Waikato region acid sulfate soils preliminary risk assessment

Report: TR 2023/24

Authors: Alex Lucas (GHD Limited)

Acid sulfate soils are naturally occurring sediments which contain reduced inorganic sulfur or sulfide minerals (Dear et al., 2014), formed in reducing redox conditions. These acidic soils are widespread, covering over 17 million hectares of land worldwide (Andriesse and van Mensvoort, 2006). When undisturbed and below the groundwater table these soils remain in their reduced state and typically do not cause adverse environmental impacts. However, when these soils are exposed to oxygen via uplift, drainage or excavation, the sulfides may oxidise resulting in the production of sulfuric acid.

While the presence of acid sulfate soils is well documented overseas, little research has been done into the extent and implications of these soils here in New Zealand. The lack of readily available information about the distribution of acid sulfate soils makes it challenging for New Zealand regulators, infrastructure owners and designers to take these potentially damaging conditions into account in a consistent and effective way.

The Waikato Regional Council (WRC) has noted several adverse environmental impacts, which have been attributed to the presence of acid sulfate soil. Impacts include infrastructure damage, localised fish kills, fluctuating pH in surface water systems and increased aluminium, zinc, and iron concentrations in surface water, as a result of (pH dependent) metals leaching. Notably, within the Waikato Region, there are large areas where acid sulfate soils could occur and persist such as the Waikato and Hauraki Basins.

This report presents the findings of a preliminary spatial assessment of the Waikato Region to inform WRC of the probability of occurrence of acid sulfate soils. The investigation included producing a draft probability map that combines an extensive range of datasets with correlation to acid sulfate soils occurrence. Examples of these datasets, also referred to herein as inputs, are geology, elevation, soil type. These datasets were divided into classes (such as geological units or soil types) and given a qualitative rating of high, medium, low, or negligible based on the assumed probability of acid sulfate soil occurrence. The selection of inputs and the division of Whaclasses was informed by a multicriteria analysis of local experience, available literature and existing GIS databases encompassing the Waikato. High and medium probability areas identified within the draft map were ground truthed in order to test the assumptions within the draft map and estimate risk associated with each sample location.