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

River and stream water quality and ecology in the Wellington region: State and trends
weather conditions, a finding reported by Milne and Watts (2008). However,
the persistent presence of dissolved copper and zinc at concentrations above
chronic toxicity guidelines at some sites during ‘base flows’ presents a more
difficult challenge for improving ecosystem health in urban streams. For
example, despite extensive riparian rehabilitation being carried out along the
Karori Stream at Makara Peak in the early 2000s, subsequent ecological
monitoring has not identified any improvements in stream health. Perrie (2008)
attributed this to the overriding impact of stormwater contaminants and habitat
degradation associated with the urban land use that dominates the upstream
catchment.
8.3.4 Microbiological contamination
Microbiological water quality data for the three-year state period examined
highlight that significant faecal contamination exists at some urban and
pastoral sites. The highest median E. coli counts were recorded at Karori
Stream at Makara Peak (1,100 cfu/100mL), Porirua Stream at Wall Park (910
cfu/100mL), Mangapouri Stream at Bennetts Road (610 cfu/100mL), and
Parkvale Stream at Weir (570 cfu/100mL). Three of these stream sites are
located in urban areas where, as outlined in Section 8.3.1, sewer/stormwater
infrastructure-related issues have been identified in the past – both in dry and
wet weather. In the case of Parkvale Stream at Weir, the elevated median
E. coli count – as well as a similar median count at Mangaone Stream at Sims
Road Bridge (430 cfu/100mL) – is in part attributed to cattle having direct
access to the stream margins and channel.
Nine RSoE sites recorded maximum E. coli counts over 10,000 cfu/100mL,
these typically being sites located in urban (Karori Stream at Makara
Peak, Kaiwharawhara Stream at Ngaio Gorge and Porirua Stream at Wall Park)
or pastoral catchments that drain the eastern Wairarapa hill
country (eg, Whareama River at Gauge, Kopuaranga River at Stewarts and
Whangaehu River 250 m from confluence). As noted above, sewer/stormwater
infrastructure-related issues likely explain elevated E. coli counts at the urban
sites. In contrast, high counts at the eastern Wairarapa sites were associated
with wet weather and are attributed to faecal matter deposited from stock on
unstable farmland and riparian margins/stream banks being washed into the
rivers. 35 Remobilisation of faecal deposits in streambed sediments may also
occur at these and other pastoral sites that recorded high E. coli counts in wet
weather (eg, Whareroa Stream at Queen Elizabeth Park). Overall, the widespread
occurrence of elevated E. coli counts during wet weather highlights the need to
address sewer/stormwater infrastructure issues in urban areas and, in rural areas,
the importance of establishing vegetated riparian margins and excluding stock
from streams. The benefits of riparian rehabilitation and stock exclusion for
stream water quality and ecological health are discussed next.
35 Faecal inputs from effluent runoff in wet weather are also expected in catchments that support dairying. As at the end of 2010, only around
40% of the 175-odd dairy farms in the Wairarapa Valley had some capacity for wet weather storage (see Perrie & Milne 2012). This indicates
that dairyshed effluent is at times applied to land when soils are fully saturated, increasing the potential for surface runoff to streams.
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