Passage VII

In a particular estuary (an inlet where ocean water and river water mix), the sediment contains the radioactive isotopes lead-210 (²¹⁰Pb) and cesium-137 (¹³⁷Cs). The isotopes are components of particles that fall from the air into the estuary. Unlike ²¹⁰Pb, ¹³⁷Cs is produced only by human activities. It has been detectable in the estuary sediment since 1954, the year it was first found in Earth's atmosphere. In 2003, 2 studies examined ²¹⁰Pb and ¹³⁷Cs concentrations in the estuary sediment.

At each of 2 sites (Site 1 and Site 2), 4 cylindrical vertical cores of the sediment, each 5.0 cm in diameter and 40 cm in length, were collected. Each core's sediment was sampled at the top and then every 2 cm down the core's length. The Site 1 samples from the same depth in each core were combined, and the Site 2 samples from the same depth in each core were combined.

Study 1

A portion of each combined sample was analyzed to determine the ²¹⁰Pb concentration in becquerels per kilogram of sediment (Bq/kg). (One becquerel is equivalent to 1 nuclear decay per second.) The results are shown in Figure 1.

Study 2

Another portion of each combined sample was analyzed to determine the ¹³⁷Cs concentration (see Figure 2). The ¹³⁷Cs data indicated the total thick ness of sediment that had been deposited at each site from 1954 to 2003. Then, the average sediment accumulation rate over that period was calculated for each site. The Site 1 rate was 0.41 cm/yr; the Site 2 rate was 0.37 cm/yr.

Figures adapted from M. C. Freitas et al., "Anthropogenic Influence in the Sado Estuary (Portugal): A Geochemical Approach." @2008 by Journal of lberian Geology.