Urban stream water quality

  • Image, stream in urban environment.

    Urban streams have unique water quality issues due to the presence of infrastructure and stormwater runoff from high levels of impervious surfaces. Zinc and copper are heavy metals that accumulate in sediment and tissue and high concentrations are toxic to aquatic life. E.coli is an indicator of disease-causing organisms, which may affect human health and the recreational values of streams. Ammoniacal nitrogen and nitrate-nitrogen can be toxic to aquatic life if concentrations in streams are high enough. High nitrate-nitrogen and dissolved reactive phosphorus can have adverse environmental impacts, such as stimulating algal blooms. We report on six indicators of urban stream water quality: zinc, copper, E.coli, ammoniacal nitrogen, nitrate-nitrogen, and dissolved reactive phosphorus for Auckland, Wellington, and Christchurch.

    We classified urban stream water quality as a case study.

    Key findings

    In general, the water quality indicators for all monitored urban streams were higher at sites with greater proportions of urban land cover in the catchments in Auckland, Wellington, and Christchurch for the period 2013–15. 

    • Dissolved reactive phosphorus concentrations improved at 59 percent of urban sites, with 8 percent of sites worsening from 2008 to 2015. 
    • Due to insufficient data for most sites, trends for zinc, copper, nitrate-nitrogen, E.coli, and ammoniacal nitrogen concentrations were indeterminate, meaning we had insufficient data to determine a trend at most sites.

    Figure 1

    Figure 2

    Note: Only urban sites, classified as more than 15 percent urban land cover in the upstream catchment, are displayed on the map.

    Figure 3

    Note: Data are for all sites monitored by the three councils for zinc and copper (including native and rural sites). Copper medians were calculated for 30 sites. The grey line (at 1.4 mg/m3) represents the Australian and New Zealand Guidelines for Fresh and Marine Water Quality 2000 (ANZECC) toxicant trigger value for 95 percent species protection for a water hardness of 30 mg/L CaCO3. The yellow line (at 2.2 mg/m3) represents the ANZECC toxicant trigger value for 95 percent species protection for a water hardness of 50 mg/L CaCO3. We do not have water hardness data for most sites, but hardness levels of 30 and 50 mg/L CaCO3 are typical for Auckland, Wellington, and Christchurch.

    Figure 4

    Note: Data are for all sites monitored by the three councils for zinc and copper (including native and rural sites). Zinc medians were calculated for 65 sites. The grey line (at 8 mg/m3) represents the Australian and New Zealand Guidelines for Fresh and Marine Water Quality 2000 (ANZECC) toxicant trigger value for 95 percent species protection at a water hardness of 30 mg/L CaCO3. The yellow line (at 12 mg/m3) represents the ANZECC toxicant trigger vale for 95 percent species protection at a water hardness of 50 mg/L CaCO3. We do not have water hardness data for most sites, but hardness levels of 30 and 50 mg/L CaCO3 are typical for Auckland, Wellington, and Christchurch. The y-axis is presented on a log scale. On a log scale, each tick mark on the scale is the previous tick mark multiplied by 10. This allows for easier viewing when there is a large range in data.

    Figure 5

    Note: NOF – National Objectives Framework.  Data are for all sites monitored by the three councils for E.coli (including native and rural sites). E.coli medians were calculated for 72 sites. The grey line (at 1,000 E.coli/100 mL) represents the National Policy Statement for Freshwater Management 2014 compulsory bottom line for wading and boating (secondary contact). The y-axis is presented on a log scale. On a log scale, each tick mark on the scale is the previous tick mark multiplied by 10. This allows for easier viewing when there is a large range in data.

    Figure 6

    Note: NOF – National Objectives Framework. Data are for all sites monitored by the three councils for nitrate-nitrogen (including native and rural sites). Nitrate-nitrogen medians were calculated for 72 sites. The grey line (at 6,900 mg/m3) represents the National Policy Statement for Freshwater Management 2014 compulsory bottom line for nitrate toxicity.

    Figure 7

    Note: NOF – National Objectives Framework. Data are for all sites monitored by the three councils for zinc and copper (including native and rural sites). Ammoniacal nitrogen medians were calculated for 68 sites. The grey line (at 1,300 mg/m3) represents the National Policy Statement for Freshwater Management 2014  compulsory bottom line for ammonia toxicity. The y-axis is presented on a log scale. On a log scale, each tick mark on the scale is the previous tick mark multiplied by 10. This allows for easier viewing when there is a large range in data.

    Figure 8

    Note: Data are for all sites monitored by the three councils for zinc and copper (including native and rural sites). Dissolved reactive phosphorus (DRP) medians were calculated for 72 sites. Phosphorus is not toxic, so there are no toxicity guidelines for dissolved reactive phosphorus. The y-axis is presented as a log scale. On a log scale, each tick mark on the scale is the previous tick mark multiplied by 10. This allows for easier viewing when there is a large range in data.

    Definition and methodology

    Auckland Council, Greater Wellington Regional Council, and Christchurch City Council collect urban stream water samples monthly at fixed locations and send them to a laboratory for chemical and bacterial analysis.

    Table 1 shows the number of stream sites measured over the period 2008–15. 

    Table 1  

    Number of stream sites measured for states and trends – selected measures, 2008–15
     Measure

    State of water quality

    Trends in water quality

     No. sites measured

    2013–15

     2008–15

     E.coli.

     54

     51

     Nitrate-nitrogen

    54

    49

     Ammoniancal nitrogen

    53

    38

     Dissolved copper

    17

    14

     Dissolved zinc

     50

     14

     Dissolved reactive phosphorus

     54

     51

    Note: For the analysis presented here, NIWA used data on the water quality of urban streams with consistent time periods and comparable methods (Gadd, 2016). An additional 13 to 18 sites in Auckland, Wellington, and Christchurch where native and rural land cover are dominant in Auckland, Wellington, and Christchurch were used for comparison; these sites were also monitored for heavy metals. Trend data were adjusted for the influence of variation in river flow, because values are strongly correlated with flow. Increasing and decreasing trends at sites are inferred with 95 percent confidence using the Relative Seasonal Sen Slope Estimator. Indeterminate trends mean there was not enough data to determine a trend.
    Sites are classified by the proportion of urban cover using the River Environment Classification, where sites with more than 15 percent urban cover in the upstream catchment are considered an ‘urban stream’.

     

    If you want detailed regional-level information, we recommend you review the relevant regional council’s environmental reports.

    This is because although our data are sourced from regional councils, we adjust some datasets to ensure our reports are nationally consistent. The adjustments may include omitting information produced by non-comparable methods. As a result, our evaluations may differ from those produced by regional councils.

    Data quality 

    Topic Classification   Relevance Accuracy 
     Freshwater quality, quantity, and flows  Case study

    Image, Partial relevance.

    Partial 

    Image, Medium accuracy.

    Medium 

    References

    Gadd, J (2016). Urban streams water quality state and trends. Prepared for the Ministry for the Environment. NIWA Client Report no. AKL2016-018. Retrieved from www.mfe.govt.nz.

     

    Published 27 April 2017

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