Water Geochemistry and Stable Isotope Changes Record Groundwater Mixing After a Regional Earthquake in Northeast India
Summary
WIVVI summary (paraphrased): Researchers monitored two bore wells in the Kopili Fault zone of northeast India, collecting 406 groundwater samples twice weekly between February 2021 and July 2023. They measured dissolved-element concentrations and stable hydrogen and oxygen isotope ratios to investigate responses to regional earthquakes. No unambiguous groundwater-chemistry changes were detected before the 2021 Assam magnitude 6.4 earthquake. Significant chemical and isotopic changes appeared afterward, including a transient anomaly at one well followed by longer-lasting changes at both wells. The authors interpret these results as evidence that earthquake-induced fracturing or rupture of a hydrological barrier allowed groundwater to mix with a new water source, potentially water associated with the Brahmaputra River.
Source
Geochemistry, Geophysics, Geosystems
Keywords
groundwater chemistry, stable isotopes, earthquake hydrology, hydrogeochemical anomolies, groundwater mixing, Kopili Fault zone, Assam earthquake, Northeast India
Source Type
research article
Key Findings
Twice-weekly monitoring detected no unambiguous groundwater-chemistry or stable-isotope changes before the 28 April 2021 Assam magnitude 6.4 earthquake. Significant changes appeared after the earthquake: a transient anomaly at one well within approximately two weeks, followed by longer-lasting changes at both wells. Multivariate analysis indicated mixing between groundwater and a newly introduced water source, potentially Brahmaputra River water, following earthquake-induced fracturing or breach of a hydrological barrier. The study therefore supports groundwater geochemistry as a recorder of coseismic and postseismic aquifer change, but not as a demonstrated earthquake precursor in this dataset.
Limitations
Monitoring began only about two months before the main earthquake, providing a short pre-earthquake baseline. Only two wells in one complex hydrological setting were studied, and one well was actively pumped by a commercial bottling plant. Lithological logs were unavailable for the monitored wells, and the proposed river-water mixing source is an interpretation rather than direct proof. Twice-weekly sampling could also miss short-lived signals. Results from one earthquake and one region cannot establish general forecasting performance.
Methods Summary
Researchers collected 406 groundwater samples twice weekly from two bore wells near the Kopili Fault zone between 18 February 2021 and July 2023. Samples were analyzed for major ions, trace elements, and stable hydrogen and oxygen isotope ratios. The team compared the hydrogeochemical time series with regional earthquakes, including the 28 April 2021 magnitude 6.4 Assam earthquake, and used multivariate statistical analyses, including principal component analysis, to evaluate compositional changes and possible mixing processes.