Assessing coastal outfall impact on shallow enclosed bays water quality: Field and statistical analysis

Abstract

Shallow bays, valued for their environmental, economic, and social significance, face increasing threats from anthropogenic stressors, primarily wastewater discharge through coastal outfalls. The slow tidal currents in these bays hinder the effective dispersion of pollutants to deeper waters, exacerbating their impact. This study assesses the spatial effects of wastewater and brine discharges on Sulaibikhat Bay's water quality. The approach combines fieldwork, laboratory analysis, and statistical techniques to identify pollutant types, sources, and concentrations near outfalls, nearshore, and offshore areas. Throughout the study, high water temperatures were observed consistently, exceeding 30 °C in summer and 18.6 °C in winter near most outfalls, with a maximum deviation from the ambient water of 11.1 °C recorded in December. In addition to elevated electrical conductivity (EC) levels peaking at 60,700 μs/cm, these elevated temperatures significantly reduce dissolved oxygen levels and have been linked to fish kill events in the Bay. NH3-N, TSS, COD, BOD5, FC, and FS concentrations exceeded allowable limits in 56.4 %, 92.25 %, 40.30 %, 32.1 %, 70 %, and 63.36 % of samples, respectively, often reaching extreme levels. Also, the BOD5/COD ratios suggest the presence of non-biodegradable organic matter, mainly from industrial sewage, and the FC/FS levels implicate human and animal waste. These results indicate a mix of domestic, industrial, and commercial wastewater discharges and underscore the severe threat coastal outfalls pose to ecosystem health and water quality in Sulaibikhat Bay. Principal Component Analysis (PCA) and Cluster Analysis (CA) delineate two distinct environmental zones with varying characteristics and contamination levels, necessitating expanded monitoring. Finally, substantial field research, numerical modeling, and establishing a sustainable environmental management system are needed to protect this fragile ecosystem. Also, converting outfalls into offshore may mitigate the environmental status of the bay by enhancing pollutant dilution and dispersion.