GEOLOGIC CONTROLS ON ARSENIC DISTRIBUTION AND MOBILIZATION IN PENNSYLVANIA GROUNDWATER

A GIS study and field program were initiated to describe the distribution, causes, and controls on arsenic (As) speciation and mobilization of an area with elevated concentrations of arsenic (>10 ppb) in the groundwater. The GIS study compares bedrock geology to groundwater chemistry using data from PA DEP and USGS groundwater databases. Elevated concentrations of As in the groundwater are spatially correlated with formations of the Gettysburg-Newark Lowland Section of the Piedmont physiographic province. Focus is placed on the Passaic formation of the Newark Basin, southeastern Pennsylvania. The Passaic formation is described as red, silty mudstone and shale interbedded with occasional coarse-grained red sandstone and thick, black shale in the lowermost units. Reductive desorption from or dissolution of iron oxides may be responsible for elevated concentrations of As; however, thick, black, pyretic shales of the lowermost Passaic formation are good candidates for release of As via pyrite oxidation.

Water samples have been collected from 53 groundwater wells in the Passaic formation and surrounding areas. Bedrock samples are being collected from local outcrops. Samples have been analyzed for As speciation (As III and As V) by coupling ion chromatography and hydride generation (HG) to ICP-MS. Samples will be analyzed for total As using HG-ICP-MS; Na, K, Ca, Mg, Fe, and Mn using ICP-MS; and SO₄, Cl, F, NO₃, NO₂, and PO₄ using IC. Redox potential, pH, and temperature were collected in the field.

Oxidizing conditions dominate the samples. Of 37 samples analyzed for As speciation, As V dominates 81% of samples under oxidizing conditions and As III dominates 8% of samples under low oxidizing or reducing conditions. Eleven percent of samples exhibit almost equal amounts of As III and As V under oxidizing conditions. There are ten results with elevated concentrations of As (>10 ppb). Nine samples were dominated by As V under oxidizing conditions. One sample was dominated by As III under low oxidizing conditions. The results show that both low and high concentrations of As tend to occur in oxidizing conditions. Preliminary plots on Eh-pH diagrams show As V species to dominate the sample set. Eh-pH relationships will be further analyzed to determine if the presence of other ions in solution are controlling factors.