Trends in Water Quality

Long-term trend analysis was carried out using a Seasonal Kendall test, which eliminates any seasonal influence on the results. The table below shows water quality trends for 18 of the RWQMN sites that have five or more year's data. The start of the dataset for trend analysis for each site is shown in brackets under the site name. The end date for the four NIWA sites is December 2008.

Only significant trends are shown. Significant trends are those with at least a 95% confidence level i.e. 95% likelihood that the trend is "real". It must be highlighted that 5 years data is a minimum for carrying out long term trend analysis and that these trends are likely to change as more data becomes available.

In the table below, an ‘up' arrow indicates an increasing trend, a ‘down' arrow indicates a decreasing trend; a green arrow indicates a positive trend and a red arrow indicates a negative trend. A blue arrow has been used where it is not yet known if the trend is positive or negative.

There were no significant trends at a 95% confidence level for conductivity, E. coli and temperature at any of the sites. Many of the sites could not have trend analysis done for ammoniacal nitrogen, as too many results were found to be below detection limits. These are shown as NA in the table below. Some sites had insufficient data for certain parameters to carry out trend analysis. These are shown as ID in the table.

Site

Cond

DO%

Ecoli

pH

Clarity

Turb

Temp

TKN

NH4

NNN

TN

DRP

TP

Awanui

Waihue channel

(Oct 1996)

 

 

 

 

 

 

 

NA

 

 

Awanui

FNDC take

(Oct 1996)

 

 

 

 

 

 

 

 

NA

 

 

Kaihu

(Jul 2002)

 

 

 

 

 

 

 

 

 

 

 

 

Mangahahuru

(Oct 1996)

 

 

 

 

 

 

 

 

NA

 

Mangakahia

Titoki bridge

(Oct 1996)

 

 

ID

 

 

 

 

 

 

 

Mangakahia

Twin Bridges

(Oct 1996)

 

 

 

 

 

 

 

 

NA

 

 

NA

Manganui

(Aug 2001)

 

 

 

 

 

 

 

 

 

 

 

 

 

Mangere

(Oct 1996)

 

 

 

 

 

 

 

 

Opouteke

(Oct 1996)

 

 

 

 

 

 

 

 

NA

 

NA

Punakitere

(Aug 2001)

 

 

 

 

 

 

 

NA

 

 

Victoria

(Oct 1996)

 

 

 

 

 

 

NA

 

 

Waiotu

(March 2000)

 

 

 

 

 

 

 

NA

 

 

 

Waipapa

(Oct 1996)

 

 

ID

 

 

 

 

 

 

Waipoua

(Jul 2002)

 

 

 

 

 

 

 

 

 

 

 

NA

NA

Wairua

(Oct 1996)

 

 

ID

 

 

 

 

 

 

Waitangi

Waimate North

(Aug 1999)

 

 

 

 

 

 

 

NA

NA

Waitangi

Watea

(Oct 1996)

 

 

ID

 

 

 

 

 

 

 

 

 

Whakapara

(Oct 1996)

 

 

 

 

 

 

 

 

 

Conductivity

Conductivity (COND) is a measure of the amount of ions in the water column and can be used as in indicator of nutrient enrichment. There were no significant trends detected at any of the sites this year.

Dissolved Oxygen

There is an increasing trend in dissolved oxygen (%) at Victoria River. This increasing trend is likely to be a positive one as dissolved oxygen levels are often outside the recommended range for aquatic life.

Water Clarity

Trend analysis indicates that water clarity has increased (improved) at four sites; Kaihu River, Mangere River, Punakitere River and Waipapa River. The improvement in water clarity at these sites could be a result of increased riparian planting or better land management in the surrounding catchments, which would both result in decreased run-off entering rivers and streams. An improvement in the quality of point-source discharges in these catchments may also be a factor.

Water clarity at the Awanui River at Waihue channel and Victoria River has a decreasing trend which may be due to land use changes in the catchment.

Bacterial Contamination

No sites have a significant trend in bacterial levels (E. coli). This is an improvement as historically, the Mangere River had an increasing (negative) trend in E. coli.

Nutrients

In general, nutrient levels are showing decreasing trends at the majority of sites, including decreasing dissolved reactive phosphorus, ammoniacal nitrogen, nitrite/nitrate nitrogen and, in turn, total nitrogen and phosphorus. This could be as a result of improving farm management practices throughout the region, including increased stock exclusion from waterways, riparian planting and better fertiliser application. It also could be as a result of better quality point source discharges, for example, discharges of farm dairy effluent or those from oxidation ponds. However, many of the sites with decreasing trends still have very high nutrient levels and there is therefore still a long way to go before desirable levels are met.

Nutrient levels are of concern in the Waitangi River at Waimate North as nitrogen levels are increasing. This could be as a result of land movement activity in the catchment.

pH

Only one site, the Wairua River, had a significant increasing trend this year. This trend is beneficial as pH is generally below the optimum range of 7.2 to 7.8 for aquatic life. An increase in pH should therefore be of benefit to aquatic ecosystems.