groundwater meeting., page-9

There were actually two American UCG trials which resulted in groundwater contamination. The other was the Carbon County burn in Wyoming in 1995.

As with Hoe Creek it was caused by (deliberately) exceeding hydrostatic pressure in the gasifier.

Blindermann says in one of his technical papers that there were over 30 UCG trials in the US and many more in Europe since the 1970s, and only these two overpressured burns ever resulted in groundwater contamination.