Comparative spatiotemporal water quality dynamics in Tappan Lake using various Water Quality Indices

Freshwater ecosystems, such as Lakes and Reservoirs, have been facing escalating anthropogenic activities across the world, leading to the decline of lake water quality. Consequently, monitoring and evaluation of Lake water quality conditions are essential to ensure a sustainable water quality ecosystem. A comprehensive assessment of the Weighted Arithmetic-Water Quality Index (WA-WQI), Canadian Council of Ministers Water Quality Index (CCME-WQI), and Nemerow’s Pollution Index (NPI) was undertaken to investigate the spatiotemporal alternation in Tappan Lake’s aquatic wellbeing. Eleven monitoring stations were established across the lake to collect key parameters at every 1-meter depth intervals of each stations, including temperature (14.3–27.8 °C), pH (7.5–9.9), dissolved oxygen (0.45–14.8 mg/L), specific conductivity (~ 400–900 µS/cm), turbidity (2.7–15.9 NTU), blue-green algae (0.43–6.19 µg/L), chlorophyll-a (4.1–24.7 µg/L). While some sections of the Lake exhibited conformity with established water quality standards, others displayed significant deviations, particularly in terms of dissolved oxygen concentrations, Specific Conductivity, and elevated levels of blue-green algae. Seasonal trends and lake stratification were also identified, with water quality showing notable fluctuations between dry and wet seasons. WA-WQI consistently reported unfit throughout the entire stations, with mean values rising from 199.32 in spring to 497.34 in summer and peaking at 1521.21 in fall, reflecting severe cyanobacterial proliferation. The CCME-WQI revealed greater variability across the lake, such as fair in spring (66.23), good in summer (81.4), and fair in fall (67.9). In contrast, the NPI Index persistently reported “seriously polluted status” across various seasons of spring (1.94), summer (4.3), and fall (13). Similarly, BGA in August was as high as 20 times the WHO guideline of 0.3 µg/L. Nonetheless, all indices converge in diagnosing significant ecological stress in terms of seasonal stratification, upstream turbidity spikes, and recurrent harmful algal blooms. These Water Quality indices will provide a unified evaluation outlook by combining various water quality parameters into a comprehensive numerical assessment and suggest clear guidelines for the stakeholders engaging in the restoration and management of the Tappan Lake.