Groundwater is water that is located beneath the earth's surface in the pore spaces of soils and in the fractures of rock formations. The U.S. Geological Survey provides unbiased, timely, and relevant information and studies about groundwater resources for the whole country.
Groundwater often moves slowly and long distances. A schematic diagram of how groundwater exists and moves is in the diagram below.
An interesting photograph was taken this past winter (2011-2012) at the Northern end of the lake. It's shown below. There was very little ice on the water, perhaps an inch or two thick, at most, and the shore surface and overall air temperature is below freezing. But, as you can see, near the shore everywhere was a band of water. This is groundwater "oozing" into the lake. The ground temperatures at shallow depths near the shoreline were high enough to keep the groundwater flowing, eventually pouring into the lake, and causing the lake ice near the shoreline to melt. With a little bit of mathematical modeling, one can probably calculate the flow rate of the groundwater entering the lake.