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 At one site, Ngarara Stream at Field Way, release of DRP from streambed sediments may be occurring during times when dissolved oxygen concentrations are very low (Figure 4.13). This phenomenon is particularly common in eutrophic lakes in summer and is linked with the breakdown of organic matter in the bottom sediment under oxygen-poor conditions. Median concentrations of total phosphorus (TP) ranged from 0.004 mg/L (Motuwaireka Stream at Headwaters) to 0.110 mg/L (Ngarara Stream at Field Way). Eighteen sites (33%) recorded a median concentration above the ANZECC (2000) lowland TV of 0.033 mg/L, all of which also exceeded the ANZECC (2000) DRP lowland TV. Dissolved reactive phosphorus (mg/L) 0.12 0.10 0.08 0.06 0.04 0.02 0.00 2009 2010 2011 Figure 4.13: Relationship between concentrations of DRP (red line) and dissolved oxygen (% saturation, black line), based on data collected monthly between July 2008 and June 2011 4.1.11 E. coli All RSoE sites located in catchments dominated by indigenous forest cover complied with the ANZECC (2000) stock water TV of 100 cfu/100mL. The 20 RSoE sites that exceeded this TV all have low elevation sources of flow and drain catchments assigned to either pastoral or urban landcover classes. At four of these 20 sites the median E. coli count also exceeded the less conservative proxy stock water TV adopted for use in this report (550 cfu/100mL): Parkvale Stream at Weir, Mangapouri Stream at Bennetts Road, Porirua Stream at Walk Park, and Karori Stream at Makara Peak (Table 4.2, Figure 4.14). Around two thirds of the 55 sites recorded E. coli counts in excess of 1,000 cfu/100mL on at least one sampling occasion over the reporting period. Nine of these sites recorded values over 10,000 cfu/100mL, these typically being sites located in urban catchments (Karori Stream at Makara Peak, Kaiwharawhara Stream at Ngaio Gorge and Porirua Stream at Wall Park) or pastoral catchments that drain the eastern Wairarapa hill country (eg, Whareama River at Gauge, Kopuaranga River at Stewarts and Whangaehu WGN_DOCS-#1100598-V3 PAGE 35 OF 160 80 70 60 50 40 30 20 10 0 Dissolved oxygen (% sat.)
River and stream water quality and ecology in the Wellington region: State and trends E. coli (cfu/100 mL) 1200 1100 1000 900 800 700 600 500 400 300 200 100 REC landcover class Indigenous forest Exotic forest Pastoral Urban GWRC modified stockwater TV ANZECC (2000) stockwater TV 0 0 10 20 30 40 50 60 70 80 90 100 % of upstream catchment in indigenous forest landcover Indigenous forest Pastoral Urban PAGE 36 OF 160 WGN_DOCS-#1100598-V3 E. coli (cfu/100 mL) 1000 900 800 700 600 500 400 300 200 100 0 GWRC modified stockwater TV ANZECC (2000) stockwater TV n = 24 n = 23 n = 7 Figure 4.14: Scatter plot (left) of indigenous forest cover in the upstream catchment against median E. coli counts for each RSoE site, and box plot (right) summarising the range of median E. coli counts across the three main REC landcover classes represented in the RSoE network. Both plots are based on data collected monthly between July 2008 and June 2011 River 250 m from confluence). As noted below, some of the highest E. coli counts recorded at urban sites were associated with dry weather; this suggests, along with the presence of elevated nutrient concentrations in dry weather flows, the presence of a point source wastewater input(s) upstream. In contrast, high counts at the eastern Wairarapa sites were associated with wet weather, when the soft erosion-prone soils in the area are readily washed into the rivers – carrying with them faecal matter deposited by stock on unstable farmland and riparian margins/stream banks. Re-suspension of streambed sediments may also be a factor at these sites, as well as at other pastoral sites that recorded high E. coli counts in wet weather (eg, Whareroa Stream at Queen Elizabeth Park). While elevated E. coli counts were typically associated with increased river and stream flow, a number of sites recorded elevated counts (ie, >550 cfu/100mL) at times when flows were relatively stable and there had been no recent rainfall. This was most commonly observed at urban sites, such as Karori Stream at Makara Peak where, for example, on 8 December 2009, a result of 11,000 cfu/100mL was recorded in the absence of any recent rainfall during clear and stable flow conditions. Some elevated E. coli counts were also recorded in rural areas in the absence of rainfall, most likely arising from stock access/crossings. For example, a herd of dairy cows crossing Parkvale Stream approximately 100 m upstream of the RSoE site (Parkvale at Weir) coincided with an E. coli count of 3,600 cfu/100mL on one occasion. 4.1.12 Heavy metals Table 4.4 summarises dissolved copper, lead and zinc concentrations recorded at the 10 RSoE sites at which these metals were monitored over July 2008 to June 2011 15 . Median concentrations of dissolved lead were below the ANZECC (2000) toxicity TV at all sites. In contrast, median concentrations of dissolved copper and zinc exceeded their respective ANZECC (2000) toxicity TVs at two sites (Porirua Stream at Wall Park and Kaiwharawhara Stream at Ngaio Gorge) and four sites (Porirua Stream at Wall Park, Karori Stream at 15 Dissolved concentrations of arsenic, cadmium, chromium and nickel were also measured at one site, Waiwhetu Stream at Wainuiomata Hill Road Bridge. The median concentrations for each of these metals were all well below their respective ANZECC (2000) toxicity TVs.
Page 1 and 2: River and stream water quality and
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Page 7 and 8: 5. Periphyton - state and trends 57
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River and stream water quality and
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IBI Score 60 50 40 30 20 10 0 River
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Water quality (n=24) Periphyton (n=
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Appendix 1: Monitoring site details
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River and stream water quality and ecology - Greater Wellington ...

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