Mapping survey of sheet Q-42 included isotope sampling to assess the formation conditions of the resources and reserves of groundwater in the zone of active water exchange. Groundwater of the upper zone has the modern recharge origin; its reserves and resources are formed directly from meteoric water or with an intermediate reservoir in the form of underground ice. Currently, melting of the formation ice does not make a significant contribution to the groundwater balance. In many cases, the recharge water that penetrates into aquifers at watersheds are partially evaporated, which is clearly visible in the change in the isotopic composition of water due to disequilibrium fractionation. The total loss of water due to evaporation can be estimated at up to 20–25%. Water losses due to evaporation for infiltration-type water intakes are small and may not be taken into account. In the northern part of the sheet Q-42, the main reserves and resources of groundwater are contained in sub-channel and sub-lake taliks. Connection of groundwater, which exploited by water intakes, with talik for the Ob River valley is reliably established based on isotope and chemical data. For the southern part of the sheet, where the permafrost loses continuity, the main reserves and resources of groundwater should be attributed to inter-permafrost aquifers.