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Wairua River at Purua

The Wairua River flowing west above Whangarei all the way into the Kaipara Harbour, is one of the major tributaries of the greater Wairoa River. Upstream of the sampling site the catchment is predominantly pastoral, with the river cutting through hard sediments along a low gradient. The Wairua River site at Purua is one of four sites in Northland that are part of the National River Water Quality Network administered by NIWA.

The 2005-2006 results for the Wairua River at Purua are summarised in the table below including the median, range and percentage of sampling occasions that comply with relevant guidelines for the 12 sampling occasions. Note: Nutrient results were only available for seven sampling occasions (July 2005 – January 2006), except for total phosphorus and nitrogen and the results for June 2006 where not available at the time of data analysis. Medians shown in red are outside the recommended guidelines.

Parameter Median Range % comply with guideline
Temperature (deg. cel.) 17.1 12.3 - 23.6
Dissolved oxygen (mg/L) 9.0 7.5 - 9.8 100
Dissolved oxygen (% Sat.) 91.8 71 - 115.3 9
Conductivity (mSm) 12.60 10.2 - 14.1
Water clarity (m) 0.58 0.18 - 1.3 45
Turbidity (NTU) 8.3 3.4 - 36 27
E. coli (n/100mL) 99 44 - 6488 64
Dissolved reactive phosphorus (mg/L) 0.024 0.011 - 0.055 0
Total phosphorus (mg/L) 0.079 0.043 - 0.144 0
Ammoniacal nitrogen (mg/L) 0.02 0.004 - 0.172 29
Total nitrogen (mg/L) 0.758 0.236 - 1.87 30
pH 7.0 6.4 - 7.9 18

 

The 2005-2006 results suggest that the Wairua River is a moderately impacted waterway, with poor dissolved oxygen levels, poor water clarity, elevated phosphorus and nitrogen levels and low pH results the majority of the time. This relatively poor water quality is a result of intensive surrounding agricultural land use.

Trend analysis on data from October 1996 to May 2006 shows several positive trends including decreasing ammoniacal nitrogen concentrations, decreasing turbidity and deceasing total nitrogen results, which is linked to a decreasing trend in nitrate (not shown on the trends table). There is also a trend suggesting that pH is increasing. This is likely to be a beneficial trend for this site as pH is often below the optimum range for the protection of aquatic ecosystems. There is also increasing trends in dissolved oxygen (both in mg/L and % saturation) but it is not known whether this is a positive or detrimental trend at this stage.

Investigation into diurnal patterns in dissolved oxygen levels will be investigated at selected RWQMN sites in 2006-2007. From this it will be determined whether trends of increasing dissolved oxygen levels at RWQMN sites is of concern. Especially as there are several sites that are showing a increasing trend in dissolved oxygen.