**2.1. Soil sampling and irrigation**

Three soil samples were taken in an agricultural area, previously conditioned with the addition of organic matter in order to ensure a high content of organic matter in the soil. Sampling was performed vertically by inserting a PVC tube 10 cm in diameter at a depth of 50 cm at a random point in the area, so that three complete soil columns were obtained. The sampling and transfer was carried out hermetically to guarantee the inviolability of the sample until the arrival at the laboratory. The first profile was used to determine the physicochemical characteristics of the soil: pH, Ce, CEC, MOS, moisture, and texture.

The other two profiles were irrigated with wastewater from an electroplating industry (559.5 mg/L Cr VI and 20.1 mg/L de Cr III); the irrigation was carried out on a single occasion in order to saturate the soil with Cr. About 10-cm deep holes were successively drilled in the soil profiles until reaching a depth of 50 cm. A sample of the saturated solution was obtained from each of these holes to observe the decrease of the concentration of chromium in the saturated solution after crossing 10 cm of soil. The ORP, Cr VI concentration [14], and 3D fluorescence spectrum were determined in the collected solution samples.
