Abstract
The aim of this study was to determine the current hydrochemical and toxicological status of two contrasting lakes in the Ayyrtau district of Northern Kazakhstan, Lake Imantau and Lake Shalkar, and to identify potential risks associated with pollution by biogenic substances and heavy metals. Field work, including water sampling and morphometric measurements, was carried out in October 2024; samples were taken in the central part of the water areas at a depth of 0.5 m. pH, temperature and dissolved oxygen were measured with a portable HQ40D device (HACH-LANGE), major ions titrimetrically, biogenic compounds with a DR 3900 spectrophotometer, and heavy metals (Cu, Zn, Pb, Cd, Co, Ni) by atomic absorption spectrometry (AA-7000, Shimadzu), with analytical accuracy verified through the ionic balance. Lake Imantau is characterised by a slightly alkaline reaction, moderate mineralisation (1231 mg/L) and phosphate concentrations exceeding the regulatory standard, whereas Lake Shalkar shows high alkalinity (pH 9.1), considerable mineralisation (17,076 mg/L) and water hardness. Heavy-metal concentrations in both water bodies do not exceed the maximum permissible values; by oxygen regime and biogenic-substance content, both lakes are classified as mesotrophic, while Lake Imantau shows initial signs of a trophic transition. The direct comparison of two hydrochemically contrasting lakes within a single district links differences in water quality to the water-balance features of their basins rather than to recreational load. The results address the data gap on the region's water bodies and provide a basis for further monitoring.