How to infiltrate nutrient-rich water via a vadose zone well without compromising groundwater quality

Infiltration via Dry Wells/Vadose Zone Wells is a cost-efficient MAR-solution, commonly applied in developing countries. Even in Germany, the standard DWA-A 138 only allows wells to be used for MAR, if they are not screened in the saturated zone, as the degradation of contaminants in the unsaturated zone is considered to act as a barrier, protecting the groundwater. The biodegradation of organic pollutants below such wells, however, is closely related to the interaction with soil air and the subsequent supply with oxygen. Hydrus2D-studies show that the infiltrating water forms a cone shape below the well with much higher saturation in the center than at the margins. The higher water content goes hand in hand with an increase in hydraulic conductivity – thus: higher flow velocities – and a decrease in oxygen entry from the soil air. These are unsuitable conditions for aerobic biodegradation, which is the actual reason for the infiltration into the unsaturated zone.
In this study, we experimentally compared different scenarios of infiltration from a dry well, evaluating the effect of drying phases and air injection. Comparing water content, DOC- and NH4+-concentrations, intermittent infiltration, especially with longer drying periods, appears to improve biodegradation significantly. Air injection, even though it lead to a ponding of water close to the well, improved biodegradation even further. The scenarios without air injection were simulated with Hydrus2D, but it turned out that entrapped air and other effects cause a change in hydraulics that cannot be reflected by this Richards-based approach. All in all, the lab scale experiments indicate that an improvement of biodegradation below dry wells can be achieved by longer drying phases and, if feasible, by air injection.