Integrated Assessment of Water Quality and Heavy Metal Contamination in Water and Aquatic Biota of the Otomona River, Indonesia

Rivers are the arteries of the landscape, carrying water, nutrients, and life from the highlands to the sea. Yet, as human activity expands, these waterways often become conduits for invisible threats. Among the most persistent of these are heavy metals—dense, naturally occurring elements like lead, copper, and mercury that do not break down or disappear. Unlike organic waste, which decomposes over time, these metals can linger in the water, settle into the riverbed, and accumulate in the bodies of fish and other creatures. This accumulation poses a dual risk: it can disrupt the delicate balance of the aquatic ecosystem and, if the fish are consumed, potentially affect the health of the people who rely on them for food. Understanding how these metals move through a river system, where they settle, and how they end up in living tissue is essential for protecting both the environment and public health.

In the tropical region of Central Papua, Indonesia, the Otomona River flows from upland catchments, influenced by mining activities, before emptying into the Arafura Sea. This river supports local fisheries and biodiversity, but until recently, there was little comprehensive scientific data on its water quality or the extent of heavy metal contamination within its ecosystem. To fill this gap, a team of researchers from Universitas Hasanuddin conducted an integrated assessment of the river, combining direct water sampling, analysis of fish tissue, and computer modeling to trace the movement of pollutants. Their goal was to determine if the river remained safe for its designated uses, such as irrigation and aquaculture, and to understand how heavy metals were distributed from the river's headwaters to its estuary.

The researchers began by collecting water samples from nine distinct locations along the river, stretching from the upper reaches down to the estuary. At each site, they measured a suite of physical and chemical properties, including temperature, clarity, acidity, and the levels of oxygen and nutrients. They also tested for the presence of five specific heavy metals: mercury, cadmium, copper, lead, and chromium. To get a clearer picture of the overall health of the water, they calculated a Pollution Index and a Water Quality Index, which synthesize these various measurements into single scores that indicate whether the water is clean, lightly polluted, or heavily polluted. The results were generally reassuring. The water temperature, acidity, and oxygen levels fell within the acceptable ranges set by Indonesian regulations for Class II water quality. The Pollution Index values across all sites ranged from 0.63 to 0.96, indicating that the river met the safety standards and was not heavily polluted. The overall Water Quality Index score of 70 placed the river in the "good" category, suggesting that, for the most part, the water was suitable for its intended uses.

However, a closer look at the specific metals revealed a more nuanced story. While mercury, cadmium, lead, and chromium remained at low, safe levels throughout the river, copper told a different tale. In the downstream sections, specifically at the two sites closest to the river mouth, the concentration of dissolved copper exceeded the safety limit of 0.02 milligrams per liter. This spike was not seen in the upper reaches, suggesting that the copper was either entering the river from local sources near the mouth or accumulating there as the water slowed down. To understand how these metals moved through the system, the team used a sophisticated computer simulation called MIKE 21. This model visualized the flow of the river and predicted where the metals would travel. The simulation confirmed the field observations, showing that concentrations of copper, lead, cadmium, and mercury tended to increase as the water moved from the upstream areas toward the lower river and estuary. The model suggested that slower currents and tidal mixing in the lower sections allowed these metals to settle and concentrate, creating zones of higher accumulation near the river mouth.

The study did not stop at the water itself. Recognizing that fish act as living filters that absorb contaminants over time, the researchers also analyzed the tissue of ten fish caught from the river. They found that even though the water often met safety standards, the fish had accumulated metals in their bodies. Mercury and cadmium levels in the fish were within the limits set for food safety in Indonesia. However, lead was a concern; while the water contained only trace amounts, several fish samples showed lead levels that exceeded the permissible limit for human consumption. Copper levels in the fish also varied widely, with some specimens showing higher concentrations. This disconnect between the water quality and the fish tissue highlights a critical reality: a river can appear clean based on a single water sample, yet still pose a risk because the metals have been absorbed and concentrated by the local wildlife. The fish essentially integrated the exposure over a longer period, revealing a hidden burden that a snapshot of the water alone might miss.

The findings paint a picture of a river that is currently in good health but faces localized pressures. The overall water quality is robust, supporting the "good" classification, yet the accumulation of copper in the lower reaches and the presence of lead in fish tissues signal that the system is not entirely free from contamination. The researchers noted that the increase in metal concentrations toward the estuary is likely driven by a combination of factors, including the transport of sediments from upstream, the slowing of water flow, and potential local inputs near the river mouth. While the current data does not point to a catastrophic pollution event, it does suggest that the lower river and estuary act as a sink where contaminants gather. The study concludes that while the Otomona River is safe for its current uses, the observed trends warrant continued vigilance. Future monitoring should expand to include different seasons and a deeper look at the riverbed sediments and the specific diets of the fish to fully understand the long-term ecological risks. For now, the river remains a vital, functioning ecosystem, but its ability to remain so depends on careful management of the sources that feed into it.