Sampling regime

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
Conductivity (mSm) - -
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).

River Water Quality Results

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
9 - Kerikeri River Kerikeri Agriculture
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
16 ND Hātea River Hātea Agriculture/urban
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
25 - Waiharakeke Stream Kawakawa 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