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

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River and stream water quality and ecology in the Wellington region: State and trends Placement of the temporal trends in water quality presented in this report in a national context is difficult. The most recent national trend analysis was undertaken by Ballantine and Davies-Colley (2010) and spanned a significantly longer time period (1989 to 2007). Based on the collective trends across the 75 NRWQN sites examined, Ballantine and Davies-Colley (2010) concluded that concentrations of NNN, TN, DRP and TP had increased. These overall increasing trends indicate deteriorating water quality at the national level that the authors considered were mainly attributable to expansion and intensification of pastoral agriculture. 8.2.2 Ecological health There are no readily available current national-scale assessments of periphyton growth or macroinvertebrate health against which to compare RSoE periphyton and macroinvertebrate data. In terms of indigenous freshwater fish, a nationwide assessment of IBI scores by Joy (2009) found similar declining trends at river and stream sites under pastoral landcover as those identified for the Wellington region in this report (as well as significant declining trends at sites under urban landcover). Allibone et al. (2010) also reported a decline across New Zealand in populations of a number of diadromous species that occur in the Wellington region (inanga, koaro, shortjaw kokopu, torrentfish and redfin and bluegill bullies); the loss of shortjaw kokopu populations from the Wellington region was specifically mentioned. 8.3 Key issues affecting river and stream health The RSoE sites in poorest condition – in particular those with small catchments dominated by urban or intensive agricultural land use – share in common one or more of the following issues or ‘stressors’: nutrient enrichment, poor clarity, toxicity, microbiological contamination, and/or habitat degradation. Overall aquatic ecological health measured at each RSoE site is a reflection of the cumulative impact of the stressors present. In the case of indigenous fish condition, these stressors may be present upstream or downstream of a given site. 8.3.1 Nutrient enrichment The occurrence of elevated nutrient concentrations in rivers and streams is recognised nationally and internationally as a key contributor to the occurrence of nuisance periphyton and/or macrophyte growth (eg, MfE 2000) that can degrade aquatic ecosystem health and other river and stream values (eg, trout spawning and contact recreation). Analysis of RSoE data here confirms an earlier assessment by Ausseil (2011) that periphyton issues are relatively widespread in the Wellington region (nuisance macrophyte growth may also be relatively widespread), with periphyton cover/biomass at RSoE sites strongly correlated with nutrient concentrations – in particular nitrogen. All 15 RSoE sites graded ‘poor’ for water quality recorded median NNN and DRP concentrations above ANZECC (2000) lowland trigger values (TVs). While these TVs are not effects-based and only serve as a general ‘bench marking’ assessment tool, it was demonstrated from the analysis in Section 6 that many of the RSoE sites with elevated median nutrient concentrations also frequently recorded nuisance periphyton cover and/or biomass. WGN_DOCS-#1100598-V3 PAGE 105 OF 160
River and stream water quality and ecology in the Wellington region: State and trends Seven RSoE sites recorded a median NNN concentration over the 2008–2011 state period that was at least an order of magnitude greater than that recorded at the other sites. Three of these sites, Parkvale tributary at Lowes Reserve (median 4.35 mg/L), Parkvale Stream at Weir (1.26 mg/L) and Mangatarere Stream at SH 2 (1.2 mg/L) drain intensive agricultural catchments in the central Wairarapa Valley and are known to be influenced by shallow groundwater (eg, Daughney 2010; Milne et al. 2010). Dairying is prominent in these catchments, as well as in several other catchments in which RSoE sites recorded elevated NNN concentrations (eg, Mangaone – also in the top seven with a median of 1.93 mg/L at Sims Road Bridge, Kopuaranga and Whangaehu catchments). 30 In the case of the Mangatarere Stream catchment, a large piggery that discharges effluent to land in the mid catchment reaches and, to a lesser extent, the discharge of treated municipal wastewater from Carterton township, are also considered to be major contributors to instream NNN (Milne et al. 2010). The other three RSoE sites that recorded the highest median NNN concentrations are classed as urban: Mangapouri Stream at Bennetts Road (median 2.5 mg/L), Karori Stream at Makara Peak (1.3 mg/L) and Kaiwharawhara Stream at Ngaio Gorge (1.1 mg/L). While groundwater nutrient inputs can not be ruled out at some sites, notably the Mangapouri Stream 31 , Greater Wellington’s pollution incident records and monitoring undertaken by the respective territorial authorities indicate that sections of two of these streams, along with sections of other urban streams that recorded elevated concentrations of NNN or DRP (eg, Porirua Stream at Glenside, Porirua Stream at Wall Park and Waiwhetu Stream at Wainuiomata Hill Road) are impacted, at least periodically, by sewer/stormwater discharges and/or faults (eg, cross connections). For example, sewer leaks were reported in the Karori Stream at Karori Park (approximately 1 km upstream of the Makara Peak monitoring site) in late December 2011 (O. Vorwerk 32 , pers. comm. 2012). Infrastructure-sourced inputs may also be significant at higher flows, particularly during heavy or prolonged rainfall when stormwater can directly infiltrate and overload the sewer network, resulting in pump station overflows to nearby streams (refer to Section 3.4.2). In a detailed analysis of the region’s RSoE nutrient concentration data, Ausseil (2011) identified a pattern of increasing median nutrient concentrations under low flow conditions at many urban sites, as well as some pastoral sites, including Mangatarere Stream at SH 2 and Ruamahanga River at Gladstone. This pattern was particularly strong for DRP, which Ausseil (2011) considered to be generally consistent with some sort of point source discharge, such as a WWTP discharge, or nutrient-rich groundwater inputs. In the case of Mangatarere Stream at SH 2 and Ruamahanga River at Gladstone, increased DRP concentrations at low flows are attributed to these sites being located 30 Fonterra records supplied to Greater Wellington in September 2011 indicate that these catchments all support more than 2,000 dairy cows. The Mangatarere Stream catchment has in the order of 7,000 dairy cows and is also home to a large piggery that, in recent years has supported in the order of 9,000 to 10,000 pigs/month (Shivas & Hosken 2012). 31 Daughney (2010) identified a chemical signature for the Bennetts Road site similar to that of groundwater in the area, suggesting that some of the nutrient enrichment may be related to inputs of nutrient-rich shallow groundwater (although the site is classified as urban, pastoral landcover makes up over half of the upstream catchment and the former RSoE site at Rahui Road (located further upstream) was characterised by very high concentrations of NNN). 32 Olivia Vorwerk, Environmental Protection Officer, Greater Wellington. PAGE 106 OF 160 WGN_DOCS-#1100598-V3
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River and stream water quality and
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DISCLAIMER This report has been pre
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observed trends were unclear. The p
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5. Periphyton - state and trends 57
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Figure 2.1: RSoE monitoring sites a
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4.2 Temporal trends River and strea
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