ARSENIC AND BORON IN GROUND WATER IN AND NEAR DIABASE INTRUSIONS, NEWARK BASIN, SOUTHEASTERN PENNSYLVANIA

The U.S. Geological Survey (USGS) conducted a study in 2004-2005, in cooperation with the U.S. Environmental Protection Agency, to help identify sources of and determine geochemical controls on elevated concentrations of arsenic and boron in ground water in and near Jurassic-age diabase intrusions in the Triassic-Jurassic Newark Basin, southeastern Pennsylvania. The USGS sampled water from 46 private supply wells in 5 areas in the Newark Basin. Wells were sampled in transects from the diabase through the adjacent hornfels and into the unaltered shales. Concentrations of arsenic to 60 µg/L (micrograms per liter) and boron to 3,950 µg/L were measured in samples from the wells. Arsenic concentrations exceeded the drinking water standard of 10 µg/L and boron concentrations exceeded the health advisory of 600 µg/L in about 20 percent of samples. Water from wells in diabase tended to be enriched in the heavier isotope of boron; values of δ¹¹B were greater than +20 per mil. The δ¹¹B values of 67 percent of well samples were greater than +10 per mil, indicating natural sources for boron. Arsenic correlated most strongly and positively with pH, boron, and molybdenum. Arsenic also correlated positively with selenium, uranium, nickel, lithium, fluoride, and strontium and negatively with total organic carbon, copper, and dissolved oxygen. Arsenic concentrations may be controlled partly by pH affecting adsorption of the anion arsenate. Generally, the ground water had relatively neutral to alkaline pH (ranging from 6.1 to 9.1) and moderate concentrations of dissolved oxygen. The drinking water standard of 10 µg/L for arsenic was exceeded in all samples with pH of 8 or higher but in no sample with pH of 7 or lower. The percentage of wells completed in and near diabase with ground-water concentrations of arsenic above 10 µg/L appears to be greater than for wells in the Newark Basin as a whole (about 20 percent compared to 10 percent), suggesting some arsenic enrichment in the rocks and(or) a favorable geochemical environment to mobilize arsenic. The amount of boron in ground water in and near diabase intrusions probably is higher than in the Newark Basin as a whole. Boron may have been introduced or remobilized during the intrusion of diabase in the basin, especially where evaporites may have been present in the host sediments.