Reconstruction of temporal variations of metal concentrations using radiochronology (^239+240Pu and ¹³⁷Cs) in sediments from Kizilirmak River, Turkey

Sediment cores retrieved from rivers, lakes, and coastal marine environment have been widely utilized to reconstruct historical variations of anthropogenic pollutants. A sediment core was collected in the Kizilirmak River, Turkey during 2014 and analyzed for a suite of metals (Cu, Pb, Zn, Ag, Ni, Co, Mn, As, Cd, Sb, V, Cr, Hg and Se) to reconstruct their temporal variations. Chronology was attempted using excess ²¹⁰Pb (²¹⁰Pb_xs), ¹³⁷Cs and ^239,240Pu in the sediment cores. The vertical profile of excess ²¹⁰Pb indicates that this core is not datable using excess ²¹⁰Pb method. The ¹³⁷Cs-based linear and mass apparent accumulation rates were estimated to be 0.84 cm year⁻¹ and 0.93 g cm⁻² year⁻¹, respectively. These values are in agreement with the ^239+240Pu peak-based linear (0.88 cm year⁻¹) and mass apparent accumulation (0.97 g cm⁻² year⁻¹) rates. There is considerable broadening in the peaks of both ¹³⁷Cs and ^239,240Pu, likely due to post-depositional mixing processes. The measured sediment inventories of ²¹⁰Pb_xs, ¹³⁷Cs and ^239+240Pu were 38, 29, and 0.42 dpm cm⁻², respectively, which were generally higher than the expected inventories based on local atmospheric depositional inventories. We attribute these enhanced inventories to potential sediment focusing and additional watershed erosional input. The Al-normalized enrichment factors (ANEF) were used to evaluate the contribution from natural versus anthropogenic sources. The ANEF of Ag, Ni, As, Cd and Se were > 1.5, which suggests a significant contribution of anthropogenic sources for these trace metals varied between 226% and 1163%. The geoaccumulation index (I_geo) values of the sediment column suggest that Cu, Pb, Zn, Co, Mn, Sb, V and Hg were in an unpolluted level and the others were in a polluted level. Besides, both the ANEF and I_geo values of Cu, Pb, Zn, Ag, As, Se, Cd and Hg showed a peak at 28–30 cm layer (corresponds to 1978–1981), which is attributed to maximum metal release during 1970s.