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4 Changes in Water Quality State (1997–2002) and Land Use Differences

4.1 Oxidised nitrogen

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Key findings

  • Concentrations of NOx-N increased sharply in the most-enriched pastoral sites (ie, the 95th percentile) between 1997 and 2002. This pattern mirrors that found by Scarsbrook (2006) for the most highly enriched sites in the NRWQN, and provides further evidence that our most enriched streams are deteriorating.

  • Urban sites had the greatest percentage exceedence of ANZECC (2000) guidelines. Around 50% of pastoral sites exceeded the guideline value for NOx.

4.2 Ammoniacal nitrogen

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Key findings

  • Concentrations of NH4-N in ‘average’ rivers were relatively stable in all three land cover classes over the 1997–2002 period. In contrast, there was a sharp decrease in concentrations in the most enriched urban streams. Again, this is consistent with improvements in point source discharge management, and mirrors similar improving trends in the National River Water Quality Network (Scarsbrook 2006).

4.3 Dissolved reactive phosphorus

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Key findings

  • Concentrations of DRP in ‘average’ rivers (ie, 50th percentile) draining urban and natural catchments decreased over the 1997–2002 period, whereas DRP increased slightly in Pastoral rivers. Note: The spike in 95th percentile DRP concentrations appear to be due to very high values in Mazengarb Drain, an urban tributary of Waikanae River.

  • The percentage of sites exceeding ANZECC and ARMCANZ (2000) guidelines trended downwards at urban sites during the 1997–2002 period. This may reflect improvements in point source discharge management. Many of the sites sampled, including a relatively high proportion of sites in natural catchments, exceeded the ANZECC guidelines.

4.4 Black disk clarity

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Key findings

  • There was little change in the percentage of sites exceeding the ANZECC and ARMCANZ (2000) recreational guidelines with regard to clarity, and little change over time in the values seen in ‘average’ and ‘dirty’ rivers.

4.5 E. coli

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Table 8: Summary giving average values across five years (1998–2002) for median, 5th and 95th percentile values (see Table 7)

Summary

Median

5th

95th

% >MfE/MoH

Pastoral

1,178.8

105.2

13,995.6

67

Natural

209.4

7.4

5,405.6

29

Urban

2558

920

7714

85

Note: This provides a long-term average of summary data (5th, 50th and 95th percentiles) for E. coli concentrations across all sites. E. coli concentrations for individual sites are calculated as the 95th percentile of values for each year.

Key findings

  • On average, 85% of urban sites, 67% of pastoral and 29% of natural sites exceeded the MfE/MoH (2003) freshwater recreation guidelines (ie, a large proportion of sites had 95th percentile concentrations greater than 550 E. coli per 100 ml, and hence, would be graded as ‘poor’ or ‘very poor’ for recreational uses). There was some indication of improvements in E. coli concentrations at ‘average’ urban sites, but there was a high level of variability in the data, which makes interpreting patterns over time rather uncertain.

  • In another national scale assessment, Scarsbrook and McBride (2004)1 found that 92% of urban sites, 85% of pastoral and 50% of natural sites exceeded the MfE/MoH (2003) freshwater recreation guidelines. Both the current study and Scarsbrook and McBride used the same compiled dataset. The variation in results of the two studies can be attributed to differences in site selection criteria and the way data was analysed (eg, values from the current study were averages across several years, whereas Scarsbrook and McBride (2004) summarised data across all years). Results from both studies show the same gradient in relative water quality across urban, pastoral and natural sites and also indicate that a majority of sites in urban and pastoral fail the freshwater recreation guidelines with respect to E. coli.

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