Reconstruction of metal(loid)s pollution history in sediments of Wami Estuary, Indian Ocean Coast of Tanzania

Abstract

Results of metal(loid)s pollution history, their enrichment and contamination factors in sediments of Wami River Estuary, Tanzania are presented. Two sediment cores (WE1 and WE2) were retrieved from the Estuary using hand-driven PVC coring tubes. Levels of metal(loid)s in sediment samples were determined using EDXRF spectrometer coupled with X-lab ProTM software. Metal(loid)s enrichment factors (EF) were determined using metal(loid)s baseline values established during this study. Contamination factors were determined by dividing measured metal(lod)s concentrations in the samples by their respective baseline concentrations. Sediment physical chemical parameters were determined in order to establish their relationship with metal(loid)s concentrations. Sediment chronologies were established using the Constant Initial Concentration (CIC) lead-210 sediment-dating model in order to convert sediment depth profile into timescales. The lowest concentrations of As in sediments was measured in sediments deposited during 1960s, followed by increase reaching the highest concentrations in sediments deposited between ca. 2015 and 2017. Lowest and highest concentrations of Cd were measured in 1960’s and ca. 2000, respectively. Cu and Zn had their maximum concentrations in ca. 1971 and their significant positive correlation (r = 0.94, p = 0.01) suggest common origin of these metals. Average metals enrichment factors in the core were established and are in the order As = Cd = Cu = Pb = Zn = 0.1. This EF value suggests that all metals in sediments of Wami originate from natural sources (EF < 1). Metal(loid)s contamination factors (CF) were low to moderate for As, Cu and Zn; moderate for Pb; and low to considerable for Cd. CIC model revealed that the sediment at the bottom of cores WE1 and WE2 date back about 33 and 55 years, respectively. Sediments of Wami Estuary is characterized by mixing as evidenced in the established chronologies. Therefore, reported radiometric dates should be considered as a preliminary chronological estimate and further research is needed focused on terrestrial records of anthropogenic activities.