Acid Sulfate Soil

6.5 FLOODPLAIN MANAGEMENT AND ACID SULFATE SOIL

Acid sulfate soils (ASS) formed naturally on the Manning estuary floodplain from 6,000 to 3,000 years ago. When waterlogged under natural conditions the soils are harmless. However, when exposed to atmospheric oxygen the soils produce highly acidic runoff (pH < 4.5) containing elevated concentrations of heavy metals such as iron and aluminium. The Manning estuary floodplain has 33,797 hectares of potential acid sulfate soil (PASS) and four areas identified by the NSW government as ASS Hot Spots.33 Cattai Creek-Pipeclay Canal is classified as one of the worst ASS hotspots on the NSW coast.34 Over the past two centuries an extensive network of drainage channels was installed on the Manning floodplain to mitigate inundation and flooding, promote dry-land pasture and prevent saline intrusion (Figure 28). As a result, prolonged drying of the floodplain allowed oxygen to penetrate the ASS sediments, acidifying soils and groundwater.33 Following rainfall events extensive floodplain areas can be impacted by acidic runoff and high concentrations of heavy metals. The drains quickly transport the acid water into the Manning River estuary. ASS pollution has significant adverse impacts on water quality, aquatic species and ecosystems, amenity, oyster production and fish stocks for commercial and recreational fishing.

The highest priority ASS areas for remediation are Moto, Ghinni Ghinni and Big

Swamp (Figure 29). These three areas contribute 81% of the overall acid drainage risk within the lower Manning floodplain. Ghinni Ghinni Creek, Dickenson’s Creek, Lansdowne River and the northern arm of the Manning River downstream of Dumaresq Island are the highest acid impacted surface water areas in the estuary. Stakeholders include: Drainage Unions, MidCoast Council, Hunter Local Land Services (LLS), DPIE, DPI; DPIE - Crown Lands; Mid Coast 2 Tops Landcare; floodplain farmers; fishers and commercial oyster growers; recreational users of the estuary. Related issues: coastal wetlands, agricultural impacts.

33: NSW Government, 1999

34: Glamore, Ruprecht, & and Rayner, Lower Manning River Drainage Remediation Action Plan, 2016 Photo: an acid event in the Manning River