River and stream water quality and ecology - Greater Wellington ...

River and stream water quality and ecology in the Wellington region: State and trends
Marua. The decreasing trend in TN concentrations observed for Ruamahanga
River at Mt Bruce is broadly consistent with the RSoE network findings;
eleven RSoE sites, including two sites on the Ruamahanga River (although not
Ruamahanga River at McLays which is located closest to NIWA’s Mt Bruce
site), exhibited meaningful declining trends in TN concentrations between July
2005 and June 2011 (refer Table 4.11).
Table 4.14: Summary of meaningful trends (ie, p1%
per year) in nutrient concentrations (mg/L) recorded at NRWQN sites over July
2006 to June 2011. MASS=median annual Sen slope (mg/L/yr) calculated using
the Seasonal Kendall test. The arrow indicates the direction of the trend
Site name Variable Median n MASS
Raw data Flow-adjusted data
Rate of
change
(%/year)
Trend Median n MASS
Rate of
change
(%/year)
Ruamahanga R
at Waihenga
Amm. N 0.009 60 -0.001 -14 ↓ No trend
Ruamahanga R
at Mt Bruce
TN No trend 0.090 60 -0.004 -4.2 ↓
Hutt R at Kaitoke DRP 0.003 60 0.0002 4.4 ↑ 0.003 60 0.0002 6.5 ↑
4.3 Synthesis
Analysis of physico-chemical and microbiological water quality data collected
at 55 RSoE sites over the three-year period July 2008 to June 2011 indicates
that more than half of the river and stream sites monitored in the Wellington
region currently have ‘good’ or ‘excellent’ water quality (according to Greater
Wellington’s WQI). Sites classified in this way tend to be located on the upper
reaches of rivers and streams that drain the Tararua, Rimutaka and Aorangi
ranges (ie, their catchments are dominated by indigenous forest cover).
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Trend
Water quality is strongly linked with upstream catchment landcover and land
use. Broadly speaking, as the proportion of pastoral and/or urban landcover
increases within a site’s upstream catchment, water quality tends to decline. Of
the 15 sites graded ‘poor’ for water quality, five are located in urban areas and
ten drain predominantly pastoral catchments (of which most support at least
some intensive agriculture, typically dairying). One of the poorest sites,
Mangatarere Stream at SH 2, also receives treated wastewater from Carterton
township. Analysis of monitoring data from this site indicates that the WWTP
discharge is having a notable impact on water quality, principally through
increased DRP and Amm N concentrations. Although more ‘subtle’ in the
monitoring record, Ruamahanga River at Gladstone also shows signs of
impacts from municipal wastewater (from the Masterton WWTP discharge
approximately 10 km upstream). This site exceeded the ANZECC (2000)
lowland TVs for DRP and Amm N on 97% and 39% of sampling occasions,
respectively. While diffuse source inputs likely account for the majority of the
nutrient load at this site, analysis by Ausseil (2011) indicates that soluble
nutrient concentrations are also elevated during lower river flows (ie, dry
weather conditions).