CONCENTRATED GROUNDWATER RECHARGE FROM MELTWATER STREAMS IN COASTAL PROGLACIAL AQUIFERS OF THE GULF OF ALASKA

Groundwater has been recognized as a valuable player in proglacial watersheds. However, as river discharge to the Gulf of Alaska continues to increase, proglacial aquifers and related groundwater processes remain severely understudied within the region. Here we provide evidence of direct links between meltwater streams and proglacial coastal aquifer systems in two glaciofluvial deltas entering Kachemak Bay, Alaska. Six temperature profile rods with vertically stacked temperature loggers are installed at each site for a total of two months that targeted the melt season. Streambed temperature profiles are combined with temperature and stage height data observed in surface waters, and temperature and water level data observed in a shallow piezometer at one experimental field site. Differential gauging with continuous stage and temperature observations are combined with the streambed temperature profiles at another stream. We incorporate measurements of streambed hydraulic conductivity for each reach. Passive seismic imaging of subsoil thicknesses constrained with the horizontal to vertical spectral analysis technique are also incorporated in profiles both perpendicular and longitudinal to stream reaches to estimate groundwater flow through the entire aquifer thickness. We find that groundwater is actively recharged by both local precipitation and snow/glacier meltwater. Heat tracing and stable oxygen and hydrogen isotopes ratios in water indicate concentrated recharge from meltwater stream losses for most of the season. Differential discharge measurements show streamflow loss to the underlying aquifer to be ~24% of streamflow. Groundwater flow through the entire estimated thickness of the aquifer is found to be a significant portion of the meltwater flux when compared to the mean streamflow. Proglacial aquifers composed of highly permeable glaciofluvial sediments are prevalent across the Gulf of Alaska and are common globally. Future alterations in the hydrologic cycle and glacier recession in these landscapes will likely influence groundwater storage and flow within them as well. Our study highlights the importance of understanding groundwater flow in these environments to accurately estimate both meltwater flux and aquifer recharge in catchments undergoing active deglaciation.