DON’T LANDFILL! STORMWATER POND SEDIMENT MAY BE BENEFICIALLY USED AS SOILS AND SOIL AMENDMENTS

R. Simcock (MWLR Auckland), Jo Cavanagh (MWLR Lincoln) & Leigh Steckler (Auckland Council)

ABSTRACT

Sediment accumulated in stormwater ponds needs to be periodically removed to reinstate pond capacity and treatment efficacy. Sediments, mixed with variable volumes of plants, are generally removed using diggers (from dewatered ponds) or dredges. The latter use a cutting head and pumps the resulting slurry to a dewatering area where flocculants may be added to facilitate dewatering in permeable geobags. In Auckland, traditional disposal of sediment from stormwater ponds involves transporting it to landfill. With over 650 ponds in the region, this process is expensive both in monetary costs and CO₂ emissions due to the large volumes involved. A fully operational desilting programme is likely to see on average 30,000 tonnes of sediment per annum requiring disposal. In accordance with Te Tāruke a Tāwhiri – Auckland’s Climate Plan Auckland Council has committed to reducing CO² emissions by 50% by 2023 and reach net zero by 2050. Additionally, it has set itself a waste minimisation target of zero waste to landfill by 2040. Consequently, the Council requires pond desilting design and physical works contractors to include methods to reduce both CO² and waste. This includes strategies which lower the water content of sediment and divert sediment from landfill. The potential for beneficial reuse of pond sediments was investigated to support these targets. Decision flow charts were informed by workshops, a literature review, sediment sampling and a plant growth field trial.

The research found:

• Excavated pond sediments generally had properties supporting plant growth with elevated nitrogen, phosphorus, and organic matter.
• Digger-dug and dredged-flocculated sediments at the field trial provided twice as much plant-available water and nutrients than local soils, increasing the resilience of planted sedges.
• Both nutrient and water storage are influenced by organic sources; i.e. the aquatic plants and/or sawdust added to increase sediment density.
• The field trial sediments were highly acidic. This may reflect acid-sulphate (marine) influences and acidic flocculant.
• Pond sediments may require dewatering and ‘ripening’ to transform them from anoxic, black sludges to brown, friable ‘soils’. This requires space and time given dominant silt and clay textures in Auckland but maybe accelerated by cultivation and root penetration.
• Contaminants have the potential to restrict the beneficial use of sediments. Zinc was the most commonly elevated metal and metalloid contaminant, and was used in the decision flow chart as this is expected to also manage organic contaminants in most cases.
• Gross pollutants may require separate assessment and management; the cost-effectiveness of retrofitting enhanced pond pre-treatment to optimise sediment quality should be assessed for individual ponds. This could include upstream treatment trains, gross pollutant traps and/or pond forebays.
• Beneficial reuse of pond sediments is enhanced by data on pH of removed sediment and in situ sampling prior to desilting operations of texture, total carbon, nitrogen, phosphorus, Olsen phosphorus (indicating plant growth benefits) along with the conventional contaminant tests.

The November 2023 report is available on the Auckland Council Knowledge site: https://www.knowledgeauckland.org.nz/publications/considerations-for-the-beneficial-use-of-sediments-from-stormwater-ponds-across-auckland/