Monthly water quality sampling is carried out at each site in the network and samples are analysed for a range of chemical and physical parameters. These are shown in the table below. The purpose of the network is to provide information about the current state of Northland's rivers, and to look at long-term trends in water quality over time.
Most results are compared to the New Zealand trigger values for the protection of aquatic environments, found in the ‘Australian and New Zealand Guidelines for Fresh and Marine Water Quality' (ANZECC 2000). It is important to note that the trigger values are used to assess the risk of adverse effects on an ecosystem and when results are outside trigger values, further investigation may still be required to determine whether or not this is having an adverse effect on the aquatic environment.
Two sets of trigger values are used by NRC to analyse results; one for upland rivers, which only includes one site in the network (Waipoua River), and one for lowland rivers (as shown in the table below). Levels of the indicator bacteria E. coli are compared to the ‘Microbiological Water Quality Guidelines for Marine and Freshwater Recreational areas' (see the Recreational Bathing Water Quality section above).
|Parameter||Guideline trigger values (NZ lowland rivers)||Guideline trigger values (NZ upland rivers)|
|Temperature (Degrees Celsius)||-||-|
|Dissolved oxygen (% Saturation)||98 - 105||99 – 103|
|Water clarity (m)||> 0.6||> 0.8|
|Turbidity (NTU)||< 5.6||< 4.1|
|E. coli (n/100mL)||< 550^||< 550^|
|Dissolved reactive phosphorus(mg/L)||< 0.01||< 0.009|
|Total phosphorus (mg/L)||< 0.033||< 0.026|
|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|
^ E. coli guideline for recreational bathing in freshwater
The results for each river from 2008-09 are detailed in the following sections. Over half of the rivers in the network now have enough data to begin long-term trend analysis. Some of the trends are discussed in more detail later in this report (Trends in Water Quality).
Rivers have been ranked from best to worst using the 2008-09 median results for all parameters (i.e. highest to lowest water clarity, or least to most nitrogen).
There have been some changes in the order of sites from overall best water quality to worst has changed in the last financial year. The Waipoua River was again ranked the best however the Ruakaka River outranked the Manganui River this year as the worst.
As in previous years, the ranking of sites is linked to catchment land use. The best sites have catchments dominated by native forest, moving through to sites with a mix of forestry, native forest and agriculture. The worst sites have catchments dominated by intensive agricultural land use. Some sites have been shown with equal ranking, as it was difficult to differentiate between them.
|Ranking 2008/09||Ranking 2007/08||Site||River system||Land use|
|1||-||Waipoua River||Waipoua||Native forest|
|2||↑ (4)||Waipapa at Forest Ranger||Waipapa||Native forest|
|3||↑ (5)||Kaihu River at gorge||Wairua||Agriculture/forestry|
|4||↓ (3)||Mangakahia at Twin Bridges||Wairua/Mangakahia||Agriculture/forestry|
|5||↑ (7)||Waimamaku River||Waimamaku||Agriculture|
|6||↓ (2)||Opouteke River||Wairua/Mangakahia||Forestry/native forest|
|7||↑ (8)||Victoria River||Victoria/Awanui||Agriculture/native forest|
|8||↑ (12)||Waitangi at Watea||Waitangi||Agriculture/native forest|
|10||-||Waipapa Stream at Kerikeri||Waipapa||Agriculture|
|11||-||Mangamuka River||Mangamuka||Agriculture/native forest|
|12||↓ (6)||Mangahahuru at Main Road||Wairua||Forestry|
|13||↑ (14)||Waiarohia at Second Ave||Waiarohia||Urban|
|14||↓ (12)||Waitangi at Waimate North||Waitangi||Agriculture/native forest|
|15||↑ (20)||Kaeo River||Kaeo||Agriculture|
|17||↑ (21)||Punakitere River||Punakitere||Agriculture|
|17||↓ (15)||Whakapara River||Wairua||Agriculture|
|19||↓ (17)||Waipao Stream||Wairua||Agriculture|
|20||↓ (18)||Mangakahia at Titoki||Wairua/Mangakahia||Agriculture|
|21||↓ (16)||Waiarohia at Whau Valley Rd||Waiarohia||Agriculture/native forest|
|22||↓ (18)||Ngunguru River||Ngunguru||Agriculture/forestry/native|
|23||↑ (28)||Hakaru River||Topuni||Agriculture/forestry|
|24||↑ (29)||Wairua River at Purua||Wairua||Agriculture|
|26||↓ (23)||Waiotu River at SH 1||Wairua||Agriculture|
|27||↓ (22)||Oruru River||Oruru||Agriculture|
|28||↓ (23)||Awanui at FNDC water take||Victoria/Awanui||Agriculture|
|29||↓ (30)||Utakura River||Utakura||Agriculture|
|30||↓ (25)||Mangahahuru at Apotu Rd||Wairua||Agriculture|
|31||↓ (27)||Paparoa Stream||Paparoa||Agriculture|
|32||-||Mangere River at Knights Rd||Wairua||Agriculture|
|33||↑ (34)||Manganui River||Manganui||Agriculture|
|34||↓ (31)||Awanui at Waihue Channel||Victoria/Awanui||Agriculture|
|35||↓ (33)||Ruakaka River||Ruakaka||Agriculture|