Depth-resolved seasonal assessment of pharmaceutical residues in sediments from Bloemfontein, South Africa

Abstract

Pharmaceutical residues are increasingly recognized as contaminants of emerging concern due to their persistence and potential ecological risks. This study investigated the occurrence and concentrations of azithromycin, prednisolone, dexamethasone, and prednisone in riverine sediments from Bloemfontein, South Africa, during summer (February 2024) and winter (May 2024). Sediment samples were collected in 5 cm depth increments (0–5 cm, 5–10 cm, and 10–15 cm) from the discharge point, upstream, and downstream zones, with seasonal differences observed across all depths. Solid-phase extraction was employed, followed by analysis using Liquid Chromatography–Mass Spectrometry (LC-MS). Chromatographic separation involved a C18 column under a gradient elution of acetonitrile and water, each spiked with 0.1% methanoic acid. Overall, higher concentrations of analytes were observed during summer compared to winter, except for azithromycin, which was higher in winter. The presence of pharmaceuticals in the deepest layers (10–15 cm) suggests earlier contamination, while detections in surface layers (0–5 cm) indicate more recent inputs. Dexamethasone exhibited the highest concentration across all sites in both seasons. Average summer concentrations were 11.30 ng/g (azithromycin), 23.01 ng/g (prednisolone), 40.13 ng/g (dexamethasone), and 7.69 ng/g (prednisone). In winter, prednisone was not detected, while azithromycin, prednisolone, and dexamethasone averaged 24.92 ng/g, 3.81 ng/g, and 4.91 ng/g, respectively. The detection of these pharmaceuticals across all sites indicates widespread contamination and potential for long-term environmental persistence. These findings raise concerns about ecological risks, including bioaccumulation and trophic transfer. The study underlines the urgent need for improved wastewater treatment and routine environmental monitoring.