Abstract

The Water Quality Index (WQI) is an essential tool for monitoring trends, conducting scientific research, and making informed decisions. It offers a comprehensive view of the spatiotemporal variation in water quality in a watershed. Water quality data from various environmental assessment sources are often scattered and contain valuable information that researchers do not use. This study addresses the challenge of analyzing temporally and spatially dispersed monitoring data to assess water quality in riverine ecosystems over time. Our objective was to investigate fluctuations in water quality by constructing an aggregate water quality index, combining data from multiple public monitoring initiatives over 40 years in the Mediterranean region of central Chile, specifically in the Mataquito River Basin. Hydrochemical data were obtained from the measurement records available through the National Water Quality Monitoring Network of the Direccion General de Aguas of the Ministry of Public Works, integrating additional information from the water quality output station in the basin, which included sources from the Ministry of the Environment. Outliers for each parameter were removed and a data availability criterion was defined in relation to the total number of campaigns. Missing data were imputed to complete the matrix of water quality parameters. A total of 550 sampling campaigns provided 6624 water quality data points distributed across 10 chemical parameters and two chemical indicators, which were used to calculate the water quality index. The aggregation WQI water quality function was calculated using a modified additive method. Our research revealed that supplemented data offer valuable insights into evaluating water quality in the river ecosystems of the Andean watershed. In the lower zone, the water quality was determined to be good, while in the middle of the basin near the Andean Cordillera, it was considered acceptable. The analysis of the WQI over a decade indicated changes in water quality, irrespective of the station location in the watershed and the level of human intervention. The WQI values suggest a decrease in the water quality over the past 40 years. This study demonstrates that integrating information from diverse environmental monitoring sources into a WQI can be used to assess water quality trends and patterns in the Andean watersheds of Central Chile over time and space. These findings have significant implications for water management and decision making, providing a solid foundation for implementing appropriate strategies aimed at preserving water quality and ensuring water security in central Chile.