Concentration of Iron(II) in Fresh Groundwater Controlled by Siderite, Field Evidence

Abstract

Iron(II) concentrations in fresh groundwater in Dutch aquifers range from absent up to 50?mg/l. Evaluation of extensive chemical data sets learned that the maximum logarithmic concentration of iron(II) in aquifers, between?±?6.5?<?pH?<??±?8, is a linear function of pH, governed by Siderite. It is a broad relation due to oversaturation with respect to Siderite and to variation in alkalinity. Iron(II) is continuously supplied to groundwater by reduction of hydrous ferric oxides (HFO), until becoming saturated with respect to Siderite, and from then on, HFO reduction and Siderite precipitation occur simultaneously. In Dutch aquifers, the electron supply rate (equivalent to the organic matter oxidation rate) apparently exceeds the HFO electron uptake rate (equivalent to the HFO reduction rate) and the excess supply is taken up by sulfate (equivalent to the sulfate reduction rate): HFO reduction, sulfate reduction and FeS precipitation occurring simultaneously, where the presence of Siderite prevents a dip in the iron(II) concentration. After sulfate becomes exhausted, the excess electron supply is transferred to methane production: HFO reduction and methane production occurring simultaneously. This evaluation also demonstrated that the organic matter oxidation rate and the HFO reduction rate decrease over time. The results of this study are also relevant for the behavior of As and of Co, Ni and Zn in groundwater, as HFO, Pyrite and Siderite may contain variable contents of these elements.