CAUSES OF VARIABILITY IN ARSENIC CONCENTRATIONS, WALLKILL RIVER, NEW JERSEY

The Wallkill River in northwestern New Jersey flows into New York State through glacial valley-fill deposits underlain by the Allentown and Leithsville Dolomites at the headwaters and lower basin, and the Franklin Marble at a middle river segment. Arsenic concentrations in river water exceed New Jersey's health-based Surface Water Quality Standard of 0.017 micrograms per liter. Consequently, a Total Maximum Daily Load (TMDL)for arsenic was developed for the river, which is used for recreation and, previously, as a drinking-water source at the middle segment. Understanding source inputs and processes (physical, geochemical, biological) that cause variability in arsenic concentrations is critical in developing a TMDL implementation plan for the river that will protect its beneficial uses. Arsenic concentrations varied temporally and spatially. Over time, concentrations in filtered samples ranged from 3 to 16 micrograms per liter at the headwaters to 0.8 to 2 micrograms per liter farther downstream, and did not vary with flow for either filtered or unfiltered samples. Arsenic enters the headwaters from a man-made lake where seasonal human interventions affect redox conditions and biological transformations of arsenic, and where major sources of arsenic do not appear to be geologic. At the river's middle segment, arsenic is contributed by arsenic- and zinc-rich sediments, some traceable to past mining activities. Downstream, arsenic concentrations varied diurnally by about 1 microgram per liter, but observations of seasonal variations were confounded by removal (dredging) of organic matter and arsenic-bearing sediments in the middle segment.