Rivers and slush fields on the Greenland Ice Sheet increasingly develop in locations where the accumulation zone hosts near-impermeable ice slabs. However, the division between runoff versus retention in these areas remains unmeasured. We present field measurements of superimposed ice formation onto slabs around the visible runoff limit. The quantity of superimposed ice varies by proximity to visible surface water and the surface slope, highlighting that meltwater can flow laterally before refreezing. We use heat conduction modelling and radar observations of autumn wetness to show that in our field area in 2022, 65% of superimposed ice formed during summer and the rest during autumn in the relict supraglacial hydrological network. Overall, 84% of melt around the visible runoff limit refroze. Ice-sheet-wide we estimate that slabs refroze 56 gigatonnes of melt (26-69 gigatonnes according to slab extent) between 2017 and 2022. Slabs are thus both hotspots of refreezing and emerging zones of runoff.