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 meters. Groundwater breakthroughs reached 1000 m3/h and more. The nature of waters of these breakthroughs remained unclear. All of them are fresh in composition. But separate water seeps 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) What waters are used to form the mine water supply? 2) From what aquifers do the largest water outbursts occur? 3) Is there a connection of mine water with the Plussa River and the Narva Reservoir?