Subsampling of Regional-Scale Database for improving Multivariate Analysis Interpretation of Groundwater Chemical Evolution and Ion Sources

Abstract

: Multivariate statistics are widely and routinely used in the field of hydrogeochemistry. Trace elements, for which numerous samples show concentrations below the detection limit (censored data from a truncated dataset), are removed from the dataset in the multivariate treatment. This study now proposes an approach that consists of avoiding the truncation of the dataset of some critical elements, such as those recognized as sensitive elements regarding human health (fluoride, iron, and manganese). The method aims to reduce the dataset to increase the statistical representativeness of critical elements. This method allows a robust statistical comparison between a regional comprehensive dataset and a subset of this regional database. The results from hierarchical Cluster analysis (HCA) and principal component analysis (PCA) were generated and compared with results from the whole dataset. The proposed approach allowed for improvement in the understanding of the chemical evolution pathways of groundwater. Samples from the subset belong to the same flow line from a statistical point of view, and other samples from the database can then be compared with the samples of the subset and discussed according to their stage of evolution. The results obtained after the introduction of fluoride in the multivariate treatment suggest that dissolved fluoride can be gained either from the interaction of groundwater with marine clays or from the interaction of groundwater with Precambrian bedrock aquifers. The results partly explain why the groundwater chemical background of the region is relatively high in fluoride contents, resulting in frequent excess in regards to drinking water standards.

Investigating the Potential Role of Geological Context on Groundwater Quality: A Case Study of the Grenville and St. Lawrence Platform Geological Provinces in Quebec, Canada

“…where Z i is the standard score of sample i, x i is the value of sample i, x is the mean, and s is the standard deviation [32]. Elements used in multivariate statistical methods should have a detection ratio greater than 75% [14]. A total of 10 elements (Ca 2+ , Mg 2+ , Na + , K + , Ba 2+ , SiO 2 , Sr 2+ , HCO 3 − , Cl − , SO 4 2− ) respected the 75% threshold and were used in the multivariate analysis.…”

“…Groundwater chemical evolution in the Grenville Province and St. Lawrence Platform can be discerned using Ca 2+ vs. Na + log-log plots and Piper diagrams [14]. The salinization of groundwater because of the dissolution of salt minerals or mixing with seawater causes groundwater to evolve along a salinization path [13].…”

“…Fluorite (CaF 2 ) is the most common mineral containing fluorine. Fluoride concentrations increase when fluorite precipitation leads to calcium-poor groundwater or when Ca 2+ is exchanged with Na + leading to a decreased Ca 2+ content in groundwater [14,58]. Fluoride concentrations above the 1.5 mg/L thresholds are mainly associated with wells in the Eastern Mauricie region within the Grenville Province, in locations characterized by a lack of dissolved oxygen, basic pH (7.4-8.8), deep bedrock aquifers (60-170 m depth), and Na-HCO 3 and Na-Cl water types.…”

“…The second involves the Ca 2+ /Na + cation exchange and mixing with the trapped fossil pore seawater originating from the incursion of the Champlain Sea in southern Quebec; the result is highly saline groundwater (Na-Cl in composition). Multiple investigations of the groundwater chemistry of the Canadian Shield belonging to the Grenville Province identified silicate weathering and Ca 2+ /Na + cation exchange as the main processes governing regional groundwater chemistry [7][8][9]13,14]. Montcoudiol et al demonstrated that water from the postglacial Champlain Sea continued to influence groundwater quality at the junction between the St. Lawrence Platform and the Grenville Province [9].…”

Hydrogeochemical and Hydrodynamic Assessment of Tirnavos Basin, Central Greece

“…Essentially, hydrogeochemical and hydrodynamic information needs to be dealt with holistically, with special emphasis put on their interactions. The methodological approach of the present work dictated the synergetic and combinational consideration of various tools and methods, e.g., classic hydrogeochemical approaches, the use of bivariate plots and/or molar ratios [2][3][4][5][6], hydrodynamic characterizations [7][8][9], hydrogeochemical modelling [10][11][12], multivariate statistics [13][14][15][16][17][18] and environmental isotopes [19][20][21][22][23].…”