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3 Water Quality State (2005)

Median values for 2005 at each site were calculated based on monthly raw data (January-December) for all parameters.

In the series of figures below, current water quality state is illustrated by overlaying site medians onto maps of New Zealand with the background coloured according to the land cover level of the River Environment Classification (REC; Snelder and Biggs, 2002). Land cover is expressed in three classes: pastoral land cover in green, natural land cover (ie, indigenous forest, unimproved tussock, scrub and bare mountain) in brown, and exotic forest in grey.

To simplify the maps, the site medians have been separated into six groups corresponding to the following percentile bands: <5th percentile, 5-20th, 20-50th, 50-80th, 80-95th and >95th percentile.

In the figure for flow, varying bubble sizes are used to represent the different percentile groups. For all other parameters the percentiles are indicated by a change in colour, with blue representing relatively good water quality and red representing relatively poor water quality.

Figure 3.1: River flow (m3 s-1) (2005 medians expressed as percentile bands)

This figure is described in the text.

River flows are shown overlain on a network of our larger rivers (Strahler order 6 and higher). The Waitaki and Clutha are our largest rivers. Across all 77 sites the median flow in 2005 was 26.7 m3 s-1.

Figure 3.2: Temperature (°C) (2005 medians expressed as percentile bands)

This figure is described in the text.

Not surprisingly there are strong latitudinal and altitudinal gradients in water temperature patterns across the country. Sites in Northland (Waitangi and Mangakahia rivers) and Gisborne (Waipaoa) had the highest water temperatures. The coolest rivers were the Motueka, Wairau and Haast. National median river temperature was 12.9°C.

Figure 3.3: Conductivity (µS cm-1) (2005 medians expressed as percentile bands)

Thumbnail of image. See the figure at its full size (including text description).

There is a strong North Island versus South Island difference in conductivity, probably reflecting the marked geological differences between the North and South Islands. National median for conductivity was 95 µS cm-1.

Figure 3.4: pH (2005 medians expressed as percentile bands)

This figure is described in the text.

Median pH was highest in the Clutha River at Luggate, the lower Hurunui and Ngaruroro rivers, and the Waikohu River (Gisborne). Median pH was lowest in the upper Oreti River, upper Waipa River and lower Rangitaiki. The national median was 7.7 and no rivers had median pH < 7.0 in 2005.

Figure 3.5: Dissolved Oxygen (% saturation) (2005 medians expressed as percentile bands)

This figure is described in the text.

Most rivers around New Zealand are well oxygenated (national median = 101%). However, sites in Northland and Auckland, and the lower Tarawera River had the lowest levels of dissolved oxygen. It is worth noting that monthly measurements of dissolved oxygen provide a relatively poor representation of conditions as there are strong diurnal cycles in DO in many rivers.

Figure 3.6: Visual clarity (m) (2005 medians expressed as percentile bands)

This figure is described in the text.

In 2005, the national median visual clarity was 2.85 m. Highest clarity sites included lake outlet sites (eg, Monowai River, Clutha at Luggate) and headwater sites (eg, upper Motueka and Ngaruroro rivers). The lower Manawatu, Waitara and Waipaoa rivers had the lowest annual median water clarity for 2005.

Figure 3.7: NOX-N (mg m-3) (2005 medians expressed as percentile bands)

This figure is described in the text.

Rivers in Southland (Mataura, Oreti), Taranaki (Waingongoro) and Waikato (Waihou) regions had the highest median NOx-N concentrations in 2005. In contrast, sites draining lakes (eg, Tarawera River below Lake Tarawera, Waitaki below Waitaki hydrolakes, Waikato below Lake Taupo) had very low NOx-N concentrations. The national median was 115.5 mg m-3.

Figure 3.8: NH4-N (mg m-3) (2005 medians expressed as percentile bands)

Thumbnail of image. See the figure at its full size (including text description).

NH4-N concentrations were low in most rivers (2005 national median = 4 mg m-3). Exceptions were sites on the lower reaches of Waingongoro, Manawatu, Waihou and Tarawera rivers.

Figure 3.9: TN (mg m-3) (2005 medians expressed as percentile bands)

This figure is described in the text.

Spatial patterns for TN were similar to those for NOx-N. National median concentration of TN were 279.5 mg m-3 in 2005. Sites in the South Island high country (eg, upper Wairau and Motueka rivers) tended to have the lowest concentrations.

Figure 3.10: DRP (mg m-3) (2005 medians expressed as percentile bands)

This figure is described in the text.

DRP concentrations tended to be higher at North Island sites than South Island sites, probably reflecting differences in geology (cf. conductivity results). Highest DRP concentrations were observed in the lower Manawatu, Waingongoro, Tarawera and Waihou rivers (where NH4-N levels were also high). National median concentration in 2005 was 4.95 mg m-3.

Figure 3.11: TP (mg m-3) (2005 medians expressed as percentile bands)

This figure is described in the text.

TP concentrations show a similar pattern to those for DRP although the Waipaoa River joins the group of rivers with highest concentrations in 2005. This probably reflects the high sediment load carried by the Waipaoa River (cf. clarity map above). National median concentration for TP in 2005 was 16 mg m-3.

Figure 3.12:E. coli (MPN 100 ml-1) (2005 medians expressed as percentile bands)

This figure is described in the text.

2005 was the first year for analysis of E. coli from NRWQN samples. Levels of E. coli tend to be higher in the North Island and especially in low elevation areas. North Island Central Plateau and South Island high-country sites had the lowest levels of E. coli. National median for 2005 was 49 MPN 100 ml-1.

Site median values for all parameters (2005 data only) were correlated with percent pastoral land cover in the catchment above each monitoring site. Land-cover data was based on flow-weighted percentage contributing area from LCBD (1997 version; see www.mfe.govt.nz).

Some of the river sites (eg, lower Waingongoro, Manawatu and Mataura rivers) have significant point source discharges upstream. This may introduce a source of error to correlations based solely on percent pastoral land use in the catchment, although rivers with major point sources are likely to have highly developed catchments.

There were significant correlations between percent pastoral land cover and median values for all but three of the parameters summarised above (Table 3.1). The strongest correlations (rS > 0.70) were for the three nitrogen species, total phosphorus and E. coli (Table 1.1). Flow, pH and % DO were not significantly correlated with pastoral land cover.

Table 3.1: Correlations for water quality parameters (annual median for 2005) versus percent pastoral land cover

Parameter

% pastoral land cover

Flow

-0.21

Temperature

0.50***

Conductivity

0.55***

pH

-0.19

Dissolved oxygen

-0.17

Visual clarity

-0.60***

Oxidised nitrogen

0.71***

Ammoniacal nitrogen

0.77***

Total nitrogen

0.84***

Dissolved reactive phosphorus

0.67***

Total phosphorus

0.74***

E. coli

0.79***

Note: '***' = very highly significant (P <0.001) and relate to the annual median for 2005.

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