INVESTIGATING THE CHROMATOGRAPHIC EFFECT OF CHEMICAL SPECIES IN GROUNDWATER

A chromatographic effect has been observed by analyzing groundwater and extracted total dissolved solid since May, 1993. Samples were taken at variable time intervals from monitoring wells installed in a confined aquifer located in southeastern Stark County, Ohio. The interbedded sandstone and shale aquifer is bound on top by the No. 6 coal underclay and by the No. 5 underclay at the bottom. The recharge area has been heavily impacted by coal mining and subsequent reclamation activities. Ion and elemental concentrations were determined using standard U.S. Geological Survey procedures and ICPMS analysis respectively.

The chromatographic effect is exhibited by the concentration anomalies of anion species SO42- , Cl-, and HCO3-, and elements As, Ca, Cd, Ni, and Pb of unknown speciation. The concentration anomalies of individual ion species passing through two monitoring wells separated by 660 m could be used to evaluate the speed of migration of that species. Based on these anomalies, the speeds for SO42- and HCO3- are estimated to be 110 m/year or 0.32 m/day. Similar migration speeds were derived for Cl- and As. A hydraulic conductivity of 110 m/year was obtained by groundwater modeling in 1995. In order to refine migration speed evaluations, a higher-resolution record of the duration of anomalies must be obtained through more frequent groundwater sampling. The migration speed of these chemical species through the aquifer follows an increasing sequence: Ca>As=Cl->HCO3-=SO42- . Other elements such as Cd, Ni, and Pb have multiple anomalies and may indicate the existence of multiple species. In addition, significant changes in concentrations of Cl- and Ca down piezometric gradient are observed while SO42 - and HCO3- remain relatively unchanged. The chromatographic effect of the migration of these chemical species through an aquifer can only be observed by a long-term study.