Benchmarking

National Policy Statement for Freshwater Management

RWQE monitoring data are assessed against attribute tables in the National Objectives Framework of the National Policy Statement for Freshwater Management.

Metals

Copper and zinc concentrations are compared against Australian and New Zealand Guidelines for fresh and marine water quality chronic toxicity default guideline values (95% level of protection) for freshwater.

Because water hardness affects the toxicity of some metals, where metal concentrations exceed trigger values, site-specific, hardness-modified trigger values are calculated based on recommendations in the Australian and New Zealand Guidelines. Since the availability of water hardness data varies across sites, the calculation of hardness-modified default guidelines values and their application is undertaken in the following two ways:

  • For the five sites where total hardness was analysed in monthly water samples, the hardness-modified trigger value is calculated for each sampling occasion and compared against the metal concentration from that occasion. A breach of the chronic toxicity guidelines is defined as occurring when a site exceeds its hardness-modified trigger value on more than 50% of sampling occasions; and,
  • For the three sites where total hardness was not analysed in monthly water samples, the hardness-modified guideline value is calculated based on the median water hardness value generated from monthly monitoring over July 2012 to June 2013. A breach of the chronic toxicity guideline is defined as occurring when the median metal concentration from monthly sampling over reporting year exceeds this site-specific, hardness modified guideline value.

Macroinvertebrates

Macroinvertebrate Community Index (MCI) scores are assessed against quality classes recommended for the Greater Wellington Region and Greater Wellington Natural Resources Plan (NRP) plan outcomes. Soft bottomed MCI scores (MCI-sb) have also been calculated for the seven sites that were sampled which have soft substrates.

MCI quality classification based on Clapcott and Goodwin (2014) and NRP outcomes for the Wellington Region. MCI thresholds in the table refer to the latest three year median MCI score at each site.
MCI score quality class
NRP outcomes
River Class Poor Fair Good Excellent All rivers Significant rivers
1 Steep, hard sedimentary < 110 110-120 120-130 ≥ 130 ≥ 120 ≥ 130
2 Mid-gradient, coastal and hard sedimentary < 80 80-105 105-130 ≥ 130 ≥ 105 ≥ 130
3 Mid-gradient, soft sedimentary < 80 80-105 105-130 ≥ 130 ≥ 105 ≥ 130
4 Lowland, large, draining ranges < 90 90-110 110-130 ≥ 130 ≥ 110 ≥ 130
5 Lowland, large, draining plains and eastern Wairarapa <80 80-100 100-120 ≥ 120 ≥ 100 ≥ 120
6 Lowland, small < 80 80-100 100-120 ≥ 120 ≥ 100 ≥ 120

Periphyton

Monthly observations of percent streambed periphyton cover (filamentous and mat-forming periphyton) are compared against the periphyton composite cover guidelines (Matheson et al. 2012). The threshold for nuisance mat cover is twice that for filamentous periphyton cover, so the periphyton weighted composite cover (PeriWCC) can be defined as filamentous periphyton cover + (mat periphyton cover / 2) with a nuisance guideline of ≥ 30%.