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Table 7.9: Water quality in 77 rivers monitored by the National Water Quality Network, showing medians, ranges and suitability thresholds.
| Parameter | Median1 | Range 10%-90% | Suitability thresholds for particular uses | |
|---|---|---|---|---|
Suspended solids |
||||
| Turbidity | 2.15 NTU | 0.45 - 31.3 NTU | No more than 2 NTU | Contact recreation (e.g. swimming) |
| Black Disc visibility | 1.3 metres | 0.17 - 5.10 metres | No less than 1.6 metres | Aesthetics |
| Biochemical oxygen demand (BOD 5)grams of consumed oxygen per cubic metre of water | 0.45 g/m3 | 0.15 - 1.2 g/m3 | No more than 12 g/m3(filtered water sample) | Contact recreation |
| No more than 35 g/m3(unfiltered, total BOD5) | Aesthetics | |||
| Dissolved reactive phosphorus (DRP) | 4 mg/m3 | 220 mg/m3 | No more than 1530 mg/m3 | Contact recreation |
| Aesthetics | ||||
| Preventing algal growth | ||||
| Dissolved inorganic nitrogen 2(DIN) | Not known but likely to exceed threshold | Not known | No more than 40-100 mg/m3 | Contact recreation |
| Aesthetics | ||||
| Preventing algal growth | ||||
| Nitrate - nitrogen (NO3 -N) | 105 mg/m3 | 10-620 mg/m3 | No more than | |
| 30,000 mg/m3for | Stock water supply | |||
| 10,000 mg/m3for | Human consumption | |||
| Ammoniacal nitrogen3(NH4-N) | 9 mg/m3 | 315.0 mg/m3 | Suitability varies with temperature and pH | Aquatic ecosystems |
| Dissolved oxygen (DO) | 100.3% | 93-108.2% | No less than 80% | Aquatic ecosystems |
| pH (acidity/alkalinity) | 7.68 (mildly alkaline) | 7.23-8.19 | Acceptable within the range 6 - 9 (i.e. slightly acidic to moderately alkaline) | Aquatic ecosystems |
| Water supply | ||||
Sources: Ministry for the Environment (1992; 1994), Smith et al. (1993), Smith and Maasdam (1994)
1 The median is a statistic which presents the middle score or measurement from all the samples. It is used in this instance to describe typical water quality. Unlike an average, the median is insensitive to changes in extreme values, making it a better indicator of normal water quality.
2 DIN is the sum of nitrate-nitrogen and ammoniacal nitrogen, but needs to be summed separately for each sample before the overall median can be calculated. Based on the nitrate-nitrogen component, however, DIN in more than half our rivers is likely to exceed the suitability threshold for contact recreation, aesthetic appreciation, and prevention of nuisance algal growth.
3 The toxicity of ammonia to aquatic life varies with pH and temperature. The threshold for long-term toxicity is a 4-day average concentration of 1,150 mg/m3 of ammonia at 20 ° C and pH 7.75.
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