TREATMENT OF NATURAL SEDIMENTS AS OPTION TO INFLUENCE REDOX ZONING

Managed aquifer recharge is a technical measure to increase groundwater resources while utilizing natural processes to improve water quality. Redox conditions play a major role for degradation and transformation processes of dissolved compounds. Trace organic substances are attenuated differently within the different redox zones. The aim is to evaluate the possibility to influence redox conditions along the flow path for optimising degradation processes for a suite of different organic trace compounds.

Laboratory soil column experiments with surface water from Lake Tegel / Berlin / Germany and natural sediments from infiltration basins were conducted. The column was equilibrated with untreated surface water at the beginning, followed by the second phase using ozonated surface water, tracer and the pharmaceutical compounds carbamazepine, primidone (both anti-epileptic drugs), and sulfamethoxazole. The column was packed with natural sediment which has been used untreated, heated at 200°C, and heated at 550°C.

Results reveal that immediately after the start of the column experiments the concentration of oxygen dropped and likewise did the redox potential. Even ozonation of the influent water did not have an effect on the redox potential at the outflow of the column over the study time of two to three weeks. However, the sediment had a decisive effect on the redox potential. The natural sediment as well as the sediment treated at 200°C lead to strongly reducing redox conditions of about - 200 mV after three to four exchanged pore volumes. The sediment treated at 550°C also showed a decrease of oxygen but the redox potential stayed at + 200 mV which was much higher compared to the sediment heated at 200°C. The fraction of organic carbon in the natural sediment was around 0.0019, even for the sediment heated at 200°C. This temperature lead to an activation of the organic carbon and increased the reactivity. Heating up to 550°C lead to a decrease of organic carbon (f_OC 0.0006) and less reactivity.

The column experiments showed that the redox potential can be controlled using a set-up of the same sediments that have been heated at different temperatures. Residence time within the redox zone may be controlled by the thickness of the layers, producing an infiltration velocity that is adjusted to optimal removal of trace compounds.