Amalnala lake is situated in the Gadchandur town of Chandrapur district in Maharashtra, India. The lake water was turning green in colour in the month of september. This study gives an insight into the causes and factors responsible for the greening of the lake adjacent to Manikgarh limestone mine in the month of September. It was noted that, phosphate concentration in lake water showed rise after July (2020, 2021) and peaked in August (2020, 2021). The phosphates were mainly contributed by organophosphate pesticides such as monocrotophos, profenofos, and chlorpyrifos, which were identified by liquid chromatography triple quadrupole mass spectrometry (LC MS/MS). The rise in phosphates showed complete correlation with the rainfall and chlorophyl a (chl a) content in September (2020, 2021). Similarly, the nitrate concentration of the lake showed peak in August (2020, 2021) followed by the rise in chl a content in September (2020, 2021). Chemical oxygen demand (COD) and biological oxygen demand (BOD) of the lake was calculated and represented in terms of COD/BOD. The COD/BOD were highest in September (2020, 2021). Positive correlation was observed between the pH, total dissolved solids, and electrical conductivity of Amalnala Lake and Amalnala Stream which flows into the lake. This stream mainly carries the contaminants from open cast Manikgarh limestone mines to Amalnala Lake. Chl a was mainly contributed by two dominant species of cyanobacteria, Phormidium species CCALA 726 which was identified by 16S rRNA sequencing and Microcystis aeruginosa which was identified by using the keys given by Desikachary and Komarek and Anagnostidis. This study thus concluded that the pesticides, nitrates, and Amalnala Stream contributed in the growth of cyanobacteria which causes the greening of the lake.