Spatial and temporal variation of dissolved heavy metal contamination in surface water of Na Thap River, Southern Thailand

Heavy metal pollution in aquatic environments represents a significant global challenge. The Na Thap River, a tidal river connected to the Gulf of Southern Thailand, represents a vulnerable tropical estuarine ecosystem subject to contamination from multiple pollution sources. This study aimed to quantify ten heavy metals; Iron (Fe), Manganese (Mn), Zinc (Zn), Cadmium (Cd), Lead (Pb), Chromium (Cr), Copper (Cu), Nickel (Ni), Arsenic (As), and Mercury (Hg) in Na Thap River surface water from 2017 to 2021, to analyze spatial and temporal distribution patterns. Surface water samples were collected from 10 sampling sites and analyzed using Inductively Coupled Plasma-Optical Emission Spectroscopy (ICP-OES) and Hydride Generation-Inductively Coupled Plasma (HG-ICP) methods. Exploratory data analysis was used to assess contamination levels and identify unsafe month-site combinations. Correlation analysis was used to measure the relationship among heavy metals. Principal component analysis (PCA) was employed to explore the variation of different heavy metals in different dimensions. Results revealed that several heavy metals exceeded Thailand’s surface water quality standards, with high toxic metals (Cd, Pb) and medium toxic metals (Cu, Ni) showing concerning levels across different sites and time periods. Specific unsafe zones and time periods were identified throughout the Na Thap River system, revealing critical contamination hotspots posing risks to ecosystem health. Temporal and month wise analyses indicated a clear seasonal pattern that contamination risk was concentrated in February, March, June, and July, with February showing unsafe conditions across all monitoring sites simultaneously. Elevated concentrations of Fe, Mn, Cd, and Pb were most frequently observed during the southwest monsoon period, suggesting that seasonal hydrological processes played an important role in controlling heavy-metal transport and accumulation throughout the river system. Spatial sites 2, 3, 5, 6, and 7 emerged as persistent contamination hotspots requiring immediate intervention. PCA identified distinct metal groupings with Dimension 1 (43.7% variance) dominated by Ni, Cu, Cr, and Cd, while Dimension 2 (17.2% variance) was characterized by Mn, Fe, and Pb, indicating different pollution sources. Spatially, recurring exceedances were concentrated at sites 2, 3, 5, 6, and 7, indicating priority locations for management action. These findings provide essential baseline data for developing targeted pollution control strategies to protect this vulnerable ecosystem and dependent communities.