Preliminary Hydrochemical Characteristics of an Urban Lake Restored Using a Sequential Application of Iron and Aluminum Coagulants

A study was carried out on Lake Mielenko in Kartuzy. It is a small and shallow waterbody (7.8 ha, max. depth 1.8 m), whose morphometric conditions qualify it as a polymictic lake (the first lake in a river—lake system). Before the reclamation began, the lake was characterized by a high concentration of chlorophyll-a (16.4 mg m⁻³ in spring to 33 mg m⁻³ in summer) and low visibility. The purpose of the study was to determine the impact of the first stage of restoration of Lake Mielenko by phosphorus inactivation using a combination of aluminum and iron coagulants (PIX and PAX). After coagulant dosing into the lake, a significant decrease in chlorophyll-a was observed, which resulted in improving water transparency in the lake and decreasing concentrations of both nutrients (nitrogen and phosphorus). The total concentration of nutrients was dominated by organic forms (constituting 71–95%). Maximum nutrient concentrations were 0.273 ± 0.01 mg P L⁻¹ and 5.52 ± 0.1 mg N L⁻¹ (in the summer period before the restoration activities were performed).