Villagers in Sutton Bridge fear there could be flooding if plans by an energy company to drill 40m below the seabank go ahead.
The work would be to bring cables ashore from a wind farm off the coast.
DONG Energy plans to insert two or three hollow ducts under the seabank between April and August subject to planning consent.
But residents and local marine watchdog King’s Lynn Advisory Group (KLAG) say it’s possible the ducts could be breached and allow water to flow under the sea defence wall and onto the landward side of the development.
KLAG has also raised concerns about “environmentally friendly” chemicals that will be used when break out occurs in the horizontal directional drilling bore.
They say chemical specification sheets for one, Diamond Seal, state it doesn’t dissolve in water and can affect the human body if inhaled, ingested or makes contact with the eyes and skin, and it should not come into contact with soil.
Resident Colin Blundell told Sutton Bridge Parish Council that hundreds of gallons of water will be needed every day for the drilling.
He said DONG is suggesting it could get the water in three ways – pumping it from the Nene, which needs a licence, taking it from a nearby drainage channel or transporting it by tankers along East Bank at the rate of three in/three out every hour on days and nights when drilling takes place.
Mr Blundell fears such heavy traffic could undermine the man-made bank on the east of the river and wants to know who will foot the bill for any repairs.
He said: “East Bank is built out of silt – with tankers going backwards and forwards along East Bank to that extent, I reckon there could be up to 21,000 movements per six month period.”
Centrica tried horizontal directional drilling (HDD) in 2010, but had to abandon it and put its cables through the sea wall.
KLAG says it broadly agrees that HDD “is the favoured method for bringing the cables ashore as this provides few disturbances to the marsh and intertidal foreshore than the cable laying/backtrack infill used previously.”
Parish councillors decided to object to the plans, incorporating some of the comments from KLAG, and voiced concerns over flooding, noise from hovercraft used in the operation and traffic on East Bank.
Parish councillor Jenny Rowe said: “We need to make a stand down here. We are not just going to roll over and keep accepting this industry. I think the planners need to look into this one properly.”
A spokesman for the energy company said: “DONG Energy is currently consulting as we consider the best option for bringing ashore power cables from the proposed Race Bank offshore wind farm.
“We are fully committed to maintaining the integrity of the local flood defence infrastructure and the sensitive local environments, such as the saltmarsh. Dialogue is now taking place with regulators and the local community to ensure that this is the case.”
The company’s main method for providing fresh water would be a hosepipe from the Nene and “water trucks would only be used as a last resort”.
DONG say it will use chemicals “specifically designed to cause no damage to the environment” and described the use of hovercraft as “the only allowable and safe method of transporting staff”.