Groundwater-surface water exchange in the proglacial zone of retreating glaciers in SE Iceland

Groundwater-surface water exchange significantly impacts proglacial hydrology and ecology. This study applies a multidisciplinary approach to investigate groundwater-surface water exchange in the proglacial zones of two retreating glaciers in SE Iceland. Mapping of decadal changes in the extent of proglacial groundwater seeps in the large outwash plain of Skeiðarársandur has shown a 97% decline, as well as substantial falls in groundwater levels. Field and laboratory measurements suggested high spatial variability in hydraulic conductivity at the Skaftafellsjökull foreland. The highest hydraulic conductivity was measured in areas underlain by glaciofluvial deposits whilst the lowest hydraulic conductivities were associated with glacial tills and lacustrine deposits.\ud Precipitation was identified as an important control on groundwater levels on various temporal scales. Automated monitoring of meltwater and groundwater levels also identified fluctuations in meltwater level as an important control on hydraulic heads, whose importance on groundwater levels has been observed during various flow regimes. The close connection between meltwater and groundwater levels suggest high meltwater-aquifer exchange. However, high meltwater-aquifer exchange is contested by significantly different geochemical and isotopic composition of groundwater and meltwater. Hydrogeological flux estimates suggest high spatial variability in groundwater seepage into the Instrumented Lake, which was attributed to the high variability in hydraulic conductivity around the lakeshores. These are also supported by high –resolution temperature mapping at the lake bed, which suggested that groundwater upwelling in the fine-grained lakeshore took place at discrete locations.\ud This study suggests climate and glacier margin fluctuations as primary controls on proglacial groundwater-surface water exchange. It also highlights the importance of groundwater contributions to water quality and ecology, with groundwater-fed bodies possibly sustaining important ecological niches. However, proglacial groundwater-fed features are transient and are threatened by changes in precipitation and glacier retreat. Further declines in groundwater-fed hydrological systems are therefore projected to adversely impact proglacial groundwater-surface water interaction