Effects of glacier retreat upon glacier-groundwater coupling and biogeochemistry in Central Svalbard

•Glacier thinning in the High Arctic leads to subglacial permafrost aggradation.•Permafrost beneath Arctic glaciers does not preclude subglacial groundwater recharge.•Intra-permafrost groundwater flow is sustained via high elevation crevasses.•Perennial springs develop in the glacier forefields as they retreat.•Methane-saturated, reduced groundwaters discharge from the springs all year.