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

River and stream water quality and ecology in the Wellington region: State and trends
Table 4.8: Summary of RSoE sites that recorded meaningful trends (ie, p1% per year) in visual clarity (m) measurements over July
2006 to June 2011. MASS=median annual Sen slope (m/year) 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)
RS04 Waitohu S at Norfolk Cres 0.61 60 -0.068 -11.2 ↓ No trend
RS08 Ngarara S at Field Way 0.42 60 -0.060 -14.4 ↓ Not assessed (no flow data)
RS11 Whareroa S at Waterfall Rd 0.62 60 -0.110 -17.8 ↓ 57 -0.064 -10.3 ↓
RS13 Horokiri S at Snodgrass 1.88 60 -0.266 -14.1 ↓ 59 -0.174 -9.5 ↓
RS28 Wainuiomata R at Manuka Track 2.60 56 -0.197 -7.6 ↓ 56 -0.273 -10.5 ↓
RS36 Taueru R at Castlehill 1.11 60 -0.091 -8.2 ↓ No trend
RS39 Whangaehu R 250 u/s confl. 0.72 60 -0.131 -18.2 ↓ Not assessed (no flow data)
RS49 Beef Ck at Headwaters 2.30 58 -0.306 -13.4 ↓ Not assessed (no flow data)
WGN_DOCS-#1100598-V3 PAGE 47 OF 160
Trend
Table 4.9: Summary of RSoE sites that recorded meaningful trends (ie, p1% per year) in turbidity (NTU) over July 2006 to June 2011.
MASS=median annual Sen slope (NTU/year) 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)
RS15 Porirua S at Glenside 2.20 No trend 60 -0.343 -15.6 ↓
RS27 Waiwhetu S at Wainui Hill Br 6.59 59 -1.025 -15.6 ↓ 59 -1.023 -15.5 ↓
RS29 Wainuiomata R u/s of White Br 2.60 No trend 59 -0.285 -11.0 ↓
RS32 Ruamahanga R at Te Ore Ore 4.35 No trend 60 -0.438 -10.1 ↓
RS33 Ruamahanga R at Gladstone Br 3.20 No trend 60 -0.136 -4.3 ↓
RS36 Taueru R at Castlehill 3.54 60 0.259 7.3 ↑ No trend
RS39 Whangaehu R 250 u/s confl. 4.78 60 0.921 19.3 ↑ Not assessed (no flow data)
Trend
The reasons for the observed trends in visual clarity and turbidity are not
known. It is noted that of the eight sites that exhibited declining visual clarity
trends, only the two reference sites (Wainuiomata River at Manuka Track and
Beef Creek at Headwaters) returned median clarity values (for the three-year
state period) above the MfE (1994) guideline of 1.6 m. It is possible that the
declining trend in visual clarity reported for Beef Creek may be due to natural
changes in the sampling reach morphology – during the latter part of the
reporting period there were a number of occasions when visual clarity
exceeded the sample reach available (ie, actual water clarity was greater than
that recorded).
In the case of the Horokiri Stream at Snodgrass, while the five-year median
listed in Table 4.9 met the MfE (1994) guideline, the more recent state analysis
suggests visual clarity at this site has continued to decline (see Table 4.2).
Soil/stream bank erosion and sediment runoff from forestry tracking activities
are possible reasons for the decline; Oliver and Milne (2012) note steep and
unstable soils are a feature of this catchment.