New Zealand microbiological guidelines cover the methods for monitoring and reporting on the public health risk associated with microbiological contamination in recreational waters.
How we derived the freshwater default guideline values
The DGVs have been derived using the methods detailed in McDowell, Snelder & Cox (2013). In the report, reference conditions are defined as the chemical and physical conditions that can be expected in rivers and streams with minimal or no anthropogenic influence.
DGVs, referred to as ‘trigger values’ in McDowell, Snelder & Cox (2013), have been defined as indicating that there is a ‘potential risk’ of adverse effects at a site.
Two percentiles have been calculated for the DGVs based on the stressor:
- 80th percentile for physical and chemical (PC) stressors (indicators) that are harmful at high values (e.g. nitrate)
- 20th percentile for PC stressors that are harmful at low values (e.g. clarity).
DGVs have been derived for these PC stressors:
- ammoniacal nitrogen (NH4-N)
- dissolved oxygen (DO)
- electrical conductivity (EC)
- filterable reactive phosphorus (FRP)
- hydrogen ion concentration (pH)
- nitrate nitrogen (NO3-)
- suspended solids
- total nitrogen (TN)
- total phosphorus (TP)
We provide DGVs for all these PC stressors in one spreadsheet.
Estuarine default guideline values
Estuarine DGVs are limited to the rate of sedimentation. Further guidance and/or guideline values may be developed in the future.
The hierarchy for deriving guideline values ideally uses local biological-effects data first but if these are not available, then local reference data can be used. In the absence of both these, the default approach is to use regional reference data or generic effects-based guidance to develop a DGV.
In New Zealand, there are insufficient analysed data examining the relationships between annual sedimentation rates and ecological condition to produce DGVs from local biological-effects data.
A considerable body of experimental data exists on the responses of soft-sediment macrobenthic communities to fine-sediment deposition in the immediate aftermath of rainstorm ‘events’. Event-based guideline values would require extensive resourcing and would likely be difficult for widespread implementation, limiting their use as a primary assessment approach. So we have adapted current knowledge of event-scale effects to develop a DGV for annual sedimentation rate, which is likely to be more practical.
The DGV is 2 mm of sediment accumulation per year above the natural annual sedimentation rate for the specific estuary, or part of the estuary.
Further information on the analysis to define this DGV can be found in ANZECC (2015) Guidance for Estuary Sedimentation.
We can define the natural sedimentation rate as the rate under native-forested catchment. It is included in the DGV as a baseline to account for estuaries or parts of estuaries with naturally high rates of sedimentation.
McDowell RW, Snelder TH & Cox N 2013,
Establishment of reference conditions and trigger values for chemical, physical and micro-biological indicators in New Zealand streams and rivers, Agresearch.