The organic chemical substances present in UCG- affected subsurface waters are formed by the thermal degradation of coal. After a UCG reactor is shut down, the sensible heat contained in the rubble pile is sufficient to continue coa1 pyrolysis. If the pyrolysis products are not removed. they become a potential source of sub surface water contamination. Heat transfer into the coal seam and migration c if chemical substances after reactor shutdown can be sub stantially reduced by a controlled influx of water through the cavity side wal1s and removal of produced fluids from the reactor.