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Takaras River is a river basin that is vulnerable to pollution, particularly from the mercury usage in small-scale gold mining. This study aims to identify water quality based on mercury (Hg)and pH levels in the Takaras River. Samples were taken at two locations using a purposive sampling method. Results showed indications of mercury use on former gold mining land around the Takaras River altought the mercury content level was below 0.075 µg/L or 0.00075 mg/L in sample 1 and 2. Based on the test results, the acidity level (pH) in the two water samples from Takaras River is 3.86 and 3.84 in sample 1 and 2, compared to the maximum pH quality standard of 6-9 for Class II water, it is concluded that the scale of acidity in Takaras River is very low, indicating that the water quality in the Takaras River is highly acidic, abnormal, and shows signs of serious pollution. The implications of low pH levels in the Takaras River can cause damage to the Takaras River ecosystem, a decline in soil quality and fertility, and an increase in heavy metal toxicity in the environment surrounding the river.

This study aims to evaluate the level of heavy metal pollution and the potential ecological and human health risks in the waters of Selangan, Bontang City. The research focuses on analyzing the concentrations of iron (Fe) and mercury (Hg) in sediment, considering that both elements are toxic, persistent, and capable of bioaccumulating within the food chain. The study was conducted over six months, from January to June 2023, at four sampling stations representing the environmental conditions of Selangan waters. The methodology included sediment sampling, sample preparation through drying and grinding, and laboratory analysis using Inductively Coupled Plasma–Optical Emission Spectrometry (ICP-OES) to determine Fe concentrations. The results were then compared with quality standards established by the Washington Annotated Code 173-204-320 and Indonesia’s National Agency of Drug and Food Control (BPOM) Regulation No. 5 of 2018. The findings indicate the presence of heavy metal contamination across all sampling stations. Fe concentrations ranged from 2.35 mg to 6.308 mg, significantly exceeding the permissible limit of 0.3 mg/L. Meanwhile, Hg concentrations ranged from 0.015 mg to 0.028 mg, also surpassing the standard threshold of 0.001 mg/L. These results clearly demonstrate heavy metal pollution in the Selangan waters, posing potential threats to marine ecosystem health and coastal communities. Regular monitoring and mitigation measures are urgently recommended to prevent further environmental degradation.