Trace metal behavior during in-situ iron removal tests in Leuven, Belgium

Subsurface iron removal (SIR) is an in-situ technique to lower the iron content of extracted groundwater. Through cyclic injection of oxygenated water ferrous iron oxidises and precipitates as iron hydroxide in a zone surrounding the extraction well, enhancing the sorptive capacity of the aquifer. During subsequent pumping phases, groundwater traverses the oxidation zone and ferrous iron sorbs to available and newly formed exchange and sorption sites, thereby retarding the breakthrough of dissolved iron. The process is well-understood in regards to the retardation of iron. Less well understood, however, is the behavior of a number of trace metals and metalloids during SIR operations, foremost arsenic (As). In this study, we analyse major and minor ion and trace metal concentrations from a number of SIR tests in a sand aquifer near Leuven, Belgium. We use reactive transport modelling to evaluate conceptual models of trace metal release and arrest. The test data, underpinned by model results, show that metal release, namely arsenic and barium (Ba), occurs through the oxidation of trace amounts of sulphide minerals during the injection phase. Sorption through cation exchange retards Ba while complexation lowers dissolved As concentrations. Arsenic is mobilized again during the pumping phase through varying phosphate concentrations in the native groundwater, despite available sorption surfaces, while Ba remains adsorbed. Concentrations, however, remain below WHO guideline values for As and Ba (10 μg/l and 0.7 mg/l), respectively. The developed conceptual model of As fate reveals a high propensity for As mobility during SIR due to desorption reactions and delivers an explanation as to why many SIR operations fail to show substantial As removal, despite efficient iron removal. Other monitored trace elements showed no mobilisation, including Zn, Al, Cd, Cr, Cu, F, Hg, Ni, Pb, Sb and Se.