Applying high-frequency surrogate measurements and a wavelet-ANN model to provide early warnings of rapid surface water quality anomalies

• Published in: The Science of the total environment Vol. 610-611; pp. 1390 - 1399
• Main Authors: Shi, Bin, Wang, Peng, Jiang, Jiping, Liu, Rentao
• Format: Journal Article
• Language: English
• Published: Netherlands Elsevier B.V 01.01.2018
• Subjects: Anomaly detection; aquatic environment; Back-propagation neural networks; case studies; chemical oxygen demand; management systems; monitoring; neural networks; Potomac River; prediction; storms; surface water; Surrogate parameters; time series analysis; Virginia; water management; Water quality; watershed management; watersheds; wavelet; Wavelet denoising; Potomac River; Virginia; Back-propagation neural networks; Surrogate parameters; Anomaly detection; Wavelet denoising; Water quality
• ISSN: 0048-9697; 1879-1026; 1879-1026
• DOI: 10.1016/j.scitotenv.2017.08.232

It is critical for surface water management systems to provide early warnings of abrupt, large variations in water quality, which likely indicate the occurrence of spill incidents. In this study, a combined approach integrating a wavelet artificial neural network (wavelet-ANN) model and high-frequency surrogate measurements is proposed as a method of water quality anomaly detection and warning provision. High-frequency time series of major water quality indexes (TN, TP, COD, etc.) were produced via a regression-based surrogate model. After wavelet decomposition and denoising, a low-frequency signal was imported into a back-propagation neural network for one-step prediction to identify the major features of water quality variations. The precisely trained site-specific wavelet-ANN outputs the time series of residual errors. A warning is triggered when the actual residual error exceeds a given threshold, i.e., baseline pattern, estimated based on long-term water quality variations. A case study based on the monitoring program applied to the Potomac River Basin in Virginia, USA, was conducted. The integrated approach successfully identified two anomaly events of TP variations at a 15-minute scale from high-frequency online sensors. A storm event and point source inputs likely accounted for these events. The results show that the wavelet-ANN model is slightly more accurate than the ANN for high-frequency surface water quality prediction, and it meets the requirements of anomaly detection. Analyses of the performance at different stations and over different periods illustrated the stability of the proposed method. By combining monitoring instruments and surrogate measures, the presented approach can support timely anomaly identification and be applied to urban aquatic environments for watershed management. [Display omitted] •Realized rapid surface water quality anomaly warnings using a data-driven approach•Wavelet, ANN, high-frequency monitoring, and surrogate methods were combined.•Anomaly thresholds for logistically distributed prediction residual error were found.•Verified via anomaly events from the Potomac River monitoring program•The method can be applied for urban aquatic (sponge city) and watershed management.