PROGLACIAL GROUNDWATER RESPONSE TO MULTI-MAGNITUDE ATMOSPHERIC/VOLCANIC EVENTS

Proglacial outwash plains provide a distinct hydrogeological environment combining recharge from both direct precipitation and glacial meltwater. In some regions jökulhlaups create thick highly permeable unconfined sand and gravel aquifers which respond dynamically to events on daily, seasonal, and multi-decadal time scales.

Both transient small-scale and more permanent large-scale modifications to the groundwater system occur at the world's largest active sandur (>1000 km2 area) Skeiðarársandur, south-east Iceland. The groundwater table lies close to the surface and so responds rapidly to daily or seasonal variations in meteoric input through varying magnitude fluctuations of the water table, which can produce fine-scale modifications to the groundwater flow path. Additionally volcanically-triggered glacier outburst floods may lead to the more permanent rearrangement of flow patterns.

A network of piezometers has facilitated the study of water table fluctuations over a spring and summer period. The stable isotopes of oxygen and hydrogen in the water, and of sulphur and strontium in the solute, have been used to trace the sources of groundwater and their subsequent flow paths, and to identify physical and chemical processes that take place within the aquifer with changing meteoric conditions.

Skeiðarársandur is subject to periodic high magnitude discharges that have been shown to affect the hydrogeology of the area. During the jökulhlaup at Skeiðarársandur in November 1996 a large outlet for flood waters was created at the margin. Melting of the outlet walls has left a large supraglacial gravel ridge, now associated with a concentration of upwellings and a strong spring line surrounding the ridge, suggesting that the jökulhlaup may have created a focus for normal ablation-controlled subglacial drainage. This demonstrates how an extreme event can alter long term hydrological and hydrogeological flow patterns. We suggest that other large-scale glacial processes such as surge events may also greatly affect groundwater flow patterns.