METAL CHEMISTRY OF SPRING SEDIMENTS FROM FORT CAMPBELL, KENTUCKY/TENNESSEE

Clastic sediments deposited in karst springs can act as reservoirs for the storage of metals. The metal concentrations and the chemical speciation of the metals depend on the source material, the sediment’s physical and chemical properties, and the ability of the spring to accumulation sediments. Deposited (bed) sediments were sampled from six springs in the karst aquifer at Fort Campbell, Kentucky/Tennessee. The bulk samples were analyzed for physical characteristics (grain size, mineralogy, surface chemistry) and total metals (via digestion). Baseline and storm-event water samples were also digested and analyzed to determine metal concentrations in the mobile suspended sediments. The total concentrations of metals in the bed sediments can be related to the metal concentrations in the suspended sediments via a power law.

The bed sediment samples were also subjected to a selective extraction procedure which separates the metals into five operationally-defined fractions: exchangeable, carbonate, oxide, organic, and residual. Each extraction fraction was analyzed using Inductively Coupled Plasma – Mass Spectroscopy (ICP-MS). Some metals are consistently present in a single fraction regardless of the spring location and characteristics, e.g., aluminum in the residual fraction. Other metals, such as calcium, split between exchangeable, carbonate and residual at all locations. While iron is mostly in the residual fraction in the bed sediments, it has an appreciable distribution in oxide and carbonate fractions in the spring with the thickest sediments. Manganese is distributed in the greatest number of fractions in the same spring. The data suggest that sediment thickness plays a key role in metal chemical distribution due to the likely presence of reducing conditions. This may cause redox-sensitive metals to be redistributed between the fractions while stored in the sediments. For example, iron appears to be dissolved from the residual fraction and redistributed into the carbonate and oxide fractions during storage.