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

River and stream water quality and ecology in the Wellington region: State and trends
Appendix 5: Water quality indices
Greater Wellington’s WQI
Greater Wellington’s WQI was first presented in Milne and Perrie (2005), derived from
the median values of the following six variables: visual clarity (black disc), dissolved
oxygen (DO, as % saturation), dissolved reactive phosphorus (DRP), ammoniacal
nitrogen (Amm N), nitrate nitrogen and faecal coliforms (see Milne and Perrie (2005)
for the rationale behind the selection of these six variables). The WQI used in
subsequent reports, including this report, differs slightly in that nitrite-nitrate nitrogen
(NNN) and E. coli are used in place of nitrate nitrogen and faecal coliforms,
respectively. These changes relate to changes in RSoE analytes and have little material
effect on the WQI; nitrite nitrogen concentrations are usually below detection in surface
waters while E. coli counts are generally similar to (or a little lower than) faecal
coliform counts.
Greater Wellington’s WQI is for comparative purposes rather than an absolute measure
of water quality. Because the WQI is based on median values (ie, 50 % compliance),
sites awarded the same water quality grade may exhibit varying degrees of compliance
(from 51 to 100%) with the different guideline values. Therefore to differentiate
between sites within a water quality class, in this report sites within each WQI grade
were ranked according to the sum of guideline compliance across all six water quality
variables (eg, an ‘excellent’ site that complied with DO%, visual clarity, DRP, Amm N,
NNN and E. coli guidelines on 90%, 80%, 90%, 90%, 70% and 75% of sampling
occasions (respectively) had an overall compliance ‘score’ of 495 and so ranked higher
than an ‘excellent’ site with an overall compliance score of 482).
Canadian WQI
As noted in Section 4.1.1(a), the Canadian WQI was also trialled in this report. The
opportunity was taken at this time to revisit Greater Wellington’s existing suite of WQI
variables (see Table 4.1). This led to the addition of water temperature into the index
and a re-arrangement of the nitrogen species, with both dissolved inorganic nitrogen and
nitrate toxicity incorporated into the index. In addition, the threshold for E. coli was
amended from 100 cfu/100mL to 550 cfu/100mL; as noted in Section 4.1.1, 100
cfu/100mL is considered a conservative threshold for stock drinking water.
Information on the calculations behind the Canadian WQI is outlined below, drawn on
material from CCME (2001). Because Greater Wellington’s WQI was used in this
report for the formal assignment of a water quality grade to each RSoE site, the index
scores generated from the Canadian WQI were only interpreted through assignment of
RSoE sites into quartiles (see Section 4.1.13). Further work is needed to determine the
thresholds for grades of ‘excellent’, ‘good’, ‘fair’ and ‘poor’ using the Canadian WQI.
The Canadian WQI is based on three elements:
Scope: the number of variables that do not meet the assigned compliance thresholds
(known as the objectives) on at least one sampling occasion.
Frequency: the frequency with which individual sample results fail to meet the
assigned compliance thresholds.
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