Quantifying co-contaminant sediment risk using geostatistical entropy and pollution indices: A transferable model from Ghana’s Birim basin

This study presents a transferable methodological framework for assessing sediment co-contamination by microplastics and heavy metals in tropical river basins, using Ghana’s Birim River as a case model. The Birim River, an essential tropical basin for drinking water, agriculture, and biodiversity, is becoming an ever-growing threat due to artisanal mining and the mismanagement of plastic waste. Fourier Transform Infrared Spectroscopy and Atomic Absorption Spectroscopy were employed to identify polymer types and quantify ten metals. Geo-accumulation Index (Ige)o, Ecological Risk Index (ERI), and Pollution Load Index (PLI), Shannon entropy were applied. Microplastic concentrations ranged from 3.75 to 4.0 particles/10 g, predominantly polyethene (PE), increasing downstream. An average Wet and Dry season metal levels peaked at arsenic (As) (87.7 mg/kg), cadmium (Cd) (0.38 mg/kg), and mercury (Hg) (0.025 mg/kg), with Ecological Risk Index scores of 50.0, 60.0, and 45.0, respectively, producing a total Ecological Risk Index of 246.66. Igeo ranked arsenic as moderately to strongly polluted (2.00), while Principal Component Analysis Isolated Arsenic as a dominant outlier. Entropy (spatial variability 2.39) revealed contamination dominance by Copper, Lead, and Arsenic. In terms of space, the downstream part of the Birim River (designated as Zone C) had continued to be high-ecological risk, unlike Zone A (upstream, low risk) and Zone B (midstream, moderate risk). A novel Entropy-Weighted Risk Transfer Model (EWRTM) simulated pollutant migration from upstream to downstream, yielding a Downstream Burden Index (DBI) of 52.9, driven by arsenic (45.4 %), Cadmium (31.7 %), and Mercury (22.8 %). In sediments, arsenic (87.7mg/kg), cadmium (0.38mg/kg), and mercury (0.025mg/kg) were detected, which gives a total Ecological Risk Index (ERI) of 246.66. Igeo rated arsenic as moderately strongly polluted (2.0), with entropy ( H = 2.39) suggesting that arsenic, copper and lead predominated in pollution. The study recommends tiered remediation and policy action by the Environmental Protection Agency -Ghana, the Water Resources Commission, and the district assemblies. This paper is among the first to combine FTIR-based microplastic profiling with AAS/ICP-MS heavy metal analysis in Ghanaian river sediments, as well as introduce the Entropy-Weighted Risk Transfer Model (EWRTM). This new framework is further than the single-contaminant analysis, as it is an instrument that can be transferred to map the risks of co-contaminants in basins affected by tropical mining.