A Satellite-based Approach to Investigating Eutrophication in Lakes Receiving Wastewater Treatment Effluent: A Case Study of Lake Windermere

Excessive phosphorus from the sewer systems in the UK is a major contributor to eutrophication in freshwater bodies. Chlorophyll-a levels are widely used as a reliable indicator of phosphorus driven eutrophication, and satellite remote sensing provides an effective means of monitoring these dynamics at high spatial and temporal resolution. This study evaluated the utility of Sentinel-2 imagery in capturing the dynamics of eutrophication in Lake Windermere from 2017 to 2022. The Normalized Difference Chlorophyll Index (NDCI), calculated from the Sentinel-2 satellite imagery of the lake, was employed as a predictor for chlorophyll-a concentration. Statistical analysis revealed no significant temporal trend in NDCI values during the time period, indicating persistent eutrophication. Spatially, the southern parts of the lake showed elevated NDCI values, driven by the higher retention time and the presence of several treated sewage effluents along the lake’s length. Notably, the NDCI values were positively correlated with the phosphorus content in the treated effluent from Ambleside WwTW, while the storm overflow showed a negative correlation, attributed to dilution effects from rainwater entering the combined sewer system.