Comparison of Dissolved and Acid-Extractable Metal Concentrations of Ground Water, Eastern Arkansas, USA

Abstract

The objective of this project was to compare dissolved and acid-extractable metal concentrations in ground water in the alluvial and Sparta aquifers in eastern Arkansas, USA. Typically, water samples for metal analyses are filtered through a 0.45 μm pore-size membrane immediately after sample collection, yielding "dissolved" concentrations. In some situations, acid-extractable metal concentrations (unfiltered samples but acidified) may better represent potential metal "availability", e.g., from cation exchange or from dissolution of Fe hydroxyoxides. For this project 41 wells were sampled in the Mississippi River Valley Alluvial and Sparta aquifers. The paired student-T test was used to determine differences in the dissolved and acid-extractable concentrations for Fe, Mn, Cu, Pb, Zn, Ca, Mg, Na and K. The test indicated that there are significant differences between dissolved and acid-extractable
concentrations for Fe, Mn, Pb and Ca at the 95% confidence level. The test indicated no differences for Mg, K, Na, Cu, and Zn (confidence level < 95%). In most cases the difference ratios for dissolved and acid-extractable concentrations were less than 25% but in about one third of samples the difference ratios were greater than 90%. Highly turbid samples had the highest difference ratios. The results indicate that acid-extractable concentrations can be important in determining metal availability for Mn, Pb, Ca and, especially for Fe. Fe concentrations show spatial variation that is related to variations in lithology, ground water flow and/or geochemical processes.

Keywords: Acid-extractable concentration, alluvial, cation concentrations, dissolved concentration, eastern Arkansas USA, field Measurements, flow modeling, ground water, heavy metals velocity, overbank flow, Sparta aquifer, spatial variation, turbulence