The provincial government grilled Enbridge on how well the proposed Northern Gateway pipeline could hold up to landslides and shifting soil as environmental assessment hearings into the project opened in Prince George on Tuesday.

Provincial lawyer Elisabeth Graff quizzed representatives from the Calgary-based pipeline company on how well it mapped out potential dangers zones along the proposed route and how it can deal with incidents if they occur.

The National Energy Board's Joint Review Panel is conducting an environmental assessment of the proposed $6 billion project to connect Alberta's oilsands with Kitimat. The final phase of the hearings, which began last month in Edmonton and continued in Prince George, allows intervenors to ask Enbridge questions on the written evidence it gave to the panel over the past two years.

Graff spent nearly four hours asking questions about mapping techniques, landslide safety and soil composition along the western edge of the route.

In one of the more dramatic exchanges, Graff pointed to areas of glacier marine sediments - clay deposits left by retreating glaciers - which she said could pose a risk to pipeline safety. She wanted to know how extensively Enbridge had mapped the soil type and how it would deal with the possible "significant risk" it could generate.

Enbridge geotechnical engineer Drummond Cavers disputed the notion that it posed a significant risk - he said there's a varying degree of soil strength in glacier marine sediments, but said Enbridge would continue to study the issue as it works on the final design of the pipeline.

"You're making a big blanket statement here," Cavers said. "We have measures that we can use to deal with them, they include changing the route if we have to. We have a variety of tools available to us, for example we can go underneath them, we can drill underneath them, we can tunnel underneath them, we can go around them."

Graff and Cavers sparred over whether it was appropriate that much of the company's studies into glacier marine sediments weren't complete and won't be complete until after the environmental assessment process is complete.

"Does B.C. have to be satisfied you'll take care of all these issues after approval?" she asked, somewhat rhetorically.

Graff also brought up a chart showing the Pacific Northern Gas (PNG) pipeline averaged more than one spill per year on its network between 1993 and 2003, with some of those as a result of landslides. Cavers replied that Enbridge has learned lessons from previous pipeline ruptures and specifically routed Northern Gateway to avoid the most dangerous routes. He dismissed the PNG comparison, saying that pipeline went through areas with different geological features.

Much of the focus of the province's questions dealt with the Kitimat River valley. Geoff Plant, the former provincial attorney general who is leading the government's legal strategy for the hearings, said to make the most of the government's allotted time they've decided to drill deep in certain areas along the route.

"The goal is to try to be focused and efficient and effective," he said.

Plant said the province is concerned about the remoteness of the terrain the pipeline will cross and if it's possible for response efforts to work effectively if leaks occur.

"It's going to be crossing some awfully pristine wilderness," he said. "It won't take much of a spill or leak into a pristine creek, that's going to have bitumen spilling into a bigger river, that's going to cause more harm than anyone wants, faster than anyone wants."

Enbridge also announced that the earliest date the pipeline could be in service is now 2018. Original estimates had the pipeline ready for the end of 2016, but Enbridge vice-president for western access Janet Holder said it had to be pushed back due to delays in the regulatory process.

The province had anticipated two hours of questioning for the current witness panel, but after nearly four hours of cross-examination on Tuesday, its lawyers still weren't done with the panel. It's expected the province will need at least another hour on Wednesday to wrap things up. The province has six hours set aside for questions of the next Enbridge witness panel.

Assuming the province is able to wrap up its questions quickly on Wednesday, Douglas Channel Watch, C.J. Peter Associates Engineering and Driftpile Cree Nation could all get their first crack at asking questions later in the day.