Stable isotope data confirms observations with a prototype "Groundwater Flow Direction Finder" that groundwater in the Toulon Sand east of the Lake Bloomington Spillway is isolated from lake water. No lateral movement of clay particles was observed in the screened interval of a two-inch diameter monitor well. The screened interval in the well is below lake level and above the spillway floor. Oxygen 18 and Hydrogen 2 values of the groundwater obtained from the monitor well are similar to values obtained from groundwater obtained from other Illinoian Glacial Stage aquifers in the area and distinctly different from values from Lake Bloomington and other surface water bodies. If lake water was bypassing the dam in the Toulon Sand, horizontal movement of clay particles should have been observed in the monitor well and isotopic values should have been closer to surface water values.