Occurrence of toxic metals in river Rwizi surface water and efficacy of wastewater stabilization ponds (WSPs) in Mbarara City, Uganda

Abstract

Toxic metal concentrations and physical chemical properties of River Rwizi surface water were determined. Aluminium, antimony, arsenic, barium, beryllium, cadmium, chromium, cobalt, copper, iron, potassium, lead, manganese, molybdenum, nickel, selenium, silver, thallium, thorium, uranium, vanadium, and zinc, were quantified using Inductively coupled plasma optical emission spectroscopy (ICP-OES). Physical chemical parameters were measured in-situ using a HANA meter. Heavy metal pollution index (HMPI), Contamination Degree (CD) and Hazard index (HI) were determined by comparing measured concentrations and reference values. Efficacy of wastewater treatment was evaluated from metal concentrations at the inlet and outlet of the Wastewater Stabilization Ponds (WSP). Selenium (up to 10.35 µg/L), iron (up to 601.74 µg/L) and aluminium (up to 435.44 µg/L), were higher than the World Health Organization (WHO) recommendations of 10 µg/L, 300 µg/L and 100 µg/L respectively. The physical–chemical parameters met the drinking water standard, except for turbidity (up to 89.1 NTU) which was higher than the WHO recommendation of <5 NTU. HMPI was 19.5–140.7 and CD was−12.0–18.5 showing moderate to very high pollution. The HI was 0.084–0.804 for children and 0.036–0.379 for adults. For chronic exposure, HI was 0.008–0.069 for children and 0.016–0.163 for adults. The risk to human health exceeded the negligible threshold of HI≤0.1 for adults. The efficiency of removal of metals by WSPs was low (5.87–57.73%), therefore, better wastewater management methods are desirable for protection of human health.