Isotopic constraints (²¹⁰Pb, ²²⁸Th) on the sedimentary dynamics of contaminated sediments from a subtropical coastal lagoon (NW Mexico)

Abstract.

Six sediment push-cores were collected at a coastal lagoon system affected by urban and agriculture wastes. The sediments were analyzed for ²²⁸Th, ²³⁰Th, ²³²Th, ²¹⁰Pb, ²²⁶Ra, and ¹³⁷Cs. ¹³⁷Cs activities were at background level for all samples. The ²¹⁰Pb_tot activities found in the area varied from 0.5 to 4.5 dpm g^–1 with ²¹⁰Pb_sup levels ranging between 1.2–1.8 dpm g^–1. Cores CHI and EPC showed flat profiles depleted of ²¹⁰Pb_xs, indicating the absence of recent sedimentation. Core CAI shows a flat ²¹⁰Pb_xs profile that seems to be bioturbated. Cores ERC and BRI show chaotic profiles with layers totally depleted in ²¹⁰Pb_xs, likely caused by resuspension triggered by storm conditions. High ²²⁸Th/²³²Th values observed at core ERC suggest that the resuspension event occurred less than 10 years ago. The contaminated sediment of the lagoon are frequently resuspended, re-oxygenated, and therefore the contaminating trace metals will continue to be easily remobilized in the food chain.