Spatial-temporal variability of metals and organic carbon in the upper awash and implications for TOC removal at Koka Water Treatment Plant (Ethiopia).

This study assessed spatial and temporal variations of metals and TOC in the Upper Awash Basin and their effects on TOC removal efficiency at the Koka Water Treatment Plant. Water samples were collected from river sites and the treatment plant influent during three dry and three wet months (May 2023-September 2024) to assess seasonal and spatial variations in water quality. Metal concentrations were quantified using inductively coupled plasma optical emission spectrometry (ICP-OES), while TOC was measured using a TOC analyzer. Spatial distribution patterns and pollution hotspots were identified using Ordinary Kriging in QGIS. Average total measured metal concentrations and influent TOC were approximately 1,546 μg/L and 29.63 mg/L, respectively, yielding a mean TOC removal efficiency of 57.52%. At the Koka Water Treatment Plant, higher concentrations of total measured metals and influent TOC were associated with lower TOC removal efficiency, indicating that elevated contaminant loading may adversely affect treatment performance. Dry-season concentrations generally exceeded wet-season levels. Spatial analysis identified the Akaki and Mojo Rivers as major metal hotspots, while Koka Lake and upstream hydropower reservoirs were key contributors to TOC. These findings highlight critical source areas that require targeted watershed management interventions to sustain drinking water treatment performance.