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

Six sediment push-cores were collected at a coastal lagoon system affected by urban and agriculture wastes. The sediments were analyzed for Th-228, Th -230, Th-232, Pb-210, Ra-226, and Cs-137. Cs-137 activities were at backgro und level for all samples. The Pb-210(tot) activities found in the area var ied from 0.5 to 4.5 dpm g(-1) with Pb-210(sup) levels ranging between 1.2-1 .8 dpm g(-1). Cores CHI and EPC showed flat profiles depleted of Pb-210(xs) , indicating the absence of recent sedimentation. Core CAI shows a flat Pb- 210(xs), profile that seems to be bioturbated. Cores ERC and BRI show chaot ic profiles with layers totally depleted in Pb-210(xs), likely caused by re suspension triggered by storm conditions. High Th-228/Th-232 values observe d at core ERC suggest that the resuspension event occurred less than 10 yea rs 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.