SOURCES AND MOBILITY OF ARSENIC AND METALS IN WATER AND SEDIMENTS OF A SHALLOW AERATED AND ALUM-DOSED LAKE, NORTHWESTERN NEW JERSEY, USA

In a 2004-05 study of the Wallkill River in New Jersey, arsenic (As) concentrations ranged from less than 4 to 34 ppb in unfiltered water samples from the outlet of Lake Mohawk at the headwaters of the river. Concentrations were lowest during winter and spring and highest during summer and fall. The shallow (7 m or less), 324-hectare lake, created in 1927, is used for recreation and is surrounded by residential development. Alum is applied and aerators operate during summer to mitigate the causes and effects of eutrophication. In a subsequent (fall 2005) study of As and metal sources and mobility in lake water and sediments, As concentrations ranged from 22 to 31 ppb in unfiltered water samples collected throughout much of the lake; 15 to 20 percent was particulate As and most dissolved As was arsenate. Concentrations exceeded 100 ppb and arsenite was the dominant species in a thin (0.3 m) suboxic layer of bottom water. Recoverable As concentrations in lakebed sediments were highest (up to 694 ppm) in highly organic material from the tops of sediment cores and lowest (less than 15 ppm) in samples containing some geologic substrate (mainly clays and sands from glacial deposits). The lakebed sediments also contained high concentrations of recoverable Al and Cu (up to 30,000 and 2,590 ppm, respectively), in part as a result of alum and herbicide applications. Higher concentrations of Cd, Pb, and Zn in sediments in core tops than in core bottoms likely reflect inputs related to suburban development. Geologic inputs of As to the lake appeared to be small; most probably is derived from anthropogenic sources. During summer and fall, reduced As in the sediment pore water is mobilized, as mixing by aerators, boat traffic, and wind causes oxidation and spreads the arsenate throughout the water column. Metals in water from the suboxic zone are mostly in particulate form and generally remain near the lake bottom, however.