# Is It Possible to Restore a Heavily Polluted, Shallow, Urban Lake?
## Material and Methods  
### Study Site  
Karczemne Lake (54\({}^{\circ}\)19\({}^{\prime}\)42\({}^{\prime\prime}\) N, 18\({}^{\circ}\)11\({}^{\prime}\)27\({}^{\prime\prime}\) E) is a strongly degraded urban lake located in the Kaszubian Lake District and belongs to the macroregion of the Eastern Pomeranian District in Kartuzy town [25]. Karczemne Lake is a shallow, polymeric, flow-through lake. Its area is 40.4 ha, and its maximum depth is 2.3 m (Figure 1). More detailed morphometric parameters of the lake are given in Table 1.  
From the early 1950s, Karczemne Lake was transformed into a receiver for domestic sewage, as well as sewage from diaries, slaughterhouses, breweries, furniture factories and municipal hospitals. Over 30 years, 60% of the raw municipal sewage went to Karczemne Lake through six sanitary sewers. With the expansion of the urban area, the amount of municipal sewage increased. Impurities from emerging single-family housing were collected in leaking cesspits, which caused some of the pollution to seep into the ground and migrate toward the lake. Some cesspits had overflows to the combined sewage network, and municipal sewage flowed directly to the lake in this way. It was also possible to illegally connect sanitary sewers from individual domestic properties to the stormwater drainage system and directly discharge sewage to the lake from homes located along the shore. The improvement of water and sewage management in Kartuzy City began in the mid-1970s. In 1982, a mechanical-biological sewage treatment plant was opened. In the 1990s, almost the entire city was connected to a sanitary sewage system, and it was only in 2010 that the management of stormwater began to be organized through the construction of settling tanks and stormwater separators. Until 2018, the stormwater drainage network in Kartuzy covered only part of the city--17%. This situation meant that during heavy rainfall, local flooding occurred, and the excess rainwater and snowmelt from streets that did not have a rainwater drainage system penetrated the sanitary sewer system, overloading it. The connection of the existing stormwater drainage system to the sanitary sewer system caused raw sewage to load into the lake. The total catchment basin of Karczemne Lake covers 5.15 km\({}^{2}\). This area is covered by two entirely different (in their ability to activate the load of nutrients in a surface flow) forms of land use: forests (57%) and urbanized areas (43%). The direct catchment of Karczemne Lake, excluding the area covered by the drainage system collecting rainwater, covers 0.45 km\({}^{2}\): wasteland constitutes 20.7% of its total surface area, and forests grow over the other 79.3%. The calculations of the amount of nutrients that are annually brought to the lake with a watercourse or from outfalls were performed based on their actual concentration in water (total phosphorus--TP and total nitrogen--TN) and flows measured at the individual stations during yearly field studies. The partial load for a given day was the product of the volume of water (momentary flow) and the concentration of given nutrients in the water. Nutrient loads were calculated with a generally accepted method of time periods. The magnitude of a lake load with nutrients originating from surface flows from the direct catchment was calculated with a method that is recommended and applied by the OECD (Organisation for Economic Cooperation and Development) that involves the use of flow coefficients. The load of nutrients introduced to a lake with precipitation was determined based on the coefficients of pollution deposition per surface unit. It was assumed that angling baits are the main sources of pollution resulting from the recreational use of lakes. Based on the data obtained from Group No 57 of the Polish Angling Association from Kartuzy and conversion factors described by [PERSON] and [PERSON] [26], the average number of anglers who used baits was calculated for each of the lakes.