Estimation of combined pollutant loads from diffuse and point sources discharged into river Rwizi and the associated environmental risks

Abstract

Surface water bodies experience significant water quality deterioration caused by nutrients, organic matter, heavy metals, pathogens from domestic and industrial wastewater effluents, agriculture and other anthropogenic activities. Estimating the relative contribution of pollution loads from diffuse and point sources and various hydrological pathways is one of the major challenges in catchment management. Understanding the transfer and estimation of pollutants discharged into water bodies is useful in designing and implementing policies that are helpful in effective management, capacity and policy needs of monitoring water resources. This study was therefore, carried out to estimate the combined pollutant loads of diffuse and point sources that are discharged into river Rwizi in Mbarara district in southwestern Uganda and the associated Environmental risks. Discharge and water quality measurements for both the river and factories were made over a period of twelve months. The Integrated environmental Impact and risk assessment method was used to determine the risks from pollutants discharged into the river. To characterise the industrial effluent and river water quality, 30 samples were collected from five factories and 60 samples from five sampling points during wet and dry seasons. The study applied statistical multivariate analysis to reveal the factors responsible for the deterioration of water quality. The principal component analysis (PCA) showed that river Rwizi water quality is largely influenced by human activities. The concentration of BOD5, COD and TSS from herbal and diary processing effluents (BOD5:185.2 ± 97.52 mg/L; 54.6 ± 15.04 mg/L, COD: 2719.8 ± 2341.01 mg/L, 539.4 ± 76.68 mg/L TSS; 387.8 ± 247.12 mg/L, 191.4 ± 11.33 mg/L respectively) were much higher than the provided discharge guidelines by the National standards of 50 mg/L,100 mg/L and 100 mg/L respectively (NEMA; 2020). The point sources contributed 5% of the total combined pollution loads, while diffuse sources account for 95% of pollutants that are discharged into the river. The results also showed that pollutant loads increased from upstream control point E1 to downstream control sampling point E5. There were higher values of TSS, 56.6 ± 15.3 mg/L, BOD5; 13.6 ± 6.3 mg/L and COD 99.5 ± 12.0 mg/L in the wet season than 44.4 ± 17.8 mg/L, 11.2 ± 5.1 mg/L and 74.8 ±18.7 mg/L respectively in dry season at the downstream sampling point E1. The results also showed that there are environmental risks posed on the river ecosystem by the pollutant loads with highest calculated environmental risk value of 120. Pollution loading into river Rwizi was highest in the highly urbanized section of the river increasing from sampling point E1 upstream to point E5 downstream by an average of 77.8%. There is a need for quantification of related public health risks and modelling of Pollution loads from diffuse sources. In addition, there is a need for enforcement of regulations to ensure effluents from industries comply with the national effluent discharge standards