Abstract
This paper evaluates how Bolton Wastewater Treatment Works (WWTW) influences the physicochemical water quality of the River Irwell using an integrated multivariate statistical approach. Weekly grab samples were collected for four weeks at three sites: upstream control, effluent discharge, and downstream recovery. Parameters measured includes BOD5, dissolved oxygen, electrical conductivity, turbidity, pH, temperature, and flow conditions following APHA standard methods with strict QA/QC procedures. Primary data were complemented by Environment Agency records, operator discharge data, and CSO events. Data were standardized and analysed using descriptive statistics and multivariate methods including. The multivariate analysis showed distinct and statistically significant division of the upstream, effluent, and downstream areas of sampling (PERMANOVA pseudo-F = 4.19, p = 0.005). Factor Analysis revealed two latent factors of the most prominent result that explained 59.28 percent of the overall variance. Factor 1 was a great chemical gradient with high positive loadings of EC (+0.82) and negative loadings of DO ( -0.96) and pH ( -0.81), which is in line with the effects of treated wastewater effluent. A rather low loading of BOD5 (−0.26) demonstrated positive removal of biodegradable organic matter during the treatment process and a minor contribution of turbidity to this main gradient. Factor 2 was an independent process related to sediments and turbidity expressed the greatest positive loading to oxygen and organic pollution dynamics (+0.55). Biplot analysis showed clear zonal clustering: upstream samples were linked to oxygen-rich, low-conductivity conditions; and effluent samples to oxygen-depleted, high-conductivity conditions; and downstream samples occupied an intermediate position, indicating partial chemical recovery and sediment influence. Dissolved oxygen was the most sensitive indicator of effluent impact, with mean depletion at the effluent site exceeding 55%, while electrical conductivity showed a consistent downstream gradient. A weighted effluent impact index (EII) classified baseline, impacted, and recovering zones without upstream false positives or effluent false negatives. Comparison of primary and secondary data showed differences in BOD5 and pH but strong agreement in temperature, highlighting the need for harmonized monitoring standards.