Acid Sulphate Soils: Identification and Management in NZ’s Largest Airport

Whilst the presence and effects of acid sulphate soils on the environment and underground infrastructure are common knowledge in regions such as Northland and the Bay of Plenty, these soils have only been identified in isolated locations within the Auckland region. During November 2016 however, Pattle Delamore Partners Limited (PDP) was called to the Auckland International Airport to investigate a ‘potential spill event’ that had occurred in the future airport area. Upon arrival to the site, it was clearly apparent from observations that there had been no spill event. The site was however being influenced from the effects of acid sulphate soils.

Preliminary water quality testing of an existing watercourse within the future northern airport area found the pH was 2.8, and the water had significantly elevated sulphate and iron concentrations. The watercourse was starting to be infilled with iron oxide precipitates, a resultant effect of acid sulphate soils caused by acidified groundwater releasing iron minerals from the soil. A wetland that receives the flows from this watercourse was also starting to accumulate iron oxide precipitates. The wetland vegetation was either dead, or looked as though it had been burnt.

Approximately two weeks after these observations, a concrete 600 mm dia culvert that allows the connection from the watercourse to the wetland collapsed as a result of the low pH waters and the elevated sulphate concentrations.

To quickly respond to the above effects and restrict the possibility of persons and other piped infrastructure being further affected, PDP undertook emergency management works (under section 330 of the RMA) by placing 8 tons of Aglime into the watercourse. This elevated the water pH from 2.8 to 6.3 pH units. In addition, a pH monitoring station was installed within the wetland to monitor discharges before they enter into a significant stormwater reticulation network of the airport.

The Auckland Airport is New Zealand’s largest and is the gateway for the greatest number of international travelers into New Zealand. The airport has significant infrastructure and stormwater reticulation networks which could be compromised as a result of acid sulphate soils. Furthermore, AIAL are in the process of constructing $1.8 billion of new infrastructure assets over the next 5 years to support new airport terminals, a new runway and auxiliary infrastructure. With such rapid growth and development of the future northern airport, it was imperative for AIAL to understand the effects, management options and infrastructure design methods to deal with acid sulphate soils effects.