The Greenland Ice Sheet (GrIS) has been losing mass at an accelerating rate since the 1990s, with a large contribution from surface meltwater runoff to the ocean. Firn, a porous transition layer between snow and ice, retains this meltwater, buffering the GrIS’s contribution to sea level rise. Depending on accumulation rates, high surface melt can lead to the formation and expansion of thick, impermeable ice slabs in the subsurface firn, reducing its capacity to retain meltwater and increasing runoff (low accumulation), or to the formation of firn aquifers, which store meltwater (high accumulation). Recent literature on ice slabs and firn aquifers has been mainly of observational nature, studying their extent and recent evolution. Here, we use a novel one-dimensional firn model, called TFM, to simulate the evolution of ice slabs and firn aquifers along glacier flowlines at the Helheim Glacier and K-Transect under different climate forcing scenarios. We show that firn aquifers were already forming in the Helheim Glacier region before the GrIS started rapidly losing mass. With warming, firn aquifers form earlier along the flowline, expanding towards the interior of the ice sheet. Firn aquifer formation remains highly dependent on surface accumulation, with higher accumulation rates favouring formation. Results further show that ice slabs, though less extensive than firn aquifers, were also present along the K-Transect in Southwest Greenland before the GrIS's rapid mass loss. With more available surface melt, ice slabs form earlier along the flowline and expand towards the interior, consistent with available observations.