Caboolture Region Water Quality Study - Moreton Bay Regional ...

Caboolture Water Quality Study
In this study, National guidelines (ANZECC & ARMCANZ, 2000) were used to define low, moderate
and high concentrations of faecal coliforms, corresponding respectively to good, moderate and poor
water quality. The guidelines recommend median faecal coliform concentrations lower than 150 CFU
per 100mL for primary contact activities (e.g. swimming) and median concentrations lower than 1000
CFU per 100mL for secondary contact activities (e.g. canoeing). Therefore, concentrations of less
than 150 CFU per 100mL are defined as “low” concentrations, and concentrations between 150 and
1000 CFU per 100mL are defined as “moderate”. Concentrations greater than 1000 CFU per 100mL
are regarded as “high”.
4.0 ASSESSMENT OF WATER QUALITY DATA
4.1 Assessment of Current Water Quality Data
A three-stage process was employed in the assessment of water quality at monitored (test) sites
throughout the Caboolture Region for water quality data (sediment toxicants and faecal coliforms were
considered separately and not included in the integrated assessment). The three steps of the
processes are:
1. assessment of test site data for individual indicators against guideline values;
2. assessment of indicator categories (comprised of multiple indicators); and
3. an (overall) integrated assessment based on individual indicators and indicator categories.
The statistics of importance in this assessment were the median, and 20 th and 80 th percentiles. The
median is the value that divides the data in half, 50 percent of the measurements being above (or
equal to) it and 50 percent being below (or equal to it). The 20 th percentile is the value below which 20
percent of the data lie. The 80 th percentile is the value below which 80 percent of the data lie. This is
graphically illustrated in Figure 2.
80th percentile: 80 percent of
values fall below this line
Median
20th percentile: 20 percent of
values fall below this line
Figure 2. Important statistics for the test site data assessment.
Step 1 of the assessment process involved an assessment of measurements of individual indicators
against guideline values for each site. For some indicators, ( pH and dissolved oxygen) both high and
low concentrations were of interest (i.e. trigger values exist for upper and lower limits). For these twotailed
indicators the median and 20 th and 80 th percentiles were assessed against guideline values. For
all other indicators where only high levels were of interest (i.e. there was only an upper limit trigger
value), the median and 20 th percentile values were assessed against trigger values.
This assessment generated a coloured rating of green, yellow or red for the individual indicator,
meaning a good, moderate or poor result for the assessment respectively (Figure 3).
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