IGRAC will present on 'Enhancing subsurface purification during Managed Aquifer Recharge (MAR)' at the UNESCO-IHP Regional Consultation Meeting 'Water Quality in Europe: Challenges and Best Practices', which will take place in Koblenz (Germany) from 1 to 4 December 2015 and will be hosted by the German Federal Institute for Hydrology - International Centre on Water Resources and Global Change, under the auspices of UNESCO. This event is organized by the International Initiative on Water Quality (IIWQ) in the framework of the implementation of Theme 3 “Addressing Water Scarcity and Quality” of the Eighth Phase of the International Hydrological Programme (IHP-VIII) of UNESCO.
'Enhancing subsurface purification during MAR'
Water quality deterioration due to interaction of infiltrated water with the aquifer is a common occurrence that increases the purification challenge during Managed Aquifer Recharge applications for drinking water production. During Aquifer Storage Recovery (ASR) for example, water quality deterioration may limit the extractable fraction of the stored drinking water (Pyne, 2005). Quality deterioration may be due to natural processes associated with the injection of oxygenated water into anoxic aquifer systems. Natural pollutants frequently associated with declining water quality are heavy metals, such as iron, manganese and arsenic. Concentrations above the aesthetic guidelines have been associated with unusual look, taste or smell of the water. Chronic exposure to manganese in drinking water above the WHO guideline of 0.4 mg/L may have neurological effects while long-term exposure to arsenic at concentrations above 50 μg/L can cause cancer and skin lesions.
This study explores the effects of reducing the reactivity of sediments around ASR wells by injection of strong electron acceptors to drastically reduce the availability of sedimentary electron donors during an aquifer pre-treatment step. The reactivity of sediments can be consumed in various ways, including the injection of oxygen-enriched water. As oxygen can achieve a maximum saturation of 5 times (100 % oxygen) compared with air, the use of “strong” electron acceptors for aquifer pre-treatment allows a significantly greater oxidation capacity per injected water volume, particularly for highly soluble ionic electron acceptors, such as permanganate (MnO4-) (Cavé et al., 2007). An additional advantage of permanganate for aquifer pre-treatment is the increase in the sorption capacity through the generation of manganese-oxide precipitates. Manganese-oxides are a by-product of the permanganate reaction with sedimentary electron donors such as organic matter and iron-sulphides. These precipitates may coat the aquifer minerals and could effectively sequester a range of heavy metals (Buamah et al. 2009) improving the subsurface in-situ purification and minimizing the post-treatment requirements of the extracted groundwater.
The effects of treating the aquifer prior to applying ASR were evaluated using bi-directional columns that simulate the evolution of the water quality in an anoxic aquifer (Antoniou et al 2014). Two air-tight columns were filled with reduced aquifer sediments and were saturated with anoxic groundwater obtained from the same aquifer. The presence of reactive mineral phases was confirmed by geochemical analyses of the sediments. Two diaphragm-metering pumps allowed the injection and extraction of water at a low flow rate. Collection of water samples was made possible via two ports, one at each side of the experimental setup (Figure 1).
Results and Discussion
Upon infiltration of oxygenated water in the anoxic columns, oxidation reactions with common sedimentary electron donors (iron-sulphides and organic matter) and the associated proton production triggered dissolution of carbonate minerals (ankerite) releasing substantial amounts of manganese in the groundwater. Simultaneously, arsenic was mobilised during the oxidation of iron-sulphides (pyrite) posing additional concerns regarding the quality of the extracted water. To neutralize the manganese production, which persistently compromised the extracted water quality after having extracted 15 % of the injected water, the aquifer sediment was treated with a 0.02 M potassium permanganate solution.
The extended oxidation reactions with sedimentary electron donors caused partial depletion and coating of their reactive surfaces (reactive organic matter and readily oxidisable iron-sulphides) along with an extensive precipitation of manganese-oxides. The high sorption capacity of these precipitates allowed a more efficient ASR application with an extractable ratio of 110 % (as opposed to 15 % prior to treatment) before encountering prohibitive manganese concentrations. Mixed results were observed for arsenic as the extended consumption of iron-sulphides was expected to reduce arsenic mobilisation while the increased pH induced by the reaction of permanganate with the sedimentary electron donors was expected to act against an efficient sorption of anions, including arsenic (Smedley and Kinniburgh, 2002).
The results suggest that the intense oxidation of sedimentary electron donors around ASR wells in combination with the resulting extended precipitation of manganese-oxides provide an efficient option for the removal of certain positively charged heavy metals (iron, manganese) from the groundwater before it is extracted. The relatively long travel times towards a well allow longer contact time for heavy metal removal than available during above-ground treatment. A field application of subsurface treatment should consider the potential negative effects on water quality and well performance. The permanganate solution, depending on its purity, might introduce certain trace elements in the aquifer. Such elements, although present in the permanganate solution introduced in the columns, were absent in the extracted groundwater confirming their efficient sorptive removal by the manganese-oxide precipitates.