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

River and stream water quality and ecology in the Wellington region: State and trends
has been a particularly large amount of earthworks associated with urban
subdivision or roading projects in recent years. 34
(Source: Environmental Regulation Department, Greater Wellington)
Figure 8.2: Sediment runoff to Stebbings Stream (a tributary of Porirua Stream) in
December 2009, resulting from a failure in sediment control measures in place on
an earthworks site
8.3.3 Toxicity
Ongoing (ie, chronic) exposure to toxic concentrations of nitrate and heavy
metals may be impacting on ecosystem health at some RSoE sites.
Mangapouri Stream at Bennetts Road, Mangaone Stream at Sims Road and
Parkvale tributary at Lowes Reserve all recorded median concentrations of
nitrate nitrogen above Hickey and Martin’s (2009) chronic toxicity TV of
1.7 mg/L. A further four sites exceeded this TV on at least 10% of sampling
occasions during the three-year state reporting period: Parkvale Stream at Weir
(42% of sampling occasions), Mangatarere Stream at SH 2 (14%), Porirua
Stream at Wall Park (11%), and Waipoua River at Colombo Road (11%).
Several urban stream sites recorded median concentrations of dissolved copper
and/or zinc above their respective hardness-modified ANZECC (2000) toxicity
TVs, indicating chronic exposure to elevated metal concentrations. These sites
were Porirua Stream at Wall Park, Karori Stream at Makara Peak and
Waiwhetu Stream at Wainuiomata Hill Bridge. Acute toxicity may also be an
issue at times; concentrations of dissolved copper and zinc exceeded their
34 Greater Wellington’s consenting records indicate that approximately 105 hectares of earthworks were consented in 2011 alone – much
of this area lies within the Porirua Stream catchment. This estimate only applies to earthworks for land >0.3 ha in area (smaller areas of
earthworks are regulated by Porirua and Wellington city councils).
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