Most South-east Queensland streams and rivers suffer from poor riparian health and, as a result, their banks are the major source of rural diffuse sediment and nutrient pollution. Waterways are in an urgent need for restoration at a time when traditional Government funding is limited. Innovative approaches are needed to fund and deliver catchment restoration.

Nutrient offsetting provides an example of a potential new funding source by optimising social investment in the health of receiving waters by expanding the economic options available to sewerage treatment plant (STP) owners to satisfy environmental outcomes to include the use of riparian restoration. The potential exists to optimise social investment in riparian restoration while reducing sewerage costs.

The Department of Environment & Heritage Protection; regional STP operator, Queensland Urban Utilities; and regional Natural Resource Management body, SEQ Catchments have come together to deliver a nutrient offsetting pilot study involving river restoration and use the learnings to help inform policy development. The pilot study investigates the potential to reduce sediment and nutrient pollution from degraded rivers in lieu of engineering solutions to reduce end-of-pipe, STP discharges.

This paper is presented in three parts: Part (a) dealt with the estimation of sediment pollution avoided: while this Part (b) focusses on the estimation of nutrient pollution avoided: and Part (c) deals with lessons for policy. The Partners engaged geomorphologic specialists, Alluvium Consulting, to assist.. 

Having estimated sediment erosion avoided through river restoration, the Partners then considered options for estimating nutrient erosion avoided. As the STP environmental licence regulated Total Nitrogen (TN), the focus was to estimate TN avoided.

Initial consideration was given to using event mean concentration (EMC), the historical relationship between sediments and TN during gauged storm flows, to convert estimate TN avoided from sediment erosion avoided. However suspended nitrogen forms tend to concentrate in storm flow as coarse sediments are deposited, with the result that EMC will over-estimate TN avoided. In theory, a sediment delivery efficiency ratio might be able to be used to estimate the component of avoided bank erosion that would be transported as sediment in storm flow. However, there is no practical way to measure this ratio.

Attention turned to the direct measurement of TN extant in the bank profile subject to bank erosion. This paper highlights the options for direct measurement. It then discusses implications of focussing on TN and of the broader nutrient and environmental benefits associated with river restoration.