Spatial extent and temporal variability of Greenland firn aquifers detected by ground and airborne radars

Abstract

We document the existence of widespread firn aquifers in an elevation range of ~1200–2000 m, in the high snow‐accumulation regions of the Greenland ice sheet. We use NASA Operation IceBridge accumulation radar data from five campaigns (2010–2014) to estimate a firn‐aquifer total extent of 21,900 km2. We investigate two locations in Southeast Greenland, where repeated radar profiles allow mapping of aquifer‐extent and water table variations. In the upper part of Helheim Glacier the water table rises in spring following above‐average summer melt, showing the direct firn‐aquifer response to surface meltwater production changes. After spring 2012, a drainage of the firn‐aquifer lower margin (5 km) is inferred from both 750 MHz accumulation radar and 195 MHz multicoherent radar depth sounder data. For 2011–2014, we use a ground‐penetrating radar profile located at our Ridgeline field site and find a spatially stable aquifer with a water table fluctuating less than 2.5 m vertically. When combining radar data with surface topography, we find that the upper elevation edge of firn aquifers is located directly downstream of locally high surface slopes. Using a steady state 2‐D groundwater flow model, water is simulated to flow laterally in an unconfined aquifer, topographically driven by ice sheet surface undulations until the water encounters crevasses. Simulations suggest that local flow cells form within the Helheim aquifer, allowing water to discharge in the firn at the steep‐to‐flat transitions of surface topography. Supported by visible imagery, we infer that water drains into crevasses, but its volume and rate remain unconstrained.

Investigation of Firn Aquifer Structure in Southeastern Greenland Using Active Source Seismology

“…We also derive the local seismic velocity of the ice by selecting the velocity 3-4 m beneath the aquifer layer. As our experimental design was not optimized for imaging the top of the aquifer, we independently obtained depth to the water table from ground penetrating radar reflections that were collected in the field in tandem with the seismic data (Miège et al, 2016). The GPR depths to the water table are estimated from the two-way-travel time of the radar electromagnetic wave between the snow surface and the water-table surface, with a calculated error of ±0.5 m. The aquifer thickness is then the difference between the GPRderived water table depth and the seismically derived base of the aquifer.…”

“…Our field site was chosen based on the presence and structure of the firn aquifer determined by NASA Operation IceBridge airborne radar measurements and ice core data from the region Miège et al, 2016). This field site was positioned to assess the impact of the firn aquifer on a tidewater glacier contributing to sea-level rise through ice discharge and melt (Enderlin et al, 2014).…”

“…This field site was positioned to assess the impact of the firn aquifer on a tidewater glacier contributing to sea-level rise through ice discharge and melt (Enderlin et al, 2014). Past field campaigns using radar soundings established that the water table is varying spatially between 10 and 20 m below the surface Miège et al, 2016), and ice-coring placed the firn-ice transition at 30 m depth (Koenig et al, 2014). Gogineni (2012) used multi-channel coherent radar sounding to determine the ice sheet is 800-1000 m above sea level (a.s.l.)…”

“…Aquifers are formed by meltwater percolating into the firn in high-accumulation, high-melt regions Munneke et al, 2014) and are located extensively in the southeastern and southern parts of the GrIS according to NASA Operation IceBridge airborne radar and ground based radar measurements (Miège et al, 2016). Though extensive in some regions of the ice sheet, the exact flux and stability of meltwater into and out of firn aquifer systems remains uncertain.…”

Recent Advances in Our Understanding of the Role of Meltwater in the Greenland Ice Sheet System

“…Here, a combination of ground and airborne radar, shallow firn cores and modelling indicate that the top of the firn aquifer ranges in depth from 5 to 50 m from the ice sheet surface, covers up tõ 70,000 km 2 and stores up to 140 km 3 of water; the aquifer thus represents a previously unknown yet significant mode of meltwater storage. The subsequent fate of the aquifer waters (refreezing or runoff) remains unclear [31] although modelling suggests that the potential water flux provided by the aquifer is sufficient to induce hydrofracture to the bed via crevasses, thereby providing a mechanism for both aquifer drainage and a potential impact on ice dynamics [32].…”