The article deals with the chemical balance and transformation of lithophile elements (Fe, Mn, Si, and Al) and heavy metals (Cu, Zn, Cd, Ni, and Pb) in Lake Onego. They were found to arrive in the lake from various sources (streamflow, precipitation, base flow, sewage, runoff from residential areas, and leakage from municipal waste landfills). The permissible input of the elements to the lake was estimated taking into account its assimilation capacity and the requirement to maintain the geochemical class of the water.