Dependence of the chemical composition of flood waters on the nature of the economic use of the catchment area

In the territory of the Pushkin district in 2023-2024, to assess the contribution of various sources to the chemical composition of flood waters, a snow—measuring survey was carried out, samples of meltwater from small watercourses, small rivers flowing into the Pestovsky reservoir and flowing through various types of landscapes - forest, field, mainly in the area of settlements, as well as samples of reservoir waters were taken. Meltwater samples were taken at the beginning and at the end of the snowmelt. In water samples from the residential area, a high content of chlorine, sodium, potassium, magnesium, calcium, ammonium and nitrate nitrogen, and sulfate ion was determined, tens of times higher than concentrations in meltwater from natural landscapes. The pH was naturally increased from 7 to 8 units. for all types of the studied landscapes, there is a tendency to increase water mineralization by the end of the flood, which is associated with thawing of the soil and removal of accumulated soluble forms. When runoff enters small rivers, due to dilution, the concentrations of elements decrease and become lower than permissible. The extremely high concentrations of chlorine, sulfate ion, phosphates, and alkali metals found in the meltwater of the residential area are obviously associated with the use of deicing reagents and the ingress of various household chemicals into drains. The determination of the heavy metal content did not reveal patterns of spatial distribution. Separate local points of contamination with lead and cadmium have been identified. The actual multiple excess of pollutants in the flood waters selected from the residential area shows the need to limit the development of reservoirs of water management importance in the catchment area, the construction of a storm sewer intercepting surface runoff for the purpose of purification.