Water quality data
Comparison of water quality data with guidelines and derivation of integrated water quality assessments
Water quality is assessed by comparison with locally derived reference values, as recommended in the Australian Water Quality Guidelines for Fresh and Marine Waters (Australian and New Zealand Environment and Conservation Council (ANZECC), 2001). These national guidelines provide a framework for assessing water quality through comparison with guidelines derived from local reference values.
The ANZECC guidelines (2001) recommend that local guidelines be based on reference values; that is, data collected from sites (reference sites) in local ecosystems that are in a substantially natural condition. It is recommended that reference values comprise at least two years of data (but preferably 5-10 years) with information collected monthly from the reference site. This provides a good estimate of the median together with a measure of the range of natural variation.
Reference values provide a reliable measure of natural water quality. However, these values do not in themselves represent water quality guidelines. To determine an appropriate guideline, the degree of departure from the reference values that can be tolerated by the ecosystem needs to be determined. The draft ANZECC guidelines (ANZECC, 1999) suggest that for physico-chemical indicators, the 20th and 80th percentiles of the reference data should form the guidelines, and that the median value for the site of interest should fall within this range.
The Queensland the department has therefore developed Queensland water quality guidelines for Queensland (QEPA, 2004). These guidelines are based on at least five years of monthly data, and are therefore unbiased with respect to time. Guidelines are given for several water types: ocean, open coastal, enclosed coastal/lower estuary, mid estuary, upper estuary, lower catchment and upper catchment. The geographic region is also given for each site (southern, central or northern Queensland).
Compliance with the Queensland Water Quality Guidelines for all indicators is assessed by comparing the median value for each indicator and site with the 20th and 80th percentiles of the reference values for that site type. If the median falls within the percentiles, the site complies with the guidelines.
Procedure for deriving Integrated Water Quality Assessments
The integrated ecological assessments provide a single rating of the current condition of each site. The rating is presented as a coloured dot on a site map, where a green dot indicates an ecologically healthy/slightly impacted site, a yellow dot indicates a slightly/moderately impacted site with some signs of ill-health and a red dot indicates a moderately/heavily impacted site.
The Queensland water quality guidelines (QEPA, 2004) and the ANZECC Water Quality Guidelines for Fresh and Marine Waters (ANZECC, 2001) are used to assess individual water quality indicators. The Queensland Water Quality Guidelines are based on data collected from unimpacted reference sites around Queensland. Guidelines are derived from the 20th and 80th percentiles of the reference data The 80th percentiles are used where high values of an indicator cause problems (eg nutrients or chlorophyll-a), the 20th percentiles where low values cause problems (Secchi depth), and both the 20th and 80th percentiles where high or low values could cause problems (pH and dissolved oxygen).
The overall assessment for each site is made using a hierarchical approach, whereby each indicator is assessed against appropriate guidelines, then the assessments for each indicator are used to derive a combined assessment for groups or categories of indicators, and finally these category assessments are used to derive a single assessment (red, yellow or green) for the site as a whole. The steps in this process are detailed below:
1. Assessment of individual indicators against guidelines
Median, 20th and 80th percentile values for each indicator at each site are compared with guidelines. Nutrients, turbidity, suspended solids, dissolved oxygen, pH and chlorophyll-a are compared with the Queensland Water Quality Guidelines.
If the median value falls within the guidelines (less than the guideline for nutrients, suspended solids, turbidity or chlorophyll-a; greater than the guideline for Secchi depth; less than the maximum and greater than the minimum for pH and dissolved oxygen), a green dot is assigned to that indicator. If the median is not within the guidelines, but the 20th or 80th percentile is within the guidelines (20th percentile less than the guideline for nutrients, suspended solids, turbidity or chlorophyll-a; 80th percentile greater than the guideline for Secchi depth; 20th percentile less than the maximum guideline or 80th percentile greater than the minimum guideline for pH and dissolved oxygen), then a yellow dot is given for that indicator. A red dot is assigned to the indicator if both the median and 20th or 80th percentile values fall outside the guidelines (20th percentile greater than the guideline for nutrients, suspended solids, turbidity or chlorophyll-a; 80th percentile less than the guideline for Secchi depth; 20th percentile greater than the maximum or 80th percentile less than the minimum for pH and dissolved oxygen).
An easy way to visually assess compliance is to produce box plots of the reference data and the data of interest using the 20th and 80th percentiles:
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Green: Median is within guidelines derived from 20th and 80th percentiles of reference values Yellow: Median exceeds but 20th percentile is within the guidelines Red: Median and 20th percentile exceed guidelines |
2. Combined assessments for indicator categories
The indicators are grouped into the following categories:
- Nitrogen (organic nitrogen, ammonia, nitrate plus nitrite and total nitrogen)
- Phosphorus (filterable reactive phosphorus and total phosphorus)
- Phytoplankton biomass (chlorophyll-a)
- Water clarity (turbidity, suspended solids and secchi depth)
- Dissolved oxygen (dissolved oxygen)
An assessment for each category is then derived by combining the ratings given to each indicator within the category. Where there are no data available for a particular category, no assessment is made. The overall assessment for each category is made by combining the results from the assessments of the individual indicators as follows:
Criteria |
Result |
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0 red
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More green than yellow?
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Yes
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Green
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No
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Yellow
|
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1 red
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More yellow than green?
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No
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Yellow
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Yes
|
Red
|
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| 2 or more red | Red |
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3. Final integrated assessment for each site
The final integrated assessment is derived for each site by combining the ratings for each category, using the same procedure as above (step 2). Only sites with enough data to derive a rating for at least three categories are given a final integrated assessment; dots for sites with ratings for less than three categories are coloured grey.
The final integrated rating may also be modified if additional information is known about the site. For example, if ammonia concentrations reached toxic concentrations at any time during the assessment period or a fish kill was recorded, the rating for a site may be changed to red. Similarly, if a site had very good water quality, but was highly modified (for example the bank was concreted), the site would not receive a rating of green, as it would be unlikely to support a diverse fauna.
For more information about the current Queensland Water Quality Guidelines visit our Queensland water quality guidelines site.
For information about indicators, including how they are measured, visit our indicators site.
Last reviewed 29 September 2011
Last updated 6 September 2007