Setting Limits for E.Coli – the Urban Allocation

The 2017 amendments to the National Policy Statement-Freshwater (NPS-FW) includes new objectives and policies that require councils to improve water quality in large lakes and rivers so they are suitable for swimming more often, and requires councils to identify which large lakes and rivers are suitable for swimming now, and which will be improved so that they are suitable for swimming in the future.

The Porirua District Council has numerous urban streams draining to the marine environment. The marine receiving environments are valued for swimming and the quality of marine waters is influenced by urban streams. Swimming occurs in some freshwater locations within the catchment.

This presentation will discuss E. coli modelling being undertaken to inform Greater Wellington’s collaborative limit setting process for the Porirua Whaitua. The Whaitua committee will use the model to test scenarios that will inform the Whaitua’s understanding of the catchment limits (catchment E. coli load) that would be required to achieve instream objectives that reflect the community’s values and the national targets set in the NPS-FW.

The E. coli catchment model is an integrated hydrological and water quality model developed using eWater Source software. Non-point E. coli sources were modelled by assigning dry weather and event mean concentrations to the predominate land uses within the catchments. The non-point E. coli concentrations for urban land were informed by the data collected from the Kāpiti Coast District Council stormwater network, because there was limited data from the Porirua stormwater network. There are no wastewater treatment plant discharges within the Porirua Harbour catchment, but there are multiple wastewater overflows, the location and frequency of wastewater overflows was modelled in MOUSE. These predicted overflows were then represented in the Source model as daily time series.

There was limited E. coli calibration data, but for the four sites where data was available a good calibration was achieved. In the Porirua Whaitua the majority of sites are predicted to be in E attribute state, which is the poorest attribute state in the NPS-FW, with a predicted average campylobacter infection risk of greater than 7%.

The Whaitua committee will consider the cost and benefit of mitigation options that allocate the catchment E. coli limit across different land use types and activities. Improving the E. coli attribute state in urban streams will require reductions in the load of E. coli from upstream rural catchments, stormwater networks and for some catchments wastewater overflows.