Arsenic Occurrence, Sources, Mobilization, and Transport in Groundwater in the Newark Basin of New Jersey

Abstract

Arsenic concentrations as high as 215 ug/L in ground water occur in bedrock aquifers of the Newark Basin in New Jersey. This basin is a Mesozoic aged half graben containing red, gray and black mudstone, shale and sandstone with basic igneous intrusions and flows. In 2000-01, random sampling of 94 domestic wells in a 200 square mile study area revealed that 15 percent have arsenic concentrations exceeding 10 ug/L. Those wells generally have low dissolved oxygen concentrations, < 3 mg/L, and pH values range from 7.5 to 8.2. Analyses of red, gray and black mudstone and shale yielded maximum As concentrations of 13, 50 and 240 ppm respectively. Pyrite (FeS 2 ) in the black shale contains up to 40,000 ppm As. A spatial association between high As in ground water and black shale is observed. Hematite and clays in red mudstone are also potential sources of As. Mobilization of As via pyrite oxidation and desorption from hematite and clays are potential mechanisms. Transport of As is aided by competitive adsorption, an alkaline pH, and suboxic aqueous environment.