Predominantly lithogenic chromium and nickel elevate sediment guideline quotients and exposure screening in a subtropical African water supply reservoir

Reservoir sediments accumulate potentially toxic elements from both catchment weathering and human activity, and concentration-based indices regularly confound the two. We evaluated this distinction in the Nandoni Reservoir, a water supply impoundment along the Luvuvhu River, South Africa. Surface sediment (0 to 5 cm) was collected using an Ekman grab at 21 sampling sitessite in October 2023, at the end of the dry season, and 26 elements were quantified by inductively coupled plasma mass spectrometry (ICP–MS). Chromium, nickel, copper and lead covaried strongly with aluminium, with element to aluminium ratio coefficients of variation of 0.36 or less. Aluminium normalised enrichment factors for Pb, Zn, Cd, As and Hg were at or below crustal proportions, whereas Cr, Cu and Ni were elevated relative to average shale. These patterns are consistent with predominantly lithogenic control, but do not by themselves establish source dominance and do not exclude diffuse anthropogenic inputs. Chromium exceeded the consensus probable effect concentration at 14 sampling sitesite and nickel at 13, and the mean probable effect concentration quotient ranged from 0.18 to 0.85. Exceedance of this kind indicates screening level concern rather than demonstrated ecological effect. Deterministic hazard indices were 0.07 to 0.34 for an adult maintenance worker and 0.36 to 1.67 for a child with incidental shoreline contact, exceeding one at nine sites for the child, falling to three sites when chromium was treated as Cr(III) rather than as the conservative upper bound Cr(VI). Replacing average shale with a pre–industrial reference inflated the pollution load index 4.0 fold, driven almost entirely by Ni, Cu and Cr. Source attribution, ecological screening and exposure assessment must be reported as separate lines of evidence.