Urban lakes are important recreational and natural resources that add to the quality of life for city residents. Unfortunately, urban watersheds often contribute contaminants to these lakes, including organic chemicals, metals, nutrients, and pathogens. Nitrogen and phosphorus are very high in urban and suburban runoff, mostly as a result of animal waste and fertilizers, although leaky sewage systems may also contribute. These nutrients promote plant and algal growth in urban lakes, ultimately resulting in hyper-eutrophic conditions. Eutrophication, in turn, may affect the cycling and mobility of contaminants, such as arsenic and other toxic metals. Spy Pond, located in Arlington, Massachusetts, was recently discovered to be heavily contaminated with arsenic of unknown origin. Surface sediment concentrations above 2,500 ppm have been measured. Subsequent investigations have also revealed that total arsenic levels in the overlying hypolimnetic waters reach over 150 ppb. However, the two interconnected basins that constitute Spy Pond have been found to differ by an order of magnitude in the concentrations of arsenic found in hypolimnetic waters. The goal of this study is to determine the mechanisms responsible for the differences in arsenic mobility in the two basins of Spy Pond, and how this may impact the potential for minimizing human and ecological arsenic exposure. Based on differences in the concentrations of chemical constituents (e.g. iron, sulfur, conductivity, etc.) measured in each basin, we hypothesize that the greater arsenic concentrations found in the bottom waters of the South Basin of Spy Pond are caused by the combined effects of eutrophication, differences in the Fe/S ratio of the two basins, and the physical and chemical impacts of salts in highway runoff.