The hydrochemical method can be used in solving hydrogeological issues arising during the development of mineral deposits. We will show the possibility of using this method on the example of the Leningrad oil shale deposit. It is characterized by the presence of several aquifers above and below the developed layer, a rather large river Plussa and the Narva reservoir. There are four mines in operation. The annual water inflow to them is 25‑28 million m³. Groundwater breakthroughs reached 1000 m³/h or more. The nature of these waters of these breakthroughs remained unclear. All of them are fresh in composition. But individual water inflows in mines have slightly different composition of macro‑ and microcomponents. This was the basis for the application of the hydrochemical method in solving three main questions: 1) Which water sources form the mine water inflow? 2) From what aquifers do the largest water outbursts occur? 3) Is there a connection between mine water and the Plussa River and the Narva Reservoir?