ARSENIC ALONG THE APPALACHIAN MOUNTAIN BELT, PATTERNS AND PROCESSES

Arsenic has been the subject of intense research activity in the Northeastern United States for the last decade. Thousands of groundwater and surface water samples from groundwater and surface waters have been analyzed to ascertain the processes at work that result in high concentrations in drinking water. In this presentation, we report on additional data from Southeast Pennsylvania that expand our knowledge of arsenic behavior, and reaffirm many of the processes observed at other locations along the Appalachian Mountain Belt.

Analysis of 52 water samples from randomly selected residential bedrock wells had detectable (>0.01 ug/L) arsenic in all samples. The highest concentrations were from a group of wells located in a Triassic lacustrine argillite and shale, locally named the Lockatong Formation. The strongest control on arsenic concentration appears to be pH, with all samples less than pH 5.75 containing under 1 ug/L arsenic. Analysis of arsenic species was conducted using ion chromatography coupled to hydride generation ICP-MS, and results indicate that 85% of the samples were dominated by As(V).

Several common themes emerge from these research results and those of many authors (e.g. Ayotte et al., Reeve et al., Serfes et al.,) who report on arsenic sources and mobility in groundwater from the Northern Half of the Appalachian Mountain belt. The first most common theme is the dependence of arsenic concentrations on groundwater pH. Nearly all data suggest that arsenic concentrations are lowest at pH values less than ~6. This observation is generally consistent with adsorption controlled mobility in oxidizing environments. The second common theme is that the rock types that one might argue to be the “usual suspects” for high arsenic can sometimes be incorrect, or more complex than initially thought. For example, arsenic concentrations are higher in the calcareous schist units in New Hampshire compared to the conformable beds of rusty weathering graphitic schist. In Pennsylvania and New Jersey, the initial suspicion of the red shales of the Passaic formation as the dominant source for arsenic is mostly correct. The black shales of the adjacent Lockatong formation may provide the original source of arsenic, which is then controlled by adsorption within the red units of the adjacent Passaic formation.