Behaviour of chromium(VI) in stormwater soil infiltration systems

- We investigated the retention capacity of stormwater infiltration soils towards chromium(VI).
- Inlet flow rates mimicked normal and extreme rain events.
- Some retention of Cr(VI) was observed even though conditions did not favour it.
- Despite the retention the Cr(VI) concentration in the effluent still exceeds the groundwater quality criteria.

The ability of stormwater infiltration systems to retain Cr(VI) was tested by applying a synthetic stormwater runoff solution with a neutral pH and high Cr(VI) concentrations to four intact soil columns excavated from two roadside infiltration swales in Germany. Inlet flow rates mimicked normal (10 mm in 2 h) and extreme (100 mm in 3 h) rain events. The objectives were to understand the behaviour of the anionic and toxic Cr(VI) in soil at neutral pH and to asses treatment efficiency towards Cr(VI). During normal rain events Cr(VI) was largely retained (more than 50%), even though pH was neutral, while under extreme rain events approximately 20% of Cr(VI) was retained. In both cases effluent concentrations of Cr(VI) would exceed the threshold value of 3.4 μg/L if the infiltrated water were introduced to freshwater environments. More knowledge on the composition of the stormwater runoff, and especially the occurrence of Cr(VI), is needed.