Samples containing nitrate had higher microbial activity than those without it
Photo for representation: iStock Many states in India has started facing a water crisis. Surface and the groundwater sources are under stress. As urbanisation is increasing, so is the dependence on groundwater.
Over extraction and mismanagement of sewage has deteriorated the quality of the groundwater. A joint article published in Water Research journal in February 2024 by Korean Institute of Science and Technology and Korea University claimed to have found a solution on improvement of aquifer storage and improvement of the quality.
The work promotes a practice called aquifer storage and recovery (ASR), which, scientists say, can be a solution in this climate-risked world. The water from surface sources is injected to the local aquifer through recharge wells / shafts and is stored there for future use.
ASR has been piloted in some states of India, under the Managed Aquifer Recharge interventions, according to media reports. The states include Karnataka, Tamil Nadu, Maharashtra, Rajasthan and Telangana, which have done pilot work on ASR.
Other states are also assessing the feasibility of ASR at different scales. “As ASR emerges as a vital solution to address environmental concerns and escalating water demands, further investigations across varied ASR scenarios are imperative to fortify our strategies for effective water management,” said Nandakumaran P, former chairman, Central Groundwater Board.
According to a 2022 research report in Frontiers of Water journal, Nalanda and Rajgir districts of Bihar unveiled the feasibility of ASR with different types of aquifers and their characteristics.
In reality, if the surface water polluted with physical, chemical and organic matter is injected into the aquifer, it will deteriorate the groundwater quality and contaminate the whole aquifer system. The problem is severe when injected surface water has high concentrations of dissolved organic carbon (DOC). DOC is a fraction of the total organic matter in water. It has a very complex effect on the quality of water.
High concentrations of DOC can reduce the pH of water, making it acidic, and also can act as a carrier for other contaminants, including heavy metals. It also deteriorates the taste and odour of water, making it unfit for consumption.
Enhanced levels of DOC act as breeding ground for harmful microbial growth. If the groundwater high in DOC feeds the freshwater lakes and water bodies, it can result in eutrophication and deteriorating water quality.
The difference in concentration of DOC between the injected surface water and existing groundwater can deteriorate the overall water quality of the aquifer. Hence, DOC attenuation is a must to improve the water quality during ASR. During DOC attenuation process, the microbes present in the water allow for biodegradation, which reduces the concentration of organic carbon.
Dissimilatory Iron Reduction (DIR) facilitates DOC attenuation by providing iron as electron acceptor to the microbes / bacteria, which they use for their metabolism. DIR is a process in which iron compounds act as electron acceptors to microbes for their microbial activity, in which ferric iron (Fe3) is reduced to ferrous iron (Fe2).
The natural availability of iron compounds affects the DIR process. The higher bio-availability of crystalline iron hydroxides (goethite and hematite) improves the DIR process and vice versa. The crystalline iron compounds are less naturally available than the amorphous ones (Iron hydrites) in aquifer systems. In anoxic conditions where oxygen is not present for microbial activity, iron hydroxides are the compounds which the microbes use for their microbial activity. The entire process is important to improve the quality of water during ASR.
During the study, a 10-day batch experiment was conducted by the experts in anoxic conditions. The results from the experiment show that the presence of nitrate in the test samples not only enhances the DOC attenuation process, but also transforms the amorphous iron compounds into crystalline ones such as goethite and hematite, which are more important for DIR process during ASR.
According to the study report, “the samples containing nitrate had higher microbial activity than those without it. In the samples containing nitrate, a new genus of Nitrate-Dependent Ferrous Oxidation bacteria appeared in high abundance”.
In the anoxic DIR process, the microbes present in the water utilise nitrate as electron acceptors for their metabolism and microbial activity. The nitrate gets consumed by the microbes, reducing its concentration too. The microbes / bacteria perform their microbial activity to degrade the concentration of organic carbon in the water.
The report also highlighted that “the bio-chemical reactions during DIR in the presence of nitrate transforms the amorphous ferry-hydrite into crystalline ferry-oxides increasing the bio-availability of crystalline compounds, which are more important for DIR. This dual process, where nitrate and iron compounds act as electron acceptors for microbes, catalyses the process of degradation of organic carbon, improving the water quality”. The reduction of organic carbon will improve the overall quality of water, making it fit for consumption.
“Though nitrate is a contaminant and deteriorates the water quality, its availability is important to reduce the DOC at a faster rate. The comparison in tests with and without use of nitrate has shown that DIR without use of nitrate is slow and bears less results (7-8 per cent). Alternatively, the samples where nitrate was used, DOC attenuation results were achieved by 15-50 percent,” the authors concluded in the report.
The concept of ASR is a solution to the arising water problems. Since there is mix of surface and groundwater in the process, it becomes important to improve the water quality along with the quantity. The results from the study can be replicated at large scales in ASR for water security in vulnerable areas.
Sudhir Srivastava, senior scientist at Central Groundwater Board, supported the study saying, ”When reactor Dissolved Oxygen concentrations are low and nitrate is present, they may couple the oxidation of organic carbon compounds to CO2 with reduction of NO3− to N2 gas.”
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