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

River and stream water quality and ecology in the Wellington region: State and trends
(e) E. coli
Thirteen RSoE sites exhibited meaningful trends in raw and/or flow-adjusted
E. coli counts over the reporting period (Table 4.13). Increasing trends were
observed in the raw data from seven sites, with sites on the Kopuaranga
and Whangaehu rivers recording the largest rates of change (43 and 85
cfu/100 mL/year, respectively). However, the flow-adjusted analysis (which
was limited to six of the seven sites) demonstrated that only two of these sites
recorded a corresponding increasing trend in E. coli counts (ie, the increasing
counts at most sites, although indicative of a deterioration in water quality, can
be attributed to river flow conditions). Further, the two sites that did also
display increasing trends in flow-adjusted E. coli counts (Totara Stream at
Stronvar and Waiohine River at Gorge) had very low median values (5 cfu/100mL),
indicating that the increasing trends are of little environmental consequence. In
contrast, the magnitude of change was more notable for the two sites that
exhibited decreasing trends in both raw and flow-adjusted E. coli counts
(Akatarawa River at Hutt confluence and Waipoua River at Colombo Road); E.
coli counts at these sites decreased in the order of 10 cfu/100mL/year between
July 2005 and June 2011 (Table 4.13). The reason for the improvements is
unclear; the median values were relatively low for the reporting period and
meet the ANZECC (2000) stock water TV.
Table 4.13: Summary of meaningful trends (ie, p1%
per year) in E. coli counts (cfu/100mL) recorded at RSoE sites over July 2006 to
June 2011. Bolded median values exceed the relevant ANZECC (2000) lowland
TV (refer Table 4.1). MASS=median annual Sen slope (cfu/100mL/yr) calculated
using the Seasonal Kendall test. The arrow indicates the direction of the trend
Site Site name Median n MASS
Raw data Flow-adjusted data
Rate of
change
(%/year)
Trend n MASS
Rate of
change
(%/year)
RS03 Waitohu S at Forest Park 6 No trend 57 -2.30 -38.3 ↓
RS09 Waikanae R at Mangaone Wk 13 No trend 60 -2.31 -18.5 ↓
RS14 Pauatahanui S at Elmwood Br 255 60 28.30 11.1 ↑ No trend
RS22 Hutt R at Boulcott 105 60 -14.47 -13.8 ↓ No trend
RS23 Pakuratahi R 50m d/s Farm Ck 66 60 14.84 22.3 ↑ No trend
RS25 Akatarawa R at Hutt R confl. 56 60 -10.84 -19.4 ↓ 60 -10.54 -18.8 ↓
RS29 Wainuiomata R u/s of White Br 100 No trend 59 -26.14 -26.1 ↓
RS34 Ruamahanga R at Pukio 105 60 25.22 24.0 ↑ No trend
RS38 Kopuaranga R at Stewarts 210 60 43.32 20.6 ↑ No trend
RS39 Whangaehu R 250 u/s confl. 285 60 84.71 29.7 ↑ Not assessed (no flow data)
RS40 Waipoua R at Colombo Rd Br 73 60 -11.49 -15.8 ↓ 52 -10.56 -19.2 ↓
RS44 Totara S at Stronvar 5 57 1.44 28.8 ↑ 57 2.76 55.2 ↑
RS47 Waiohine R at Gorge 5 60 0.99 22.0 ↑ 60 1.19 26.5 ↑
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Trend
(f) National River Water Quality Network (NRWQN) sites
Just three meaningful trends were observed across the five NRWQN sites for
the six water quality variables examined (Table 4.14). Only the increase in
DRP concentration of 0.0002 mg/L/year at Hutt River at Kaitoke (a reference
site) was evident in both raw and flow-adjusted data sets. No such equivalent
trend was observed at Greater Wellington’s site further downstream at Te