PHYSICOCHEMICAL AND BIOLOGICAL PARAMETERS OF DOMBROVSKE PIT LAKE — LEGACY OF OPENCAST POTASSIUM SALT MINE (KALUSH, UKRAINE)

Abstract

Complex monitoring of physicochemical and biological parameters of water bodies as holistic ecosystems allows developing their right management strategy in future. Dombrovske Pit Lake (49°01′34′′N, 24°19′25′′E)  was formed in an abandoned opencast potassium salt mine in 2008, as a result of ceased mining. The lake is characteri­zed as meromictic with the salt gradient of approximately 30 g/L at the surface to 400 g/L at the bottom, with the prevalence of Na⁺, K⁺, Mg²⁺, Cl⁻ and SO₄²⁻ ions. A slight heliothermal effect occurs during summer (the temperature of the hemocline is approximately 5 °C higher than that of the myxolimnion and monimolimnion). Major anions and cations (Cl^–, SO₄^2-, Na⁺, K⁺, Mg²⁺, Ca²⁺, HCO₃⁻) profiles are very similar to the mineralization curve, the content of microelements (Cd, Ni, Fe, Mn, Zn, Pb, Co) and pH also correlate with salinity. Concentrations of most heavy metals ions in the surface waters of the lake meet the Ukrainian quality standards for drinking and surface water; a small excess of Cd content compared to the Ukrainian standards is expected to disappear along with a decrease in the overall mineralization when the lake is filled to the planned level. Chlorophyll concentration (0.5–4 µg/L), pH values (≈ 7.5) and dissolved oxygen content (8–12 mg/L) point to a high biotic activity within the 0–5 m surface water level. Such concentrations correspond to oligotrophic lakes with a low plankton level, poor nutrient content and minimal contamination. Chl B constitutes the largest part of the total mass of chlorophyll, which indicates the predominance of green algae representatives. The relatively high proportion of Chl C is due to the considerable biomass of diatoms and dinoflagellates, which play a crucial role in the photosynthesis of seas and oceans. Due to low content of Chl A, the current conditions in Dombrovske Lake are not favorable for the development of blue-green algae responsible for polluted water bloom. The current stratification, physicochemical and biological characteristics of Dombrovske Lake clearly correspond to the previous forecasts about the formation of a lake with an area of approximately 100 ha, a shoreline of 8 km, and a top freshwater layer up to 17 m. Flooding to the final level will depend mainly on the amount of precipitation, and is expected to occur within the next 5–10 years.