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

River and stream water quality and ecology in the Wellington region: State and trends
4.2.2 Results
An overview of the temporal trend analysis outputs, for both raw and flowadjusted
data sets that contained 30% or less censored values, is presented in
Table 4.7. Full details of all other trend analysis results can be found in
Appendix 7.
The majority of RSoE sites were typically stable across the five-year period,
based on both raw and flow-adjusted data for the 14 variables assessed.
However, pH was an exception, with around half of the sites exhibiting
decreasing trends over the reporting period. The reason for the declining pH
trends is unclear but it should be noted that there were some changes in
sampling personnel (and field meters) during the early part of the reporting
period. 18 Further, along with water temperature and DO which also exhibit
significant diurnal variation, changes in site sampling time during the reporting
period confound interpretation of the trend results. The focus of the following
sections is therefore placed on describing the meaningful trends (ie, statistically
significant and with a rate of change >1% per annum) observed in water
clarity, turbidity, nutrient concentrations, TOC concentrations, and E. coli
bacteria counts. A total of 78 meaningful trends were identified in the raw data
sets across these nine variables (equating to 14.1% out of a total of 411
possible trends), with half of these trends associated with just three variables:
TP (15 sites), NNN (13 sites) and TN (11 sites).
(a) Visual clarity (black disc) and turbidity
Eight RSoE sites exhibited meaningful declining trends in raw black disc
(visual clarity) values, three of which also exhibited these trends in flowadjusted
data (Table 4.8). Rates of decline ranged from 0.06 m/year (Ngarara
Stream at Field Way) to 0.31 m/year (Beef Creek at Headwaters). These eight
sites comprised a mixture of hard and soft-bottomed sites and all have lowelevation
sources of flow. Two of these sites are classed as reference sites
(Wainuiomata River at Manuka Track and Beef Creek at Headwaters), while
the remaining six sites are located in catchments dominated by pastoral (five
sites) or urban landcover (one site).
Only two RSoE sites (Taueru River at Castlehill and Whangaehu River 250 m
from confluence) that recorded deteriorating trends in visual clarity recorded
corresponding deteriorating (ie, increasing) trends in turbidity (raw data)
(Table 4.9). Five sites recorded meaningful decreasing trends in turbidity
which were typically only evident in flow-adjusted data (Table 4.9).
18 From July 2005 up until January 2007, monthly water sampling was undertaken by external contractors. Issues were identified with the
quality of some field measurements reported during this time.
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