Study of Impact of Groundwater Cascading on Bio-Geochemical Parameters of Lake Michigan

Groundwater Cascading (GC) is a specific type of thermohaline circulation, in which dense water formed over the continental shelf descends down the continental slope to a greater depth. This process is a major component of ventilation of intermediate and abyssal waters, hence affecting thermohaline circulation and global climate. The resulting flows produce an irreversible exchange of oceanic and shelf waters and takes an important role in bio-geochemical cycles by removal of phytoplankton, carbon and chlorophyll from productive areas. Because it can take decades or more for the subducted water to re-surface, water cascades contribute to long term climatic variability. It is common to consider formation of dense water by cooling, evaporation or freezing in the surface layer. GC can provide an alternative mechanism of dense water formation on the shelf. We are working on the estimation of the impact of GC on the bio-geochemical parameters of Lake Michigan. GC and groundwater discharge (GD) through permeable sediments is an important pathway for fluid, solute, and energy transport including freshwater, nutrients, trace metals, bacteria, and other land based pollutants. Given the vast population, agriculture, and industry surrounding Lake Michigan including Chicago area, there is high potential that the groundwater exchange in this environment can significantly contribute dissolved nutrients, heavy metal contaminants and organic pollutants to the lake. These pollutants are likely to impact both water quality and ecosystem health and must be considered by area managers and ecosystem modelers in order to fully understand the water, nutrient, and metal budgets of the lake under influence of climate change. It is important to obtain reliable quantitative estimates of both in coming and outgoing fluxes of fresh cold waters. In situation that incoming groundwater is cold enough to occupy the lower part of the lake water column, transport processes in the bottom boundary layer dominate the removal of discharged water from the coastal zone. Physical mechanisms of such removal seem to be similar to the mechanisms of dense water cascades off the continental shelf over the shelf break. We investigate the physical mechanisms of the removal of groundwater from the coastal zone of Lake Michigan to try to get a quantification of off-shore fluxes and transport pathways for GD/GC and chemical species from the shelf through numerical modeling of the processes in the bottom boundary layer. We are using numerical modeling of the GD fluxes and transport pathways of dense water flow in the bottom boundary layer of Lake Michigan using a set of process oriented numerical models of GC. Some results of this study will be presented during the session.