River and stream water quality and ecology - Greater Wellington ...
River and stream water quality and ecology - Greater Wellington ...
River and stream water quality and ecology - Greater Wellington ...
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<strong>River</strong> <strong>and</strong> <strong>stream</strong> <strong>water</strong> <strong>quality</strong> <strong>and</strong> <strong>ecology</strong> in the <strong>Wellington</strong> region: State <strong>and</strong> trends<br />
Seven RSoE sites recorded a median NNN concentration over the 2008–2011<br />
state period that was at least an order of magnitude greater than that recorded at<br />
the other sites. Three of these sites, Parkvale tributary at Lowes Reserve<br />
(median 4.35 mg/L), Parkvale Stream at Weir (1.26 mg/L) <strong>and</strong> Mangatarere<br />
Stream at SH 2 (1.2 mg/L) drain intensive agricultural catchments in the<br />
central Wairarapa Valley <strong>and</strong> are known to be influenced by shallow<br />
ground<strong>water</strong> (eg, Daughney 2010; Milne et al. 2010). Dairying is prominent in<br />
these catchments, as well as in several other catchments in which RSoE sites<br />
recorded elevated NNN concentrations (eg, Mangaone – also in the top seven<br />
with a median of 1.93 mg/L at Sims Road Bridge, Kopuaranga <strong>and</strong> Whangaehu<br />
catchments). 30 In the case of the Mangatarere Stream catchment, a large<br />
piggery that discharges effluent to l<strong>and</strong> in the mid catchment reaches <strong>and</strong>, to a<br />
lesser extent, the discharge of treated municipal waste<strong>water</strong> from Carterton<br />
township, are also considered to be major contributors to in<strong>stream</strong> NNN (Milne<br />
et al. 2010).<br />
The other three RSoE sites that recorded the highest median NNN<br />
concentrations are classed as urban: Mangapouri Stream at Bennetts Road<br />
(median 2.5 mg/L), Karori Stream at Makara Peak (1.3 mg/L) <strong>and</strong><br />
Kaiwharawhara Stream at Ngaio Gorge (1.1 mg/L). While ground<strong>water</strong><br />
nutrient inputs can not be ruled out at some sites, notably the Mangapouri<br />
Stream 31 , <strong>Greater</strong> <strong>Wellington</strong>’s pollution incident records <strong>and</strong> monitoring<br />
undertaken by the respective territorial authorities indicate that sections of two<br />
of these <strong>stream</strong>s, along with sections of other urban <strong>stream</strong>s that recorded<br />
elevated concentrations of NNN or DRP (eg, Porirua Stream at Glenside,<br />
Porirua Stream at Wall Park <strong>and</strong> Waiwhetu Stream at Wainuiomata Hill Road)<br />
are impacted, at least periodically, by sewer/storm<strong>water</strong> discharges <strong>and</strong>/or<br />
faults (eg, cross connections). For example, sewer leaks were reported in the<br />
Karori Stream at Karori Park (approximately 1 km up<strong>stream</strong> of the Makara<br />
Peak monitoring site) in late December 2011 (O. Vorwerk 32 , pers. comm.<br />
2012). Infrastructure-sourced inputs may also be significant at higher flows,<br />
particularly during heavy or prolonged rainfall when storm<strong>water</strong> can directly<br />
infiltrate <strong>and</strong> overload the sewer network, resulting in pump station overflows<br />
to nearby <strong>stream</strong>s (refer to Section 3.4.2).<br />
In a detailed analysis of the region’s RSoE nutrient concentration data, Ausseil<br />
(2011) identified a pattern of increasing median nutrient concentrations under<br />
low flow conditions at many urban sites, as well as some pastoral sites,<br />
including Mangatarere Stream at SH 2 <strong>and</strong> Ruamahanga <strong>River</strong> at Gladstone.<br />
This pattern was particularly strong for DRP, which Ausseil (2011) considered<br />
to be generally consistent with some sort of point source discharge, such as a<br />
WWTP discharge, or nutrient-rich ground<strong>water</strong> inputs. In the case of<br />
Mangatarere Stream at SH 2 <strong>and</strong> Ruamahanga <strong>River</strong> at Gladstone, increased<br />
DRP concentrations at low flows are attributed to these sites being located<br />
30 Fonterra records supplied to <strong>Greater</strong> <strong>Wellington</strong> in September 2011 indicate that these catchments all support more than 2,000 dairy<br />
cows. The Mangatarere Stream catchment has in the order of 7,000 dairy cows <strong>and</strong> is also home to a large piggery that, in recent years<br />
has supported in the order of 9,000 to 10,000 pigs/month (Shivas & Hosken 2012).<br />
31 Daughney (2010) identified a chemical signature for the Bennetts Road site similar to that of ground<strong>water</strong> in the area, suggesting that<br />
some of the nutrient enrichment may be related to inputs of nutrient-rich shallow ground<strong>water</strong> (although the site is classified as urban,<br />
pastoral l<strong>and</strong>cover makes up over half of the up<strong>stream</strong> catchment <strong>and</strong> the former RSoE site at Rahui Road (located further up<strong>stream</strong>)<br />
was characterised by very high concentrations of NNN).<br />
32 Olivia Vorwerk, Environmental Protection Officer, <strong>Greater</strong> <strong>Wellington</strong>.<br />
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