Acid sulfate soils (ASS) are naturally occurring soils and sediments containing iron sulfides, most commonly pyrite. When ASS are exposed to air the iron sulfides in the soil react with oxygen and water to produce a variety of iron compounds and sulfuric acid. Initially a chemical reaction, the process is accelerated by soil bacteria. The resulting acid can release other substances, including heavy metals, from the soil into groundwater and the surrounding environment.
A detailed site investigation for ASS will be required prior to the commencement of any development site works to identify the presence or absence of ASS likely to be disturbed and define the nature and extent of ASS (including existing and potential acidity) in order to determine appropriate management options. Where ASS is present, these management options will be outlined in an ASS Management Plan (ASSMP). An ASSMP may be required if a development includes any dewatering or drainage works, and/or the excavation of more than 100 cubic metres of soil.
The kind of activities that might generate acid through the disturbance of soils includes the following:
- Major earthworks—large scale excavations for canal developments and estates.
- Infrastructure earthworks—digging for clearways, roads, tunnels and railways.
- Excavating for underground services or drainage channels.
The objective of an ASSMP is to outline a strategy to effectively manage the determined extent and severity of ASS on the project site. An ASSMP should provide for ongoing management and monitoring of the effects of ASS disturbance through the entire construction or operation period of a project.
An ASSMP must provide:
- Evidence of practical, achievable and auditable plans for the management of the project to ensure that environmental impacts are minimised - including comprehensive monitoring and control of construction and operational impacts;
- A framework to confirm compliance with approval conditions stipulated by regulatory authorities; and
- Evidence that the project management will be conducted in an environmentally acceptable manner.
Stay tuned for Part 2 of this article where we dig deep into the format and content required within an ASSMP.