Despite some researches indicating the possibility of correlation being induced by the common variable effect, correlation plots of ionic ratio (Na+/Cl−) versus ionic concentration (Cl−) still remain popular for interpreting the causes of groundwater salinization. There were doubts about relevance of spurious correlation in groundwater and its detection using the randomization process, owing to the fact that groundwater is charge-balanced and randomization would result in abnormal ionic ratios. In this context, the relevance of spurious correlation and its detection using randomization of common variable was established in this study, which was missing from the literature. The study used qualitative and quantitative tools for detecting the possibility of induced correlation and demonstrated the efficiency of the proposed method using published datasets from a variety of geochemical processes of groundwater salinization. In five out of the eight cases examined, the correlations observed in the plots appeared to be induced by the common variable effect and, as such, were deemed unreliable as positive indicators of the stated salinization processes. Even when the correlations appear not to be induced, it is recommended to always support the inferences with other independent evidence(s).
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The authors are grateful to all the researchers (refer Table 1), whose data were used to demonstrate the limitations of Na/Cl versus Cl common ion plot in different scenarios of groundwater salinization. Authors would like to thank the Heads of the respective Divisions for supporting this study. The authors sincerely appreciate the comments received from the anonymous reviewers which contributed to improving the scientific content and clarity of our presentation.
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Keesari, T., Dauji, S. Groundwater salinization processes: pitfalls of inferences from Na+/Cl− versus Cl− correlation plots. Environ Geochem Health (2020). https://doi.org/10.1007/s10653-020-00622-2
- Groundwater salinity
- Spurious correlation
- Na–Cl ratio plot
- Correlation coefficients