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

River and stream water quality and ecology in the Wellington region: State and trends
Although no new consent requirements are in place as yet, the current river
based discharges from the Carterton, Greytown and Martinborough
Wastewater Treatment Plants (WWTPs) are all in the process of being assessed
by the relevant territorial authorities for their long-term viability. Similar
approaches to that of Masterton are being explored, the aim being to ensure
wastewater is only discharged to land when river/stream flows are low.
In addition to the municipal WWTP discharges, there are several other
wastewater discharges to water in the Wellington region. For example,
Rathkeale College has consent to discharge treated wastewater to a tributary of
the Ruamahanga River north of Masterton township and Castlepoint township
discharges treated wastewater to Castlepoint Stream during the winter months
when land disposal is not possible.
(ii) Industrial wastewater discharges
There are only a small number of industrial wastewater discharges to water in the
Wellington region, the majority of which are associated with quarries, cleanfills/
landfills or sawmill yards. Treated wastewater associated with an animal research
facility in Whitemans Valley is discharged to a tributary of the Mangaroa River
and several municipal water treatment plants intermittently discharge waste
products into rivers and streams (typically suspended sediment and aluminiumbased
coagulants associated with filter backwash water). Receiving waters for
these backwash water discharges include the Waikanae, Hutt, Wainuiomata and
Waingawa rivers, and the Waitohu and Kaipaitangata streams.
(iii) Stormwater and wet weather wastewater discharges
Generally defined as rainwater collected from roofs, driveways, roads, carparks
and other sealed surfaces, stormwater in the Wellington region is piped directly
into rivers and streams, generally without any treatment. During its travels, this
stormwater picks up sediment, rubbish and a variety of other contaminants,
including metals, hydrocarbons, herbicides, pesticides, nutrients and pathogens.
Although general stormwater discharges are a permitted activity under Greater
Wellington’s existing RFP (WRC 1999) and so do not require a resource
consent 9 , Kapiti Coast District Council and Carterton District Council both
hold ‘global network’ resource consents to discharge stormwater to surface
waters in their respective districts. Several territorial authorities also hold
consents authorising the discharge of diluted untreated or partially treated
wastewater to rivers and streams during times of very heavy or sustained
rainfall. During these conditions, stormwater can directly infiltrate the sewer
network, resulting in overflows at selected stormwater or ‘emergency sewer’
outfalls (eg, to the lower reaches of the Hutt and Wainuiomata rivers and the
Waiwhetu Stream). High volumes of ‘diluted’ wastewater also arrive at the
WWTPs in wet weather conditions where, on occasion when storage at the
plants is exceeded, a portion of these ‘wet weather flows’ bypass treatment and
are discharged directly to streams (or the coast) in close vicinity of the
treatment plant (eg, the lower Waiwhetu Stream and Karori Stream in the cases
9 Stormwater discharges must meet certain criteria to be permitted, a key one being that the discharges do not adversely affect aquatic life.
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