A South Dakota state geologist says he's not worried by an aquifer that underlies all or most of the site for a planned $10 billion crude oil refinery north of Elk Point.

Derric Iles says a team that's been drilling test holes has found a highly impermeable protective layer between the Lower Vermillion-Missouri Aquifer and the surface. He told the Sioux City Journal that the layer would make it difficult for substances to leak through it.

Iles says ground water monitors need to be installed. He says the monitors will provide data showing whether his opinion is warranted.

Drilling started in mid-July but was interrupted by an equipment breakdown. It's expected to continue for another week.