A quarry that has been backfilled with quake debris.

Christchurch's world-class drinking water be at risk if quarries are allowed to dig deeper into the ground, an expert hydrogeologist says.

A group comprising 10 Christchurch quarries, calling themselves the Canterbury Aggregate Producers Group, have applied for resource consent to excavate into the groundwater table.

Current rules require they remain one metre above the highest groundwater level.

SUPPLIED Pooling groundwater at the base of a quarry.

Operators say digging deeper will allow them to excavate up to 30 million tonnes of extra aggregate, and they will backfill excavated areas with cleanfill material.

READ MORE:

* Contamination fears prompt call to stop quarries digging deeper

* Yaldhurst quarry accused of digging too close to groundwater

* Irresponsible operators 'tarnishing' New Zealand quarry industry's reputation

Christchurch's drinking water, sourced from aquifers beneath the city, is among the best in the world, with a quality similar to bottled mineral water.

It is naturally filtered through gravel and sand from the Waimakariri River, which means it can be consumed untreated.

Helen Rutter, a senior groundwater hydrologist who will provide expert evidence on the applications, said quarrying closer to the aquifers would come with risks.

She said the quality of the material used as backfill would need to be carefully monitored to ensure contaminated material did not leach into the groundwater.

Other issues included spillages from machinery and waterfowl contamination.

"When you're going close to the water table, there's always an issue with potential contamination," she said.

"As long as you're above the groundwater, and you've got a spill, you can clean it up quickly. If you've got groundwater inundating the base of the quarry, then that can be an issue, because things can get straight into it."

Any contaminants present in backfill could leach into the groundwater, she said.

In most cases, the quantity of groundwater would dilute any contamination, but the risk increased with the number of quarries proposing to dig deeper.

"If you've got one quarry that's got fill with contaminants in it, then it will get diluted. If you've got a whole load of them . . . every time you add another one, it has a cumulative effect."

Some residents near quarries have vehemently opposed the application, as have environmental groups concerned about the threat to the city's aquifers.

Robert Wynn-Williams, the former chair of the Canterbury Aoraki Conservation Board who lives near a quarry, cited international examples of unpredictable effects of quarrying into groundwater.

"Due to the very unpredictable nature of the outcome of quarrying into the water table both internationally and in Canterbury in particular, it is impossible to predict the long term outcome of quarrying in the Groundwater Protection Zone," he wrote in his submission.

"The applicants cannot guarantee that quarrying below the known highest groundwater level and backfilling will be safe in perpetuity."

Bob Cross, chair of the Yaldhurst Residents' Association, echoed concerns about water quality and the impact on residents near quarries.

Public submissions on the application closed on Friday.

The final tally of submissions was yet to be counted, but it was believed to be about 600, an Environment Canterbury spokeswoman said.