Long Oral Presentation 11th Australian Stream Management Conference 2024

Putting it all together: synthesis of multiple long-term monitoring programs to inform effective water quality offsets (#87)

Michael Newham 1 , Jon Olley 2 , David Orr 1 , Ian Ramsay 1 , Joanne Burton 1 3
  1. Queensland Department of Environment and Science, Dutton Park, QLD, Australia
  2. Australian Rivers Institute, Griffith University, Brisbane, QLD
  3. Resilient Rivers Initiative, SEQ Council of Mayors, Brisbane, QLD
  • The Point Source Water Quality Offset Policy (2019) provides an alternative mechanism for managing the water quality impacts of point-source nutrient discharge. The policy allows for the load of nutrients emitted from a point source to be offset by reducing nutrient inputs at another location. Streambank erosion control has been the primary method of offset applied under the policy.
  • The Brisbane River estuary receives the greatest load of point-source nutrients in Queensland and streambank erosion in the catchment is a known threat. This situation provides both the demand and potential supply of nutrient offset actions. Using 21 years of comprehensive monitoring of ambient nutrient concentrations, catchment loads, and point-sources discharge, we assessed the relative influences of point and diffuse sources of nitrogen (N) in the Brisbane River estuary. This informs the effectiveness of catchment offset actions to reduce N in estuary waters.
  • We found that under low flow conditions, diffuse sources of N from the catchment had little influence on the estuary, however, point source loads could be used to model the ambient concentrations. During flood flows, diffuse N dominated load, was transported through the estuary without processing and delivered to coastal waters. Low flow conditions quickly re-established following floods.
  • Nutrient offset methods of diffuse N reduction, such as streambank erosion control in the catchment, do not effectively mitigate point-source impacts downstream in estuaries. To effectively mitigate water quality impacts of point-sources, nutrient offset actions must be effective for the same timing and location of impact as point-source discharge.
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