Identification and quantification of redox and pH buffering processes in a heterogeneous, low carbonate aquifer during managed aquifer recharge

Abstract

Managed aquifer recharge of aerobic water into deep aquifers often induces the oxidation of pyrite, which can lead to groundwater acidification and metal mobilization. As circumneutral pH is often maintained by the dissolution of sedimentary calcite or high injectant alkalinity little attention is generally paid to potential alternative pH buffering processes. In contrast, this study analyzed water quality evolution from a 2 year long groundwater replenishment trial in an anaerobic, mostly carbonate free aquifer. While injection of aerobic, very low salinity water triggered pyrite oxidation, the comprehensive field data showed that in many aquifer zones pH was buffered without substantial release of inorganic carbon. A numerical analysis was performed to test and evaluate different conceptual models and suggested that either proton buffering or the dissolution of aluminosilicates, or a combination thereof, can explain the observed, rapid buffering at locations where carbonates were absent. In contrast to many previous managed aquifer recharge [MAR) studies, the oxidation of sedimentary pyrite by nitrate was found to be of minor importance or negligible. The study also highlights that the depositional history of the aquifer, and the associated differences in mineralogy and geochemistry, need to be considered when estimating groundwater quality evolution during the injection of various water types for aquifer replenishment or other management purposes. ] describe sequential buffering during ongoing pyrite oxidation in a carbonate-bearing mine waste. After the initial consumption of primary and secondary carbonates (calcite and siderite), they observed the dissolution of previously precipitated gibbsite at pH 4.9-5.2, and subsequent ferrihydrite dissolution at pH < 4.5. Salmon and Malmstr€ om [2004] found that the pH of 4.9 in the groundwater of a carbonate-free mine waste deposit could largely be explained by a balance of acid generation by pyrite oxidation and acid consumption by chlorite and ferrihydrite dissolution.

Fluoride and phosphate release from carbonate-rich fluorapatite during managed aquifer recharge

“…This is consistent with its previous detection in fine-grained Leederville Formation sediments (Descourvieres et al, 2011). A number of other carbonate minerals were under-saturated (calcite median SI = -1.58, dolomite median SI = -2.88, ankerite median SI = -3.12) along with gypsum (median SI = -3.01), as discussed in more detail in Seibert et al (2016).…”

“…To obtain a good agreement between the simulated and observed fluoride and phosphate concentrations, it was important that the simulated pH closely matched the observed pH. The pH was well matched when a proton exchange reaction was included in the model, which agrees well with the previous study of Seibert et al (2016), who suggested that proton buffering was the main buffering mechanism at the study site. The best match between simulations and observations was obtained when a cation exchange capacity of 75 mmol/L was used.…”

“…Over the trial period, a total of 3.90 × 10 6 m 3 of this pre-treated wastewater was injected at an average injection rate of 2800 m Time lapse temperature and induction logging of the monitoring wells and subsequent solute and heat transport modelling demonstrated that flow and transport occurred preferentially in the interlayered sand beds (Seibert et al, 2014). Previous column experiments and reactive transport modelling identified pyrite oxidation as a key reaction that occurred when the injectant displaced the highly reducing NGW, particularly in the vicinity of the injection well (Descourvieres et al, 2010a;Descourvieres et al, 2010b;Seibert et al, 2016). Acidity generated from pyrite oxidation was found to be buffered by a combination of proton exchange on cation exchange sites and, to a lesser extent, the dissolution of trace carbonate minerals, ankerite and siderite, as well as dissolution of glauconite and chlorite (Seibert et al, 2016).…”

“…A comprehensive field-scale injection experiment was conducted between November 2010 and September 2014 to investigate the feasibility of recharging highly treated wastewater into the Cretaceous siliciclastic Leederville aquifer of the Perth Basin, Western Australia ( Figure 1) (Seibert et al, 2016;Seibert et al, 2014). The highly treated wastewater was injected through a single injection well screened between 123.7 and 224.4 m below ground level (bgl) ( Figure 2).…”

“…Previous column experiments and reactive transport modelling identified pyrite oxidation as a key reaction that occurred when the injectant displaced the highly reducing NGW, particularly in the vicinity of the injection well (Descourvieres et al, 2010a;Descourvieres et al, 2010b;Seibert et al, 2016). Acidity generated from pyrite oxidation was found to be buffered by a combination of proton exchange on cation exchange sites and, to a lesser extent, the dissolution of trace carbonate minerals, ankerite and siderite, as well as dissolution of glauconite and chlorite (Seibert et al, 2016). Fluoride and phosphate pulses were observed during breakthrough of the injected pre-treated wastewater.…”