Standard Ion Index for Groundwater Quality Evolution

Abstract

Spatial and temporal groundwater quality variations present distinctive characteristics for the hydrochemical composition of any aquifer. Hydrochemical characteristics of groundwater quality can be depicted by the standard ion index (SII). This is equivalent to the statistical standardization procedure, which is robust, assumption free and a dimensionless parameter with zero mean and unit standard deviation. Its application is suggested for eight major ion compositions (Ca₂, Mg, Na, K, Cl₂, HCO₃, NO₃, and SO₄) in a given water sample. Individual and successive SII procedures are available, each with distinctive exposition of groundwater quality. Individual SII graphs expose standard ion values on the vertical ratio axis versus the nominal scale axis on the horizon in sequence of Ca₂, Mg, Na, K, Cl₂, HCO₃, NO₃, and SO₄ variables for each sample. Successive SII graphs indicate the scatter of standard major ions values of one sample versus the next sample values. Theoretically two samples with the same hydrochemical composition yield scatter points that fall on the 45° straight-line, which is referred to as the “ideal similarity line” (ISL). In the successive SII graph the minimum ion concentrations take place at the lower end and the maximum concentrations at the upper end of the same line. Other ion clusters are aligned around the line depending on the groundwater quality composition. The application of this methodology is presented for a set of groundwater samples taken from a major well near Jeddah, Kingdom of Saudi Arabia, during 2000–2005 periods.