Technological advancement and the quest for power have gradually contaminated our environment, as industrialization, agriculture, and nuclear activities progressed. In the year 1986, a terrible nuclear incident occurred in Ukraine, in which the nuclear reactor exploded. The explosion resulted in the release of radionuclides with a massive concentration of 137Cs and 131I. Among these,137Cs was the most persistent one having a half-life of 30.2 years. The137Cs spread to Sweden due to advection, dispersion,and diffusion by wind, fallout over by precipitation via adsorption, and absorption in northern parts of Sweden experiencing a heterogeneous distribution. This fallout affected both the terrestrial and aquatic environments. This research aims to investigate where the 137Cs went and how its concentration changed with time in Swedish mushrooms, lakes, and in the Gulf of Bothnia. Also, to search for the environment that holds137Cs for a longer period and why it is so. More than 30 years have passed since the incident, and it isexpected that 137Cs would have decayed to values of its half-life. Swedish food authority recommends that up to 10,000 Bq/kg of 137Cs in mushrooms is safe for human consumption. The advocated limitsfor fish and seawater are 1500 Bq/kg and 40 Bq/kg, respectively.The method embraced to answer the research questions is based on data from the Swedish Radiation Safety Authority, the Radiant Earth project between Uppsala University and SLU, and also the data published in the literature. Decay models were generated for 137Cs.These models help in explaining the processes of accumulation and extraction of 137Cs. A closer look is also given to processes that have animpact on factors such as weathering, erosion, wind, washout, and bioturbation, and the rate of intake in environments. The aquatic environment, because Gävleborg was most affected during the incident, the highest value in the fish measured in the year 1988 was 23,508 Bq/kg. However, after 31 years (2016), the value was 4.5 Bq/kg. The values for Jämtland, Västerbotten, and Västernorrland were also almost equivalent to those for Gävleborg, estimated in the year 2016 to be around 4.5 Bq/kg. Observations of water data in Gävleborg show a wide range between 2-996 Bq/kg. In the Gulf of Bothnia, the highest 137Cs value recorded for sediment data in the year 2003 was 310 Bq/kg, and the lowest value of 181 Bq/kg was observed in the year 2015. In terrestrial environments, the maximum value recorded for Craterellus tubaeformis mushrooms was 522,000 Bq/kg in the year 1987. On the other hand, the lowest value recorded for Chanterelle mushrooms was 101.5 Bq/kg in the year 2017. Observations from a sample collected in Uppsala in the year 2017 indicates a value of 15 Bq/kg. Several mushroom species (Chanterelle, Craterellus tubaeformis) had distinct concentrations within the same geographical area and time, i.e., the Västernorrland in the year 2017.Due to weathering and erosion, when 137Cs reaches a water body (sea, ocean, estuary, river, lake or wetland), it dissolves in the water increasing its concentration. As a result, it increases the overall concentration of 137Cs in the fish. The results show that most of the 137Cs is locked in the sediments, with time buried under new sediment. However, due to resuspension and bioturbation, the contaminants may end up in the aquatic environment again. Models for lakes show irregular behaviour since their sizes are not as large as seas and small intakes depict high concentrations. Based on the mushroom results, 137Cs concentrations have been greatly reduced compared with Chernobyl levels. Different mushroom species exhibit different levels of 137Cs in the same area. Most radionuclides are found in the top layer of organic soil, making this layer crucial for taking up 137Cscontamination. Over time, these radionuclides, including 137Cs, are buried beneath the soil layers and cease to contribute to the system.