Gosford City Council Historical Water Quality Review & Analysis

RESULTS OF WATER QUALITY DATA ANALYSIS 3-13
3.1.3 Compound Water Quality Index
Considerable insight to the water quality of the Gosford waterways can be obtained through
investigation of the various water quality parameters, as discussed in the preceding Sections of this
report. However, it is most useful to simplify the information into a single parameter, which can be
reported to the community and Councillors to give an overall indication of the general water quality
condition.
In order to obtain a ‘Compound Water Quality Index’, the data from a range of water quality
parameters needs to be synthesised and considered concurrently. For each major water quality
parameter, the measured concentration is compared to a criteria set, and categorised into one of four
different ranges. The overall water quality score can be determined by applying a value to each of the
ranges (1 = good; 4 = highly degraded), summing the individual scores, and then dividing by the
number of parameters included in the assessment.
Table 3.22 presents the key water quality parameters and the interim relative ranges (and scores) for
each as applicable to the Gosford waterways. Although Chlorophyll-a was not included in the most
recent water quality monitoring program (as described in the previous Sections of this report), it has
been included in the matrix below for future reference, as it is an extremely valuable water quality
indicator that is likely to be included as part of any future monitoring program.
Table 3.22 Gosford Water Quality Index Range Definition Matrix
Parameter Range 1
(value = 1)
Range 2
(value = 2)
Range 3
(value = 3)
Range 4
(value = 4)
Turbidity 0 – 5 NTU 5 – 15 NTU 15 – 30 NTU > 30 NTU
Dissolved Oxygen > 6.5 mg/L 4.0 – 6.5 mg/L 2.0 – 4.0 mg/L < 2.0 mg/L
Ammonia 0 – 0.01 mg/L 0.01 – 0.03 mg/L 0.03 – 0.05 mg/L > 0.05 mg/L
Oxidised Nitrogen 0 – 0.02 mg/L 0.02 – 0.05 mg/L 0.05 – 0.10 mg/L > 0.10 mg/L
Total Nitrogen 0 – 1.0 mg/L 1.0 – 1.5 mg/L 1.5 – 2.0 mg/L > 2.0 mg/L
Orthophosphate 0 –0.01 mg/L 0.01 – 0.03 mg/L 0.03 – 0.05 mg/L > 0.05 mg/L
Total Phosphorus 0 – 0.05 mg/L 0.05 – 0.10 mg/L 0.10 – 0.20 mg/L > 0.20 mg/L
Chlorophyll-a 0 – 2 µg/L 2 – 5 µg/L 5 – 10 µg/L > 10 µg/L
Waterways that exhibit water quality concentrations within Range 1 can be considered to be in
relatively good condition (with respect to water quality at least). If waterways start to exhibit a large
number of Range 3 or Range 4 values, then it would tend to indicate a more degraded system (which
is likely to have follow-on effects to the general aquatic ecosystem). The values provided in Table
3.22 are intended to provide a more sites-specific assessment of water quality, as recommended by
ANZECC (2000). In determining the values, consideration was given to the default trigger values for
physical and chemical stressors (as defined in ANZECC, 2000), along with an appreciation of the
historical water quality results.
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