Organic matter characterization of sediments in two river beaches from northern Portugal for forensic application

In a forensic investigation, the analysis of earth materials such as sediments and soils have been used as evidence at a court of law, relying on the study of properties such as color, particle size distribution and mineral identification, among others. In addition, the analysis of the organic composition of sediments and soils is of particular value, since these can be used as complementary independent evidence to the inorganic component. To investigate the usefulness of organic indicators in sediment characterization and discrimination, seventy-seven samples were collected during a period of one year in two river beaches located at the southern bank of the Douro River estuary in the North of Portugal. Isotopes of total carbon, pollen and plant wax-marker analyses were performed. In both beaches, an increase of the organic matter concentrations was noticeable, moving landward, related with the higher cover of associated plant material. The results obtained showed that the combination of all the techniques adopted showed a clear discrimination between samples from the two beaches, and also showed a differentiation of samples in relation to distance from the river in both beaches. The results also show that seasonality in these beaches was not a determining factor for discrimination, at the times considered. In addition, the effects of time was not marked.