Controlling the formation of the reaction zone around an injection well during subsurface iron removal

Abstract

Tracer and pump tests including depth dependent water sampling were performed to investigate the flow conditions inside and in the vicinity of an injection well with two screen segments used for subsurface iron removal (SIR). A high resolution groundwater flow model of the well and the adjacent aquifer with vertically varying dissolved iron concentration was calibrated and used to plan measures to manipulate the vertical outflow distribution of injected oxygen enriched water. The optimized injection regime was adopted in a pilot SIR test with the aim of increasing the treatment efficacy through a depth specific injection of water using an inflatable packer. When water was injected conventionally above the pump, the outward migration of the oxygen enriched water was non-uniform and disproportional to the iron concentration and resulted in an early iron breakthrough in the lower screen. The proportion of water injected into the lower iron-rich part of the aquifer increased as a packer was placed inside the well to seal 4/5 of the upper well screen length. Thereby, the efficiency coefficient increased by 50% and iron removal by 25%. The treatment efficiency at the site suffered from low alkalinity and pH-values below 5. Higher efficiency coefficients may have been achieved by the addition of alkalis prior to injection.