Superfund Research Program
Hydrodynamic Controls on Metal Remobilization from Sediments of The Mystic Lakes
Project Leader: Heidi Nepf
Grant Number: P42ES004675
Funding Period: 1995 - 2000
The water quality of many lakes is degraded by pollution, excess nutrients and pathogens present in the watershed and carried to the lake by the watershed river network. For example, the Upper Mystic Lake, located near Boston, has been contaminated by many years of arsenic loading from its urban watershed. Much of the arsenic has been retained in the sediments of the lake. This toxic heavy metal continues to enter the lake's water column both from its feeder river and by remobilization from lake sediment. It poses an ongoing threat to human users of the lake. This year investigators used both numerical and field studies to continue investigation of the physical processes described below.
Littoral Wetlands: Controlling the Impact of Inflow Fluxes to a Lake or Reservoir
As in many lake systems, the Aberjona River enters the Mystic Lake through a littoral wetland. The researchers demonstrated how and to what degree a littoral wetland affects the temperature of the river water when it ultimately reaches the lake. Because its temperature determines the depth at which the river inflow enters a lake, it is a controlling factor in determining the fate of the pollutants it carries. Field studies using both dye and temperature as tracers were conducted to determine how the river interacts with the waters of the wetland, specifically to identify the river trajectory and the extent of water exchange. Based on these observations a numerical model of the river-wetland circulation was created and used to examine the thermal evolution of river water as it passes through the wetland. This work has shown how the degree to which the river's temperature is altered depends on the circulation within the wetland and on the magnitude and direction of diurnal and seasonal heat fluxes. By affecting the river water temperature and thus intrusion depth, the presence of the wetland impacts the initial fate of pollutants (here arsenic) within the lake.
Internal Waves (Seiches) and Remobilization from the Bed
Seiches are basin-scale oscillatory motions of the water column generated by the wind. These motions are similar to the sloshing one can create in a pot of water by tipping it back and forth. Previously, investigators examined the timing and strength of these motions using thermistor chains, a set of temperature probes hung at different depths from a single line. When seiching is present in a stratified lake the temperature records at each probe oscillate in time. Differences in the magnitude of these temperature oscillations provide information on the spatial structure of the seiche-motion. This year researchers completed a numerical model that predicts this spatial structure for specific bathymetry and density profiles. The model was verified by comparison to records of seiche motion from the Mystic Lake. The model was then used to examine the near-bed velocity, which can control boundary mixing, resuspension, and remobilization of arsenic from the sediment. This study has suggested that localized regions of the bed experience enhanced mixing and potentially larger fluxes of arsenic from the sediments. The position of these 'hot spots' depends on both the bathymetry and the stratification within the lake, and migrates seasonally as the position of the thermocline shifts in the water column.