CONTRIBUTIONS OF ARSENIC FROM BED SEDIMENTS TO WATER IN THE WALLKILL RIVER, NORTHWESTERN NEW JERSEY, USA

Arsenic (As) concentrations in unfiltered water samples from the Wallkill River in New Jersey range from 0.2 to 34 ppb, depending on season and sampling location along a channel that runs through carbonate rocks veneered with glacial deposits. Tributary waters and sediments from the surrounding gneiss highlands and shale lowlands contribute little As to the river; As contributions in water from Lake Mohawk, a man-made lake at the river's headwaters, vary seasonally (3.5-34 ppb). At the lake outlet, bed sediments contained about 25 ppm of As. Sequential extractions show that most of this As was adsorbed to iron hydroxides. Less than 10 percent of the As in water downstream from the lake was particulate under base-flow conditions. Concentrations of As in streamwater and sediments decreased with distance downstream to the river reach adjacent to an abandoned mine in the Franklin Marble, which is host to rare zinc (Zn) ores and As minerals. Near the mined area, where the channel crosses a fault between dolomite and marble, concentrations of As and Zn in bed sediments were as high as 100 and 6,900 ppm, respectively, and up to 22 ppb of dissolved As was present in ground-water discharge to the river. Most As in ground water apparently adsorbs to bed sediments, because concentrations in streamwater typically have been 5 ppb or less. Some As in bed sediments near the mined area was present in the minerals arsenopyrite and loellingite; willemite (a Zn ore) also was found. Bed sediments with high concentrations of As (190 ppm) were found at least 2.5 km downstream from the mined area, although As concentrations in streamwater generally were low (0.5-4.1 ppb). The bed sediments from the mined area and downstream serve as a sink for adsorbed As; about 20 to 30 percent of the As in streamwater at low to moderate flow is particulate and contributes to the river's total As load. The dissolved load contains As desorbed from sediments with diurnal increases in pH; weathered As minerals also may contribute.