All sample and field measurement collection methods follow those documented in the councils Field Monitoring Procedure Manual (NRC, 2010). All laboratory sample analysis was carried out following the procedures in the ‘Standard Methods for the Examination of Water and Wastewater‘ (APHA, 1998).
The ANZECC guidelines have been used to assess water quality for aquatic ecosystems, while the levels of then indicator bacteria (Escherichia coli) have been used to assess water quality for recreational bathing and drinking water. The water clarity guideline from the Regional Water and Soil Plan (RWSP) for Northland (NRC, 2007) has also been used to assess water quality for recreational bathing.
The results are compared to the Australian and New Zealand Environmental and Conservation Council (ANZECC) trigger values for the protection of aquatic ecosystems in New Zealand (ANZECC, 2000).
It is important to note that the trigger values are used to assess the risk of adverse effects on the ecosystem and when results are outside trigger values further investigation is recommended to determine whether there is adverse effects on the environment and to what extent. There are two sets of trigger values; one for upland rivers, which only includes one site in the network (Waipoua River), and one for lowland rivers, as shown in table 1 (below).
Table 1: Trigger values for NZ lowland and upland rivers (ANZECC 2000)
|Parameter||Trigger values for lowland rivers||Trigger values for upland rivers|
|Dissolved oxygen (% Saturation)||98 - 105||99 – 103|
|Water clarity (m)||> 0.6||> 0.8|
|Turbidity (NTU)||< 5.6||< 4.1|
|Dissolved reactive phosphorus (mg/L)||< 0.01||< 0.009|
|Total phosphorus (mg/L)||< 0.033||< 0.026|
|Nitrate, nitrite nitrogen (mg/L)||<0.444||<0.167|
|Ammoniacal nitrogen (mg/L)||< 0.021||< 0.01|
|Total nitrogen (mg/L)||< 0.614||< 0.295|
|pH||7.2 – 7.8||7.3 – 8.0|
The levels of the bacteria, Escherichia coli are used as indicator for the presence of pathogen causing bacteria, which can be a health risk for humans and stock. The levels of E. coli can be compared to the microbiological water quality guidelines for recreational users (MFE 2003), to determine whether the water is safe for recreational use. The majority of rivers in Northland are used for recreational purposes. The guideline is less than 550 E. coli/100mL of sample.
Similarly the levels of E. coli can be compared to the NZ drinking water standard of less than 1 E.coli/100mL (MoH, 2005) to determine whether the water is safe for human consumption. Note this standard is the Maximum Acceptable Value (MAV) for microbiological contamination for drinking water leaving a treatment plant. Untreated water from surface water systems will rarely meet this standard and therefore it is recommended that surface water is treated to ensure it meets the drinking water standards. On the other hand groundwater is rarely contaminated by harmful pathogens and therefore more likely to be suitable for human consumption without treatment.
Water clarity is also important for recreational users, partly for aesthetic reasons but also because elevated bacterial levels are often associated with turbid water. Water clarity readings can be used as a measure of the aesthetic appeal of water. Public perception of water quality is typically based on colour and clarity. The Resource Management Act (1991) requires that there be no conspicuous change in colour and clarity under sections 70 and 107. The Regional Water and Soil Plan has a guideline for the management of waters for contact recreation purposes of a visual clarity greater than 1.6 m (NRC, 2007, p53). This guideline comes from the ‘Water Quality Guidelines No. 2: Colour and Clarity' (MFE, 1994), which is currently the only water clarity guideline for the purposes of contact recreation and aesthetics in New Zealand.
For the trend analysis all values below detection limit or denoted with a less than value, were replaced with half of the detection limit. Because of this, caution should be taken when interpreting results where more than 15% of the samples were below detection (Gilbert, 1987).
The trend analysis was carried out on both the raw water quality data and flow adjusted data. All data was flow adjusted in TimeTrendsTM. Where there was no flow record for a sample, the sample was deleted from the analysis. The Seasonal Kendal test in TimeTrendsTM was used for the trend analysis.