Relevance. The necessity to assess current geo-ecological situation of peaceful underground nuclear explosions sites near cities and mining deposits by the risks of radionuclides entering the human environment. Aim. To reveal processes and interconnections in geotechnogenic system "underground nuclear explosion cavity – host geological environment – surface ecosystems" by analyzing chemical composition of water runoffs at the explosion epicenter. Object. The research focuses on the peaceful underground nuclear explosion "Crystal" site, carried out in 1974 in the frozen rocks at the depth 98 m, which resulted in an upthrust dome with a collapsed central part at the epicenter and the ejection of a 12-meter-long fragment of the casing column from the emplacement hole. Methods. Field radiometry, liquid scintillation spectrometry, capillary zoned electrophoresis, titration, atomic emission spectrometry with microwave plasma, mass spectrometry with inductively coupled plasma. The results of chemical analyses of surface water, sampled at the “Crystal” epicenter and its adjacent area in 2019–2021, meet the requirements for quantitative analysis. Results. Due to probably free air convection in the artificial cover, condensation of atmospheric moisture in the cover is main factor of water provision at the epicenter. Leaching of the limestone and dolomite material of the artificial cover, the technogenic materials of the emplacement hole, pushed onto surface, and underground gas-brine fluids, most probably rising along the open emplacement hole, lead to formation of mixing chloride-bicarbonate-sulfate magnezium-calcium brackish ultra-hard waters with ³H, Br, Sr, Li, I, K, Na at the “Crystal” epicenter. It is admitted that Li⁺ could be a component of the nuclear explosive device.