Acid Sulphate Soils – what are they?


Acid Sulphate Soils (ASS) are soils and sediments which occur under specific environmental conditions. When these soils are exposed to oxygen and water, a chemical reaction takes place producing sulphuric acid. That sulphuric acid can then interact in an unfavourable way with the environment, buried infrastructure, and building foundations.


Scientifically speaking, Acid Sulphate Soils (ASS) are soils, sediments or other materials that contain sulphide minerals, mainly pyrite (iron sulphide, FeS2). In New Zealand these soils are most likely to be found in coastal and near-coastal areas. Acid Sulphate Soils are harmless when left undisturbed in a waterlogged oxygen-poor (anoxic) environment. However, when these soils are exposed to oxygen and water, the sulphides in soil break down and sulphuric acid is produced. The decrease in pH results in mobilisation of soluble iron and potentially other elements (e.g. aluminium, lead, zinc etc.) if present in soil.


Why is assessment of Acid Sulphate Soils important?


Acid Sulphate Soils can affect land use and development as they have a potential to adversely impact land, water and ecosystems. In relation to development, these soils can have a negative impact on engineering works such as site infrastructure and buildings. For this reason their classification and management needs to be considered prior to development of land.


Acid Sulphate Soils received a fair amount of attention in recent years in Northland. The main reason for this sudden interest in the topic is the problem encountered at Marsden City development near Whangarei. This new subdivision was developed in the late 2000s and at the completion of the works the infrastructure assets were vested into Whangarei District Council’s care. Significant problems were discovered in the area in 2013 when some of the assets, in particular buried concrete structures, began to fail. A subsequent investigation has found that Acid Sulphate Soils are present in the area and the degradation of the stormwater and wastewater assets is the direct result of the disruption of these soils. These activities resulted in the production of acid which attacked the concrete structures. The consequences of this situation are very significant in monetary terms as the site requires remedial works and the life span of the remaining assets in the network are significantly reduced. The Whangarei District Council has determined that the assets have been damaged by more than $8.7m (WDC Annual Plan 2014-2015).


Following the developments at Marsden City the Whangarei District Council ordered for the region to be risk mapped in order to determine which areas have the potential for Acid Sulphate Soils’ presence. This work has also shown potential Acid Sulphate Soils existing in both Far North and Kaipara District Council’s areas.

The Whangarei District Council also issued a planning policy guide which classifies activities to be undertaken as part of a proposed development as low, moderate or high risk. If the area of the proposed new development is located within the mapped zones characterised by Acid Sulphate Soil potential, the guide outlines the type of investigations that should be undertaken if high risk activities are planned.


What needs to be done and how can we help?


Vision has in-house expertise to undertake Acid Sulphate Soils investigations from the desktop study stage, through field investigations all the way to management planning and implementation, to allow for your planned development to go ahead. Our team of specialists are able to tackle small-scale to large complex projects.


If your property is suspected to have acid sulphate soil potential we are able to inform and assist you of the requirements to satisfy all three Councils. Our work is professionally recognised with best practice procedures implemented to save you time and costs.


The field testing involves primarily a qualitative method. A selection of samples collected from different depths in the ground are analysed in the field and usually a small percentage of these samples are sent to a laboratory for confirmatory analyses.


If Acid Sulphate Soils are identified on your site, we can produce an appropriate management plan which will be required as part of the Assessment of Environmental Effects in a Resource Consent application. We are also available for technical advice, overseeing and management of works at the site.


If you find that your existing site is affected by Acid Sulphate Soils, our specialist can assist you with investigation, options assessment, and development of a management or site remediation plan.


We hope this has helped you learn about and understand a little more on Acid Sulphate Soils.