Uranium and plutonium in anoxic marine sediments of the Santiago River mouth (Eastern Pacific, Mexico)

Abstract

The uranium (U) and plutonium (Pu) content with depth in a sediment core collected in the continental shelf off the mouth of the Santiago River in the Mexican Pacific was studied to evaluate the contamination effects of the effluent of the Santiago-Lerma River as it moves into the sea. The large mass of terrestrial detritus delivered by the river influences the physicochemical and geochemical processes in the seafloor. Abnormal concentrations of U and Pu in sediments were examined as indicative of the effects of anoxic conditions. One of the indicators of pollution of seawater is the bacterial activity of the shallow seabed layer; and among the prevailing bacteria, the magnetotactic ones induce the formation of euhedral and framboidal shapes (pyrite). These pyrite entities are by-products of anoxic environments loaded with decomposing detrital material and are very abundant in the surface layers of the sediment core analyzed. The pyrite formation is the result of a biochemical reaction between iron and organic sulphur reduced by bacteria, and the pyrite entities precipitate to the seafloor. In the same upper zone of the profile, 238U is readily immobilized, while 234U is oxidized and dissolved in seawater by the effect of hot atom chemistry. This may cause the activity ratio (AR) 234U/238U disequilibrium (near 0.41). Furthermore, in the shallow layer of the sediment core, an abnormally high concentration of 239+240Pu was detected. In this upper layer, the activity concentrations found were 3.19 Bq kg−1 for 238U, 1.32 kg−1 for 234U and 2.78 Bq kg−1 for 239+240Pu. In the lower fractions of the sediment core, normal values of AR 234U/238U (≈1) were found, with traces of 239+240Pu.