Abstract
The study was conducted on Długie Lake in Olsztyn, which for 20 years since the mid-1950s served as a domestic and storm wastewater receiver, which led to its complete degradation. The discontinuation of wastewater inflow in 1976 caused a change in the trophic state from saprotrophic to hypertrophic. Evident improvement of water quality was possible only after the implementation of proper restoration techniques. Długie Lake was subjected to artificial aeration with thermal destratification (1987–2000). After all opportunities to improve water quality in the lake by artificial aeration (low phosphorus sorption capacity of sediment) had been exhausted, it was decided that a phosphorus inactivation method using the coagulant PAX 18 be used (2001–2003). Before restoration, the nutrient concentration in the near-bottom water layer of Długie Lake was very high at 22.9 mg TN L−1 and 3.50 mg TP L−1. The average amount of chlorophyll a was ca. 200 μg L−1, and the Secchi disc visibility did not exceed 1 m. In 2017, 14 years after termination of the lake restoration process, the total phosphorus concentration at the bottom was 0.21 mg P L−1 on average, and the total nitrogen was 1.5 mg N L−1. The mean transparency of the water oscillated around approximately 5 m, and the amount of chlorophyll a was 1.86 μg L−1. Studies have shown that the most important step in reclamation is to prevent pollutants from entering the lake to the maximum extent possible and to use a combination of several reclamation methods as a matter of good practice. Stable environmental conditions have developed in the lake, and the values of chlorophyll a and the Secchi disk visibility indicate that the lake has reached a mesotrophic state.
Highlights
One negative aspect of the urbanization, industrial and agricultural development, which occurred after World War II in Poland and throughout the world, especially in the mid-1950s, was the transformation of urban water bodies into wastewater receivers
The water properties selected for analyses were TN, TP, BOD5, chlorophyll a, and water transparency
An alternative hypothesis tested was the presence of significant differences in mean annual values of nutrients between the control year (1972) and subsequent stages of lake protection and recultivation (1975, 1984—after limiting and after cutting off the inflow of domestic sewage; 1987, 1998, 2000—during artificial aeration; 2001, 2002, 2003—during phosphorus inactivation) and 2011, 2013, and 2017
Summary
One negative aspect of the urbanization, industrial and agricultural development, which occurred after World War II in Poland and throughout the world, especially in the mid-1950s, was the transformation of urban water bodies into wastewater receivers. Symptoms of degradation are especially visible as algal blooms, deteriorated water transparency, deoxygenation of water, or the occurrence of unwanted decomposition products such as ammonium and hydrogen sulfide near the lake bottom (Grochowska and Tandyrak 2007). (Inoue et al 2015; Miyagi et al 2016) In such cases, protective measures are necessary. The first and foremost step is to cut off the flow of sewage into the lake and redirect it beyond the catchment area. If lakes are surrounded by agricultural areas, it may be necessary to change the land use practices in the catchment area and reduce the amount of arable land in favor of grassland (Grochowska et al 2011, 2014)
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