SOURCES OF ARSENIC TO STREAMS OF THE INNER COASTAL PLAIN OF NEW JERSEY, USA

In New Jersey, arsenic concentrations can exceed 3 micrograms per liter in streamwater in the western (“inner”) part of the Coastal Plain Physiographic Province. The arsenic-affected streams, which drain to the Delaware River, traverse Cretaceous through Tertiary sediments of marine origin that contain minerals, such as glauconite, enriched in arsenic. In addition to natural sources, arsenic also has been contributed to soils in some watersheds through arsenical pesticide applications in orchards. Suboxic ground water discharging through the beds of some streams can contain up to about 90 micrograms per liter of arsenic, much of which typically is present as arsenite. In most cases, concentrations of nitrogen species (mainly ammonia) and dissolved organic carbon (DOC) of this water also are high—up to 10 and 20 milligrams per liter, respectively. Assimilable organic carbon below the streambeds may stimulate microbial synthesis and growth that may in turn reduce arsenate (the oxidized form of arsenic) present in geologic materials to arsenite, which is then transported to streams in ground-water discharge. On mixing with oxygenated streamwater, the arsenite apparently is oxidized and sorbs to streambed sediments. Because of these reactions, as well as dilution from upstream water, arsenic concentrations in the water column at a sampling site typically are substantially lower than those in the ground water discharging to the stream.

Surface runoff carrying glauconite-rich particles and (or) arsenical pesticide residues from former and existing orchards is another mechanism for arsenic transport to streams. During high-flow conditions in a basin with residential development of former orchard land, concentrations of total (unfiltered) arsenic in the stream were higher (2.3-3.1 micrograms per liter) than those (1.3-2.3 micrograms per liter) during similar high-flow conditions in the stream of a larger, less-developed basin. Arsenic-rich sediments and soils are major contributors of arsenic to Inner Coastal Plain streams. Human activities probably contribute to the arsenic loads (1) through past pesticide applications, (2) through soil disturbance and (3) through introduction of assimilable organic carbon to ground water that enhances microbially mediated release of arsenic from geologic materials.