Sample collection was performed in two sites of the study area,identified as sites A and B (Fig. 1). Samples of soil and of different types of lettuce were collected in 2013 and stored in plastic bags for posterior analysis. All samples were taken to the Environmental Analysis Laboratory of the Northeastern Brazilian Regional Center for Nuclear Sciences/Brazilian Nuclear Energy Commission (CRCN-NE/CNEN) for processing and analysis. Forty centimeters long soil cores were collected randomly using a 7 cm-diameter, 50 cm-long stainless steel corer with 10 samples per site, which were subsequently sectioned into two layers of 20 cm each (0–20 cm; 20–40 cm), totaling 20 samples per site. All samples of the first layer of site A and of the first layer of site B were then mixed and macerated to a fine particle size so as to obtain two composite samples of the first layer of soil. The same procedure was performed for the samples of the second layer of both sites