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

River and stream water quality and ecology in the Wellington region: State and trends
As well as a comparison against the guideline values listed in Table 4.1, the
River Environment Classification (REC) was also used to help interpret water
quality across the 55 RSoE sites. The main REC class used was landcover (see
Appendix 2). Information on the proportion of different landcover types in
each catchment was derived from MfE (2010).
(a) Water quality indices
Two water quality indices are presented in Section 4.1.13 to summarise overall
water quality and facilitate inter-site comparisons. First, to provide a measure of
consistency and allow comparisons with earlier technical reports on river water
quality in the Wellington region, Greater Wellington’s water quality index (GW-
WQI) developed by Milne and Perrie (2005) – and adapted by Perrie (2007) – was
applied to median results from the 55 RSoE sites. The second index uses the
methodology defined in the Canadian Water Quality Index (CCME 2001).
CCME-type indices have already been trialled in several regions (eg, Auckland
and Southland); more recently, this index has been recommended for further
investigation in relation to national reporting of river water quality (Hudson et al.
2011). Further background and information on the application of the two water
quality indices are provided in Appendix 5. The water quality variables and
associated threshold values used in each index are listed in Table 4.1.
(b) General data processing, analysis and presentation
Data were initially screened for outliers and where there was cause for concern
around the quality of a data value (eg, the field meter performance had been
questioned by the field staff undertaking the sampling), these values were
removed. During data processing, any water quality variables reported as less
than or greater than detection limits (ie, censored data) were replaced by values
one half of the detection limit or the detection limit respectively (eg, a value of
400 became 400).
Considerable use of scatter plots and box-and-whisker plots (box plots) is made
in this section to graphically summarise the median and range of measurements
for various water quality variables and compare these between sampling sites
and/or major REC landcover classes. All plots were generated in SigmaPlot
(version 11.0). In terms of interpretation of the box plots:
the lower and upper boundaries of the box represent the 25 th percentile and
75 th percentile of the data set, respectively (a minimum of three data points
are needed to generate the box);
the horizontal line within the box represents the median value;
the ‘whiskers’ (error bars) extending above and below the box
(interquartile range) represent the 90 th and 10 th respectively; and
percentile values,
the black dots represent outliers.
4.1.2 Summary statistics and compliance with guideline values – overview
Summary statistics for selected ‘core’ water quality variables over the three-year
period examined are presented for each RSoE site in Table 4.2. Compliance with
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