**3.4. Irrigation with wastewater**

It was observed that the concentration of chromium (VI) in the wastewater decreased along the soil column. **Table 4** shows the concentration of chromium (VI), the oxidation–reduction potential and pH along the soil column. The samples show a direct effect of dissolved organic matter on the soil conditions that facilitate the accumulation of chromium. The irrigation results show that dissolved organic matter improved the soil reduction conditions, which promoted the accumulation by precipitation of chromium species. The soil reduction conditions are represented by the ORP values in the soil-saturated solution at the outlet of the soil column during direct irrigation.

The initial ORP value of the irrigation water was 274.9 mV; it decreased by up to 66% with each additional 10 cm of depth. The pH of the irrigation water changed from 3.4 to 5.6. It should be mentioned that the wastewater used for irrigation stayed in the soil column for 15 min, which shows that the soil had good drainage.
