Assessment of major ions in groundwater supplied to Monterrey metropolitan area, Mexico: quality assurance, technical analysis, and addenda.

Performing quality assurance checks on hydrochemical data prior to interpreting these data is important as shown in this case study. The informative hydrochemistry paper authored by Mora et al. (Environ Monit Assess 189:394–408, 2017. https://doi.org/10.1007/s10661-017-6096-y) presents a significant amount of hydrogeochemical data for 30 large-yield wells, interpretation (including advanced statistics), findings, and conclusions for the Monterrey metropolitan area (MMA), Mexico. The author concludes that the probable analytical inaccuracy (notable bias and imprecision) and data conversion errors contained in the Mora et al. (2017) paper do not appreciably impact negatively their hydrogeochemical findings and conclusions. The author performed the cation–anion balance (CAB) quality assurance (QA) check method and discovered systematic error (bias) ranging from − 20.8% (Well S1) to 16.5% (Well S9), with 14 well analyses associated with CAB errors ≥ 5.0%. Coupling this simple but effective QA check with other QA algorithms, the author determined for nine well major-ion analyses, HCO₃ concentrations were notably low, and 10 well analyses were identified as possessing anomalously low Ca concentrations. These QA check methods will enhance: (1) the accuracy and defensibility of analytical results, and (2) potentially the interpretation and findings of hydrochemical investigations. Additionally, this paper provides new hydrogeochemical evaluation, interpretation, and findings for the MMA, i.e., Na/Cl molar ratios ≤ 0.88 for six studied wells strongly suggest that the groundwater supplying these wells has been impacted by seawater intrusion most likely sourced by the Western Interior Seaway. At least four of these wells occur in an area underlain by marine sedimentary rocks. [ABSTRACT FROM AUTHOR]