The Taranaki Regional Council aims to maintain and enhance the quality of surface water in the region.
This objective is set out in the Regional Fresh Water Plan for Taranaki (Issue 6.2). Go to the Fresh Water Plan index
To determine whether waterway quality is improving or deteriorating, scientists need to analyse long-term trends.
Ecological trends
As is increasingly the case elsewhere in the world, the primary measure of the quality of our region's streams is their ecological health. This is measured by assessing the richness of the macroinvertebrate communities (tiny animals including insects, crustaceans, molluscs, worms and leeches).
The measurement tool is known as the Macroinvertebrate Community Index (MCI), which ranks species found in streams according to their tolerance for pollution. Species in a sample of macroinvetebrates are each given a score (the higher their tolerance for pollution, the lower their score). These scores are combined with assessments of the abundance of each species to derive an overall score indicating the ecological state of the stream. A high score indicates good health.
The MCI was developed in New Zealand, with scientists from the Taranaki Regional Council playing a significant role. The index is used widely and is accepted as evidence in resource consent hearings, and in the Environment Court. Similar scoring is used worldwide.
The Council has been using the MCI to monitor the ecological health of Taranaki waterways since 1995, and reports annually. Typical patterns include:
- MCI scores decline down the length of a waterway. Naturally occurring factors include the more open and exposed stream beds in lower reaches, lower river flows, and more sedimentation on stream beds. Another factor is accumulation of some contaminants.
- MCI scores are generally lower in summer than in spring. This is because river levels are lower and water temperatures are higher.
The latest annual report (2009-2010) includes MCI results from 57 sampling sites, and also includes analysis of trends over the 16 years between 1995 and 2010. Results include:
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MCI scores have improved at 40 (77%) of the 52 sites surveyed in 2009-2010, with declines at 12 sites. These respective values were also better than in the previous year.
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Applying a strict statistical analysis of trends, seven sites show a very significant positive trend, 11 sites show a positive trend that is not definite enough to be described as “significant”, and no sites showed significant negative trends.
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Most of the improvements are occurring at mid-catchment sites, in the midst of areas of intensive land use.
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Overall, the decline between spring and summer in 2009-2010 was not as pronounced as in previous years.
As in previous years, the Stony River was in the poorest ecological health despite being one of the region’s “cleanest” rivers in terms of contamination from point or diffuse sources. This is because of the effects of erosion in its headwaters releasing sediments into the river. Similar erosion also had an impact on MCI scores in the upper reaches of the Maketawa, Waiwhakaiho, Katikara and Timaru streams and rivers.
Looking at trends over 15 years, waterways with both the strongest statistical evidence of improvement (‘there definitely is an improvement’) and the greatest change in ecological state (‘there is a definite improvement’) are:
- Kaupokonui Stream above Kapuni
- Timaru Stream
- Mangaoraka Stream
- Kapoaiaia Stream
- Mangati Stream
- Kurapete Stream
- Katikara Stream
The improvement at Kurapete Stream, Inglewood, reflects the diversion of wastewater from the treatment plant at Inglewood to the treatment plant in New Plymouth.
The improvements at Mangati Stream, Bell Block, reflect a great deal of work in the locality, including the pre-treatment of stormwater and industrial discharges by the New Plymouth District Council.
15-year trends
Click on map for large version.
The conclusion: Regioanl water quality in Taranaki is definitely improving.
For the full report, ee our environmental monitoring technical reports (scroll down to "Fresh Water Macroinvertebrate Fauna Biological Monitoring Programme").
Physical and chemical trends
The Council also monitors the physical and chemical characteristics of the region’s waterways. These measures indicate the pressures upon water quality and ecological health.
This physicochemical monitoring began in the mid-1990s, and the Council has built up a database which can now allow robust analysis of trends.
Council officers have analysed 15 years of data (1995-2010) from 11 monitored sites, and compared these trends with those revealed in an analysis of the most recent eight years of data (2002-2010). The latter period covers the time the Council's Regional Fresh Water Plan for Taranaki has been in effect.
The analyses include levels of nutrients, levels of bacteria and physicochemical parameters. Read more about physicochemical parameters
The conclusions refer to "stastically significant" and "statistically very significant" trends. In physicochemical analyses, a trend is regarded as significant if there is 95% confidence that the data represents a real trend and not just a random outcome. "Very significant" means there is 99% confidence that the trend is real.
The results of the latest trend analyses are presented graphically in the colour-coded chart below (click on each chart for a large version). A detailed report, which includes graphs and data supporting this chart, can be downloaded in PDF format.
Click on chart for big version.
Click on chart for big version.
Nutrients
Considering both the 15-year and eight-year trends, the following conclusions have been made:
- Most (88%) measures of nutrients at most monitored sites are stable or improving across the region.
- The 2002-2010 period has fewer deterioration trends in most nutrients across most sites and catchments than the 1995-2010 trend analysis. That is, where deterioration was occurring, it has largely (although not completely) been halted. The only exception is the Mangaoraka Stream at Corbett Rd, and a possible reason for this could be poor agricultural management practices (stock access) in the Awai Stream, a tributary immediately upstream of the monitoring site.
- The 15-year analysis shows a significant improvement in total nitrogen levels in the upper catchment sites of the Patea River, the Punehu Stream and the Stony River, while the eight-year analysis shows no significant change. That is, the earlier improvement is now being maintained.
- The eight-year analysis (2002-2010) shows a reduction in very significant mid-catchment deterioration trends, from 11 down to just one, which may indicate that there has been a stabilisation of nutrient levels in the latter years of the monitoring programme.
- With the exception of the Mangaoraka Stream, no site has more than one nutrient parameter (out of five measures) showing significant or very significant deterioration in the 2002-2010 period.
- There are no statistically very significant deterioration trends in nutrients at the upper catchment level sites during the 2002-2010 period and only one in the 1995-2010 period.
- Nitrate levels in the Waiwhakaiho River at SH3 and the Waingongoro at River Eltham Rd steadily increased from 1995 to about 2003, then decreased. Trends measured over the eight-year period from 2002 illustrate the decreases more prominently, as earlier increasing trend data is not included. However, the values are still relatively high when compared to the early monitoring data in the 1995 to 2003 years.
- The trend comparision reveals no reversal of early improvements in mid-catchment nutrient levels, and many cases where deterioration has been arrested. Other than for dissolved reactive phosphorus in the Waingongoro River, there have been no further statistically significant increases in any nutrients in the mid catchments in the past eight years.
Faecal coliforms and enterococci (bacteria)
- The analyses reveal no very significant deteriorations in faecal coliform trends at any of the monitored sites. However, the eight-year analysis shows a deterioration at the "significant" level in the Mangaoraka Stream at Corbett Rd and the Punehu Stream at SH45.
- Enterococci levels are stable throughout all the catchments except at the Mangaoraka Stream at Corbett Rd.
Physicochemical parameters
- Five sites show no deterioration for any parameter at any level of significance during the 2002-2010 period.
- The Mangaoraka Stream at Corbett Rd site has had more statistically very significant deterioration trends in the physical parameters than any other site in both periods, followed by the Waingongoro River at SH45 site.
- Biological oxygen demand (BOD) is generally not an issue in Taranaki waterways, . However, the eight-year trend analysis reveals a very significant deterioration at the Waingongoro River at SH45 site and significant deterioration at the Mangaoraka Stream at Corbett Rd the Punehu stream at SH45 site over the 2002-2010 period. The 15-year analysis reveals only one significant deterioration, at the Waingongoro at SH45. The BOD5 levels, although deteriorating, remain at low levels and are not cause for concern in Taranaki.
Conclusions
- The Council's objective is to "maintain and enhance the quality of the surface water resources of Taranaki".
Issue 6.2, Regional Fresh Water Plan for Taranaki (53 KB) - Most water quality measures are stable or improving at most sites in Taranaki. Of 13 measures at the 11 sites discussed here (143 possible trends), 126, or 88%, show either improvement or no change in the past eight years.
- The eight-year period shows far fewer statistically very significant deterioration trends than the 15-year period, particularly for nutrients at the middle catchment sites. This is probably due to a range of factors such as stabilisation of water quality in recent years, a shorter time period and/or absence of higher nutrient concentrations, which were apparent in the early monitoring history (1995-2004).
- The 15-year period is more robust than the eight-year period for trending purposes, so care must be taken when comparing trends in the eight-year period. The latter may be taken out of context when not evaluated in conjunction with the 15-year period.
Downloads
Trends in physicochemical water quality data: a comparison of 1995-2010 with 2002-2010 (553 KB)
Freshwater Physicochemical Programme State of the Environment Monitoring Annual Report 2009-2010 (1.3 MB)
What do we mean by 'physicochemical parameters'?
The physical parameters used to help determine water quality include clarity and temperature, conductivity and pH (acidity) levels, and the levels of oxygen and biological oxygen demand, or BOD.
Clarity
Water clarity is determined by measuring transparency and turbidity. Transparency is measured in the stream using a black disc, while turbidity is a laboratory measurement (expressed as NTUs) that is related to the suspended material in the water.
Conductivity and pH
Conductivity, a measure of the ability of water to conduct electricity, provides a useful indication of dissolved solids in surface waters reflecting the geology and aspects of run-off and discharges within catchments. Surface water pH levels are influenced by geology, but are also affected by the photosynthetic activity of periphytons (slimes and algae found on riverbeds) particularly in warmer summer low-flow months.
Dissolved oxygen and BOD
Well oxygenated water is an important requirement for aquatic life and is a feature of the short, steep ring plain streams descending from the slopes of Mt Taranaki/Egmont. Biochemical oxygen demand (BOD) is a measurement of the amount of oxygen consumed in the breakdown of organic matter. BOD in uncontaminated waters is generally lower than one gram per cubic metre.