A Novel Toxicity Index for Assessing Heavy Metal Risks in Freshwater Sediments: Application to Inle Lake, Myanmar (Indo-Burma biodiversity hotspot)

Abstract Metals in water and sediments of aquatic ecosystems pose significant ecological risks. However, existing methods to integrate and effectively communicate the overall toxicity risks of multiple metals are limited. This study introduces a toxicity factor (Tf) and toxicity index (TI) to comprehensively evaluate and communicate sediment metal pollution and associated risks to biota in a single figure. Furthermore, they are applied to Inle Lake, Myanmar, a crucial component of the Indo-Burma biodiversity hotspot, impacted by untreated sewage, uncontrolled waste disposal, agriculture, and artisanal mining. Sediment metal concentrations (mg/kg) decreased in the order Al (mean: 16,049) > Fe (11,191) > Mn (411) > Cr (34.7) > Zn (33.2) > Pb (22.4) > Ni (14.9) > As (9.69) > Cu (8.17) > Se (1.83). Contamination factor analysis indicated very high pollution by Al (6.99), considerable pollution by Cr (5.26), Pb (4.85), Ni (4.50), Cu (3.64), and Fe (3.09), and moderate pollution by Zn, Se, Mn, and As. The pollution load index (PLI) was highest (7.26) downstream of textile-weaving industries (site S1). Tf analysis identified possible toxicity risks for As, Cr, Ni, and Pb at points S1 and S5, while TI values (S1: 0.99; S5: 0.90) suggested these points were close to the threshold of possible toxicity. Acid-extractable fractions according to BCR-701 protocol revealed lower bioavailability of Cr despite its elevated total concentration, suggesting a primarily lithogenic source. Metal uptake by water hyacinth exhibited no direct correlation with sediment levels, emphasizing variability in metal bioavailability within the lake ecosystem.