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 Ausseil (2011) also demonstrated that nutrient concentrations remain consistently elevated at lower flows at other RSoE sites, including many urban streams (eg, Porirua Stream at Wall Park, Karori Stream at Makara Peak, Kaiwharawhara Stream at Ngaio Gorge and Waiwhetu Stream at Wainuiomata Hill Road Bridge), consistent with nutrient inputs from point source discharges (eg, wastewater) and/or nutrient-rich groundwater. As will be discussed further in Section 8, it is likely that both sources are applicable in the Wellington region. Certainly Daughney (2010) has demonstrated that water quality at many RSoE sites is influenced by groundwater inputs, including some of the highly impacted rural streams in the central Wairarapa (eg, Mangatarere and Parkvale streams). The most common water quality variables to exceed guideline values over the three-year state analysis period were visual clarity and DRP; median values only met guidelines at 44% and 55% of sites, respectively. At some sites, the exceedances may reflect natural geology. This is considered to be the case for DRP at Waikanae River at Mangaone Walkway (a ‘best available’ reference site with a high proportion of indigenous forest cover) and highlights the limitations associated with region-wide application of the ANZECC (2000) TVs. This limitation has been documented previously for the Wellington region (eg, Milne & Perrie 2005) and is currently being addressed through the development of more specific trigger level guidance for Greater Wellington’s new Regional Plan intended to better reflect the natural diversity of the region’s river and stream types (eg, Ausseil in prep a–d; Greenfield in prep a & b). Other water quality variables with median values that frequently exceeded guidelines included E. coli (36% of sites exceeded the ANZECC (2000) stock water TV) and NNN (30%). In terms of nitrate nitrogen, median concentrations at three sites (Mangapouri Stream at Bennetts Road, Mangaone Stream at Sims Road and Parkvale tributary at Lowes Reserve) exceeded the recommended trigger value for aquatic toxicity. Water temperature also appears to be an issue at some sites during the warmer months; 17 sites exceeded the 19°C guideline on more than 10% of sampling occasions, of which two (Awhea River at Tora Road and Totara Stream at Stronvar) exceeded this threshold on more than 25% of sampling occasions. Metal toxicity also appears to be a concern for some of the region’s urban streams. Three of the ten RSoE sites routinely monitored for heavy metals (Porirua Stream at Wall Park, Karori Stream at Makara Peak and Waiwhetu Stream at Wainuiomata Hill Bridge) consistently recorded median concentrations of dissolved copper and/or zinc above their respective ANZECC (2000) hardness-modified toxicity TVs. Metal concentrations at some of these sites also exceeded USEPA (2009) acute toxicity guidelines on a few sampling occasions. This is discussed further in Section 8. The results of temporal trend analysis illustrated that the majority of RSoE sites were largely ‘stable’ across the July 2006 to June 2011 period assessed, based on both raw and flow-adjusted data for the 14 variables examined. Of the selected key variables of interest, most of the meaningful trends were observed across five variables: TP (15 sites), NNN (13 sites), TN (11 sites), DRP (10 sites) and E. coli (10 sites). In regards to nutrients, more sites exhibited WGN_DOCS-#1100598-V3 PAGE 55 OF 160
River and stream water quality and ecology in the Wellington region: State and trends improving trends than deteriorating trends (ie, decreasing concentrations), including all nitrogen-related trends. The reason for the improving trends is not clear but given that these trends were evident at RSoE sites located across all three main landcover types represented in the RSoE network (indigenous forest, pastoral and urban), including a number of reference sites, they likely reflect (at least in part) broader regional scale patterns, such as natural climate variation. For example, it is noted that Keenan et al. (2012) reported a slight drying trend over 2005/06 to 2010/11, where summer (defined as November to April) ‘low flows’ generally occurred more frequently and for a longer duration across the region. The influence of this, if any, on water quality at RSoE sites is unclear but climatic variability/patterns have been shown to exert a strong influence on water quality trends in New Zealand rivers. For example, Scarsbrook et al. (2003) reported trends in water quality for both baseline (reference) and impact river sites that were generally consistent with trends in the Southern Oscillation Index. The results of trend analysis performed on monitoring data collected from the five NRWQN monitoring sites in the Wellington region support the general finding that, on a regional basis at least, water quality has largely remained stable in rivers and streams in the Wellington region. While this can be considered a positive finding (ie, water quality generally has not deteriorated), the RSoE state analysis has identified that a number of pastoral and urban sites have impaired water quality. The principal water quality issues and their implications for aquatic ecosystem health are discussed in Section 8. Although not discussed further, a comparison of the water quality grades assigned by Greater Wellington’s WQI with the CCME (2001) WQI showed that both indices generally produced comparable rankings. This was despite some differences in the water quality variables, guidelines and calculations applied. However, the CCME WQI is considered to be a superior index given its greater sophistication (in particular its consideration of the magnitude of guideline exceedances). As such, its use in reporting on the state of water quality in the Wellington region should be continued – especially given that it can readily accommodate site-specific guidelines. PAGE 56 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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Appendix 1: Monitoring site details
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