Observations on Vertical Variability in Groundwater Quality: Implications for Aquifer Management

This work demonstrates significant vertical variability in the chemical composition of groundwater (Cl^- from 150 to 550 mg/L, NO₃^- from 3 to 70 mg/L, trichloroethene from 350 to 55,000 μg/L and Cr_Total from 3 to 2,900 μg/L) along a 130 m thick vertical section passing through two subaquifers of the Costal Plain aquifer of Israel. Water samples were obtained by multilevel sampler under natural gradient flow conditions from a monitoring well which penetrates the entire aquifer. The vertical chemical variability detected at a single point in time in this well was found to be similar to the range of concentrations detected for Cl^- in pumping wells located over an area of about 1,000 km² in the same aquifer. Similarly, vertical variations in NO₃^- concentration in the single monitoring well represent more than 50% of the total variation in nitrate levels in pumping wells sampled across the entire aquifer. These results graphically illustrate that data from pumping wells, which extract unknown mixtures of groundwater from different depths, cannot provide adequate information about groundwater quality for management purposes.