Anion Exchange in Saprolite

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Many studies have shown that subsoils in the Piedmont region of the southeastern USA have anion exchange capacity (AEC) and can adsorb anions such as nitrate NO−3 Groundwater discharging to wells and streams often passes through thick layers of saprolite beneath Piedmont soils, but little is known about the AEC of saprolite. Our objective was to determine the AEC on saprolite samples collected with a split‐spoon sampler during the installation of seven groundwater monitoring wells in north Georgia. Forty four samples were collected from depths ranging from approximately one to 15 m below the surface and analyzed for AEC, extractable iron, and pH. The saprolite thickness was at least 15 m at all seven sites as verified by drilling an adjacent 7.6‐cm hole to bedrock. The average AEC for 37 saprolite samples was 0.80 cmolc kg−1 This was slightly less than half of the AEC observed in typical subsoil horizons of Piedmont soils. There was a significant positive correlation (r = 0.64) between saprolite AEC and iron oxide content. Taking into account the differences in thickness of the soil and saprolite, saprolite had about six times the AEC of subsoil. The expected adsorption coefficient (Kd) for NO−3 in saprolite was approximately 0.81 cm3 g−1, based on the relationship between AEC and Kd Using an estimated bulk density and saturated water content based on texture, this corresponds to a retardation coefficient (R) of 4.4. Models and remediation efforts involving NO−3 in shallow Piedmont groundwater need to account for anion adsorption in saprolite.