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Monitored river water quality

Nitrate concentrations at monitored sites are getting worse in some areas

Median nitrate-nitrogen concentration over the 2009–13 period was 18 times higher in the urban class and 10 times higher in the pastoral class, compared with the native class (Larned et al, 2015).

For the 175 monitoring sites in the pastoral class, trends in nitrate-nitrogen were worsening at 61 percent, improving at 22 percent, or indeterminate at 17 percent of sites between 1994 and 2013 (see figure 11). Similar results were obtained over a shorter period (2004–13), where trends in nitrate-nitrogen for 340 sites in the pastoral class were worsening at 39 percent, improving at 23 percent, or indeterminate at 39 percent of sites.

Figure 11

trends in nitrate-nitrogen concentrations at monitored river sites grouped by land-cover class between 1994 and 2013
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This graph shows trends in nitrate-nitrogen concentrations at monitored river sites grouped by land-cover class between 1994 and 2013. Visit the MfE data service for the full breakdown of the data.

Note: Monitored sites are classified by land cover class (eg pastoral). However, monitored sites can be downstream of a mix of land covers within the catchment, meaning that the water quality at a particular site can be influenced by different land covers.

For more detail see Environmental indicators Te taiao AotearoaRiver water quality: nitrogen [Stats NZ].

Phosphorus concentrations at monitored sites in some urban and pastoral rivers are improving

Median dissolved reactive phosphorus concentration over the 2009–13 period was 3 times higher in the urban class and 2.5 times higher in the pastoral class, compared with the native class (Larned et al, 2015).

For the 145 monitoring sites in the pastoral class, trends in dissolved reactive phosphorus were improving at 46 percent, worsening at 21 percent, or indeterminate at 34 percent of sites between 1994 and 2013 (see figure 12). Similar results were obtained over a shorter period (2004–13), where trends in dissolved reactive phosphorus for 277 sites in the pastoral class were improving at 57 percent, worsening at 15 percent, or indeterminate at 29 percent of sites.

A number of factors have been explored that may be associated with improvements in dissolved reactive phosphorus concentrations over time. In urban areas, one of these may be due to improved wastewater treatment (Julian et al, 2017).

In rural areas, the following factors may have contributed to improvements:

  • Various strategies, in addition to those such as stock exclusion, developed since 2003 to mitigate phosphorus loss from land to water (eg McDowell & Nash, 2012).
  • The targeting of critical source areas of contaminant loss since 2008 in 77 documents (industry guidelines, farm environment plans, and regional policy). A farm or catchment accounts for the majority of contaminant loss to water. Targeting critical source areas improves the cost-effectiveness of strategies (McDowell, 2014).
  • Improved education of farm consultants, fertiliser company representatives, and regional council staff since 2002 on mitigating phosphorus loss, for example through Massey University’s Intermediate Sustainable Nutrient Management course.

The following factors may have contributed to improving phosphorus concentrations at specific sites, but there is insufficient evidence of large-scale improvements:

  • Decreased sales of phosphorus fertiliser. However, sales have recovered since the 2008 price spike.
  • Reduced soil Olsen phosphorus concentrations.
  • Increased sales of low water-soluble phosphorus fertilisers (fertilisers less likely to cause phosphorus loss in run-off) (McDowell et al, 2010).
  • An increase in nitrogen concentrations causing phosphorus uptake in periphyton.
  • Twenty-one percent of sites with increasing nitrate-nitrogen concentrations had decreasing concentrations of dissolved reactive phosphorus (1994–2013; Larned et al, 2015). Less phosphorus loss in leachate from grasslands was noted where nitrogen use increased (Dodd et al, 2014).
  • An increase in nitrate-nitrogen concentrations in groundwater causing the oxidation of iron, which sorbs phosphorus in streams. However, only 1 groundwater site (out of 22) that had improving dissolved reactive phosphorus trends also had worsening nitrate-nitrogen concentrations (2005–14; analysed by Ministry for the Environment and Stats NZ).

Figure 12

trends in dissolved reactive phosphorus concentrations at monitored river sites grouped by land-cover class between 1994 and 2013
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This graph shows trends in dissolved reactive phosphorus concentrations at monitored river sites grouped by land-cover class between 1994 and 2013. Visit the MfE data service for the full breakdown of the data.

Note: Monitored sites are classified by land cover class (eg pastoral). However, monitored sites can be downstream of a mix of land covers within the catchment, meaning that the water quality at a particular site can be influenced by different land covers.

For more detail see Environmental indicators Te taiao AotearoaRiver water quality: phosphorus [Stats NZ].

E.coli trends at monitored sites are largely indeterminate

Median E.coli concentration over the 2009–13 period was 22 times higher in the urban class and 9.5 times higher in the pastoral class, compared with the native class (Larned et al, 2015).

For 268 monitoring sites in the pastoral class, trends in E.coli were indeterminate at 65 percent, improving at 21 percent, and worsening at 14 percent of sites between 2004 and 2013 (see figure 13).

Figure 13

trends in E.coli concentrations at monitored river sites grouped by land-cover class between 2004 and 2013
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This graph shows trends in E.coli concentrations at monitored river sites grouped by land-cover class between 2004 and 2013. Visit the MfE data service for the full breakdown of the data.

Note: Monitored sites are classified by land cover class (eg pastoral). However, monitored sites can be downstream of a mix of land covers within the catchment, meaning that the water quality at a particular site can be influenced by different land covers.

For more detail see Environmental indicators Te taiao AotearoaRiver water quality: Escherichia coli [Stats NZ].

Visual clarity of rivers is better at monitored sites in native areas

Median visual clarity over the 2009–13 period was approximately two times greater in the native class, compared with the pastoral and urban classes (Larned et al, 2015).

For 70 percent of monitoring sites in the native class, median visual clarity was two metres or more (84 of 120 sites), compared with 15 percent of sites in the pastoral class (46 of 308 sites) (see figure 14).

Figure 14

median river visual clarity at monitored river sites grouped by land-cover class for the period 2009–13
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This graph shows median river visual clarity at monitored river sites grouped by land-cover class for the period 2009–13. Visit the MfE data servicee for the full breakdown of the data.

Note: Monitored sites are classified by land cover class (eg pastoral). However, monitored sites can be downstream of a mix of land covers within the catchment, meaning that the water quality at a particular site can be influenced by different land covers.

For the 130 monitoring sites in the pastoral class, trends in visual clarity were worsening at 46 percent, improving at 33 percent, or were indeterminate at 21 percent of sites between 1994 and 2013. However, over a shorter period (2004–13) trends in visual clarity across 241 sites in the pastoral class were improving at 31 percent, worsening at 17 percent, or were indeterminate at 51 percent of sites.

For more detail see Environmental indicators Te taiao AotearoaRiver water quality: clarity [Stats NZ].

Macroinvertebrate community index scores were excellent or good at most monitored sites

For 512 monitored river sites, median macroinvertebrate community index scores (2009–13) were excellent or good at 63 percent, fair at 26 percent, and poor at 11 percent of sites (Larned et al, 2015). For 308 sites in the pastoral land-cover class, 54 percent had scores that were excellent or good, compared with 90 percent of 152 sites classed as native (see figure 15).

Figure 15

shows median macroinvertebrate community index scores at monitored river sites grouped by land-cover class for the period 2009–13
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This graph shows median macroinvertebrate community index scores at monitored river sites grouped by land-cover class for the period 2009–13. Visit the MfE data service service for the full breakdown of the data.

We assessed 10-year trends (2004–13) in median macroinvertebrate community index scores (Larned et al, 2015), and found that 83 percent of 462 monitoring sites were indeterminate, with 5 percent of sites improving and 13 percent of sites worsening.

For more detail see Environmental indicators Te taiao AotearoaRiver water quality: macroinvertebrate community index [Stats NZ].

Zinc and copper concentrations are elevated in some urban streams

Run-off from roofs and roads to rivers and stormwater systems contains heavy metals, such as copper and zinc. Results from monitoring urban streams for water quality in Auckland, Wellington, and Christchurch indicated positive relationships between median concentrations of copper and zinc and the proportion of urban land cover in the upstream catchment (see figure 16 and figure 17; Gadd, 2016).

We did not have enough data to determine trends in copper concentrations for 12 of 14 monitoring sites in Auckland, Wellington, and Christchurch for the eight-year period 2008–15 (Gadd, 2016). Trends in zinc concentrations were improving at six sites, worsening at two sites, and were indeterminate at six sites over the same period.

Figure 16

median zinc concentration by proportion of urban land cover in the upstream catchment for the period 2013–15
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This graph shows median zinc concentration by proportion of urban land cover in the upstream catchment for the period 2013–15. Visit the MfE data service for the full breakdown of the data.

Note: A site with greater than 15 percent urban land cover in the upstream catchment is considered ‘urban’.

Figure 17

median copper concentration by proportion of urban land cover in the upstream catchment for the period 2013–15
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This graph shows median copper concentration by proportion of urban land cover in the upstream catchment for the period 2013–15. Visit the MfE data service for the full breakdown of the data.

Note: A site with greater than 15 percent urban land cover in the upstream catchment is considered ‘urban’.

For more detail see Environmental indicators Te taiao AotearoaUrban stream water quality [Stats NZ].