acid sulphate soil

Chances are, that you (or someone you know) has obtained a LIM for a property which alerts the buyer to the possibility of acid sulphate soil. When you consider that New Zealand has approximately 15,000 kms of coastline, it’s actually not surprising, but what does it actually mean?

Acid Sulphate Soils are naturally occurring, having been deposited when the sea level was around 5 metres higher than it is at present - 5-10 thousand years ago. While the land was under the sea, salts in the seawater mixed with this land. Now that the sea level has dropped, this land which was under the sea and mixed with salty sea water is now available for use. In some situations the salts (most commonly sulphates) that were mixed with the land got trapped there. Over time, bacteria convert these sulphates into sulphides. If left undisturbed, sulphides remain in the soil without causing any major issues. If the soils are disturbed, such as through drainage which lowers the groundwater or earthworks which aerate the soil, the sulphide react with oxygen releasing sulphuric acid which can cause the groundwater to be acidic. Acidic groundwater can have a number of implications:

• Minerals in the soil can become more soluble, which can cause them to leach out of the soil and be discharged into the environment. An example is iron rust stains on surfaces which is commonly associated with acidic groundwater

• An increase in metal concentration can be toxic to ecosystems. Therefore excavation and drainage of acid sulphate soils need to consider the potential impacts on the environment.

• Acidic groundwater can corrode buried infrastructure such as concrete, steel and other sensitive materials. Consideration needs to be given to what material is used if installed in ground containing acid sulphate soils.

Generally, the impacts resulting from acid sulphate soil disturbance reduce over time. Freshly disturbed soils are likely to generate the greatest increase in acidity, with the acid release tapering off over a 50 year period. Over this time the sulphide concentration in the soil is exhausted, and acid is slowly flushed from the groundwater table by continuous rainfall flows.

https://www.wdc.govt.nz/Council/Council-documents/Policies/Acid-Sulphate-Soil-Planning-Policy

If you are concerned about the implications of acid sulphate soil on a property you are interested in, I would recommend that you seek a specialist’s advice - at a minimum, run it by your lawyer, before you sign an offer!