Partition Coefficients and Metals Quality Index of Calabar and Great Kwa Rivers, Niger Delta, Nigeria

Partitioning of toxic metals between suspended particulate matter (SPM) phase and dissolved phase in Calabar and Great Kwa rivers was investigated from October 2021 to August 2022, to determine the metals' mobility, bioavailability and potential toxicity. Metals in surface water were differentiated between dissolve and particulate matter forms, based on whether or not they passed through 0.45µm filters. Total and dissolved metals concentrations were determined using Atomic Absorption Spectrophotometer (Shemadzu, model 6800, Japan) after wetting. Total metal concentrations ranged from 0.212 mg/l - 0.494 mg/kg, 0.011-0.053 mg/kg, 0.004-0.008 mg/kg, and 0.014-0.049 mg/l for lead, cadmium, mercury and arsenic, respectively. The total metals concentrations of both rivers were above Nigerian Drinking Water Quality Standards (NDWQS) and World Health Organisation guidelines for drinking water quality, with mercury being the only exception. The percentage of lead, cadmium, mercury and arsenic concentrations in dissolved phase ranged from 14.80 – 23.54%, 8.88 - 20.00%, 16.67 – 20.00% and 6.45 – 17.24%, respectively. The total, dissolved and SPM concentrations of all the metals were significantly higher (P≤ 0.05) in Calabar River than the Great Kwa River. Heavy Metal Pollution index (HMPI) indicated that, both rivers are not fit for drinking and the status corresponds to high contamination. Partition coefficients revealed that cadmium and arsenic displayed affinity to bind and remain preferentially in suspended particulate matter phase, while lead and mercury showed tendency to be more easily mobilized from suspended particulate phase toward the dissolved phase. The study concluded that, studied metals were predominant in suspended particulate matter phase. Both rivers are not fit for domestic and agricultural purposes; however, filtering the water before application could reduce human health risk significantly. Further investigations into the chemical speciation of the metals in the two rivers is hereby recommended