A council has admitted it does not know if contamination has been cleared up at the UK’s first luxury sustainable holiday location, despite it having stated 15 years ago that action needed to be taken at the site.

As Richmondshire District Council said it remained a mystery whether health concerns over heavy metals at Natural Retreats’ site at Aislabeck, west of Richmond had been addressed, it emerged the authority may face investigations by the Local Government Ombudsman over alleged failures relating to the site.

A meeting of the authority’s planning committee last week, which saw councillors grant the firm’s latest expansion plan at the site, heard when the council granted permission for the holiday park in 2003, it had stipulated that contamination needed to be cleared before it could be used.

The former Army rifle range and council landfill site has been found to have highly elevated concentrations of lead and concentrations of copper, nickel and zinc.

A report on the site by consultants Amec Foster Wheeler states “there are potential unacceptable outstanding risks to human health should the remediation and mitigation measures recommended to the council in 2003 not have been implemented”.

When asked to clarify the situation, a council spokesman did not offer an explanation for why the mystery surrounding whether the planning condition had developed.

He said: “Whilst some site investigation work was undertaken in 2004 it is unclear precisely how extensive this investigation was, or to what extent precisely any previous mitigation measures were undertaken.”

Government guidance to councils on ensuring planning conditions are fulfilled states: “Effective enforcement is important as a means of maintaining public confidence in the planning system.”

Responding to claims that nearby land and water courses, including the River Swale, had become polluted with contaminants as a result of council inaction, the council said testing of the water courses surrounding the site had found no evidence of contamination from the site. A spokesman added the Environment Agency was responsible for dealing with water pollution incidents.

The council has also dismissed claims it buried the consultants’ contamination report for more than a year, saying it posted it on its website a month after receiving it. Campaigners said it remained unclear whether the report had been properly labelled before December last year.

The meeting also heard criticism that the council’s failure to ensure a warden was living on site – as part of the 2003 planning conditions – had led to antisocial behaviour continuing on site, affecting both nearby businesses and residents.

When asked about the situation, the authority spokesman did not state why the planning condition had not been followed up, but added it was being investigated by its enforcement officers.

Following the meeting, a group of residents said they had launched complaints over the council’s handling of planning applications at the site with the Local Government Ombudsman.

They also said it remained unclear whether conditions attached to the latest expansion plan meant action needed be taken to clear contamination across the 53-acre site or a smaller section of it.

The council spokesman said planning permission would be subject to any contamination risks identified across the entire site as part of fresh investigations would be subject to remedial works.