlaup, whose deposits altered approximately 18% of the area of groundwater seeps. The surface areas of groundwater seeps and lakes in the study area have declined by ~97% between 1978 and 2012. Most of the decline took place after 1997, when there was a threefold increase in the mean rate of annual retreat. Groundwater levels also declined substantially between 2000 and 2012, although this trend varies spatially. Changes in precipitation and groundwater flow, alongside glacio-isostatic uplift, are suggested as possible causes for the observed declines in groundwater levels and seep extent.