Probabilistic approach to evaluation of metal(loid) fate during stormwater aquifer storage and recovery

The fate of metal(loid)s is a key consideration when recycling urban stormwater via the aquifer, due to their potential within urban stormwater from anthropogenic sources and also the potential for mobilisation from the aquifer sediments. A probabilistic modelling approach incorporating the use of probability density functions was applied to assess the capacity of a confined, anoxic limestone aquifer to treat or generate metal(loid)s. Iron was abundant in the aquifer storage zone and had a large influence on the fate of many trace metal(loid) species. Reductive iron(III) dissolution and the associated loss of sorption sites, along with displacement from sorption sites by phosphate present in stormwater, resulted in consistent gains of arsenic in the recovered water across four ASR operations. Lead and zinc were predominantly removed, aside from one scheme where the concentration in stormwater was considerably lower than the remaining schemes. Sorption of copper in preference to nickel explained the removal of copper and mobilisation of nickel in two ASR operations. The fate of aluminium was independent from that of iron, instead dictated by aqueous speciation and the insolubility of Al(OH)3 and consistent removal was evident. The results suggest that ASR operations can provide some level of treatment for metal(loid)s in stormwater, but the treatment provided can be site specific and is influenced by the stormwater quality itself and variability in the geochemical conditions experienced within the storage zone.