Superfund Research Program
Understanding the Physical Processes Involved in Metal Transport in the Upper Mystic Lake
About 10 miles northwest of Boston, the Aberjona River winds its way through the community of Woburn, Massachusetts, a once highly industrialized city that is now the location of two Superfund sites. For over two centuries wastes from a variety of industries, including leather tanneries and pesticide manufacturers, were released into the river resulting in elevated levels of arsenic and other heavy metals throughout the Aberjona watershed. A significant portion of the arsenic contamination has been transported to the Upper Mystic Lake - some 5 miles downstream - where this toxic heavy metal has been accumulating in the lake sediment.
As the terminus of the Aberjona watershed, the Upper Mystic Lake suffers from chronic and cumulative contamination by arsenic and other heavy metals. Arsenic from the Aberjona River continues to enter the shallow forebays (i.e., the littoral wetlands) and the main basin of the Upper Mystic Lake at a steady rate. This arsenic contamination not only builds up in the lake sediment over time, but also periodically resuspends from the sediment to the overlying water. Because the Upper Mystic Lake is a popular recreation area used for swimming, sailing, and fishing, there is concern about potential human exposures to the lake's metal contamination. Understanding the transport of metals in the lake is essential for assessing the health risks of the lake's metal contamination. However, the transport and cycling of heavy metals in this and other lake systems are still poorly understood.
To provide a better understanding of the timing and magnitude of heavy metals in the surface waters of contaminated lakes, scientists at the Massachusetts Institute of Technology (MIT) are investigating the physical processes that influence the transport of arsenic within the Upper Mystic Lake system. They are focused on understanding two specific physical processes: the hydrodynamic processes controlling the exchange of contaminants between the Aberjona River, the littoral wetlands, and the main basin of the lake; and the internal mixing of water in the lake induced by what are known as "seiche motions." Studying the first process will provide a better understanding of how littoral wetlands mediate river-born fluxes to control the timing and fate of metal loading to the lake. Investigating seiche motions (wind-induced, wavelike oscillations of the lake surface that increase turbulence in the lake) is important to understanding the mobilization of contaminants from lake sediment. This second study will improve our knowledge of how, where, and when metal contaminants remobilize within lakes.
These studies include intensive field measurements to characterize both the temporal and spatial variability of the hydrodynamic processes involved in contaminant transport. Tracer studies have been used to investigate the degree of interaction between the river and forebay during both the low flow conditions characteristic of the drier summer months, and the high flow conditions associated with fall rainstorms. These investigations have shown that the forebays mediate the flux of arsenic entering the lake. Specifically, the wetlands trap the arsenic during high-flow conditions, but release it during lower flow conditions. These findings contribute new knowledge of toxic metal transport in the Upper Mystic Lake system, knowledge that may also be applied to similar lake systems.
The seiche motions or internal waves in the Upper Mystic Lake were measured using thermistor chains placed at different locations within the lake. Each chain consists of six temperature probes hung vertically in the water column to record the temperature at different depths. The wave motion can be measured in terms of temperature because the lake is thermally stratified (i.e., contains distinct layers of water of different temperatures). When the stationary thermistor registers a lower or higher temperature, it is actually recording a wave of cooler or warmer water passing by.
Field measurements made from 1995 to 1997 indicate seasonal shifts in the behavior of the lake's internal wave field. The seiche activity was found to be strongest in late summer and fall when the stratification and wind climate are most conducive to internal wave generation. In collaboration with Professor Harry Hemond, also of MIT, this increase in wave motion was found to coincide with the observed seasonal rise in arsenic levels in the hypolimnion, the deepest water layer in the lake. This coincidence of peak chemical and physical activity highlights the fact that a full understanding of contaminant transport can only be accomplished by coupling physical and chemical studies, as has been done at MIT through cooperative research programs that link the study of lake physics and chemistry.
In addition, localized regions of the lake were found to have enhanced mixing and potentially larger fluxes of arsenic between the sediments and water column. Further studies will investigate whether upwelling of the deeper water layers occurs and whether arsenic is transported to the surface of the lake. By exploring the mechanisms that release metals from lake sediments, scientists can better predict the long term threat posed by this contamination.
The results of this research are being used to develop a conceptual model of the physical processes active in determining the arsenic distribution in the lake system. This model will provide the predictive capacity needed to develop management principles that could minimize hazardous exposure to humans. Because these hydrodynamic processes, wetland mediation, and seiche-induced remobilization are also important to the fate and transport of nutrients and passively mobile pathogens, these studies will also contribute to management strategies for controlling the eutrophication of lakes and quality of reservoir supplies.
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To learn more about this research, please refer to the following sources:
- Nepf H, Oldham C. 1997. Exchange dynamics of a shallow contaminated wetland. Aquatic Sciences 59(3):193-213. doi:10.1007/BF02523273
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