Water quality indicators
The following is a brief summary of the indicators and sampling methods the EPA uses when monitoring water quality in Queensland. For a full description of the techniques see the Water Quality Sampling Manual 3rd Edition (1999, Queensland Government).
Water quality is assessed by measuring physical, chemical and biological indicators. The EPA takes these measurements at a number of sites on a monthly basis. Some readings are taken at the site (in situ) and others from water samples; both data are from water at a depth of 0.2 metres. At estuarine sites, all samples and readings are taken on the ebb (falling) tide. Water samples that are collected for further analyses are collected, stored and delivered in accordance with the EPA Water Quality Sampling Manual 3rd Edition (1999), and analysed using standard methods.
Sediment samples are taken for the assessment of concentrations of trace elements (metals and metalloids), organic compounds (pesticides, polychlorinated biphenyls and organophosphates) and nutrients (total nitrogen and phosphorus). Sediment sample collection, preparation and storage are undertaken according to methods outlined in the EPA Water Quality Sampling Manual 3rd Edition (1999).
The table below describes the indicators used in the regular water quality monitoring programs. Click on their names (highlighted in blue) for details of the methods used.
| Category | Indicator | Explanation |
| Nitrogen
Organic Nitrate plus nitrite Ammonia Total Phosphorus |
The nutrients nitrogen and phosphorus
are essential for plant growth. High concentrations indicate potential
for excessive weed and algal growth.
Total nutrients are made up of a dissolved component (e.g. nitrate plus nitrite, ammonia and filterable reactive phosphorus) and an organic component, which is bound to carbon (e.g. organic nitrogen). Nutrients in the dissolved state can be readily used by plants. |
|
| Microalgal Growth | Chlorophyll-a | An indicator of algal biomass in the water. An increase in chlorophyll-a indicates potential eutrophication of the system. Consistently high or variable chlorophyll-a concentrations indicate the occurrence of algal blooms, which can be harmful to other aquatic organisms. |
| Water Clarity | Suspended solids | Small particles (soil, plankton, organic debris) suspended in water. High concentrations of suspended solids limit light penetration through water, and cause silting of the benthic (bottom) environment. |
| Turbidity | A measure of light scattering by suspended particles in the water column, provides an indirect indication of light penetration. | |
| Secchi depth | The depth to which the black and white markings on a Secchi disc can be clearly seen from the surface of the water provides an indication of light penetration. | |
| Oxygen | Dissolved oxygen | Essential for life processes of most aquatic organisms. Low concentrations of dissolved oxygen usually indicate the presence of excessive organic loads in the system, while high values can indicate excessive plant production (i.e. eutrophication). Many aquatic organisms will suffocate if there is insufficient volume of dissolved oxygen in the water. |
| pH | pH | A measure of the acidity or alkalinity of the water. Changes to pH can be caused by a range of potential water quality problems (e.g. low values due to acid sulfate runoff). Extremes of pH (less than 6.5 or greater than 9) can be toxic to aquatic organisms. |
| Salinity | Conductivity | A measure of the amount of dissolved salts in the water, and therefore an indicator of salinity. In fresh water, low conductivity indicates suitability for agricultural use. In salt waters low conductivity indicates of freshwater inflows such as stormwater runoff. |
| Toxicants in sediments | Trace elements in sediments. | Trace elements are present in the environment naturally and derive principally from weathering of rocks and soils. Many elements are essential for aquatic organisms. However, high concentrations of some elements in sediments can be toxic to aquatic organisms and may indicate contamination from domestic or industrial sources. |
| Pesticides in sediments | Commonly used pesticides accumulate in the sediments of aquatic environments and may reach concentrations toxic to aquatic organisms. | |
| Recreational health | Faecal coliforms | The National Health and Medical Research Council guidelines (National Health and Medical Research Council, 1990 Australian guidelines for recreational use of water. AGPS, Canberra) are widely used to assess suitability of water for recreational purposes. The guidelines specify appropriate counts for primary contact (e.g. swimming, and diving) and secondary contact (e.g. boating and fishing). These guidelines, however, overestimate the health risk to humans if the major sources of faecal pollution are animals such as livestock, pets and wildlife including birds. |
Methods: water and sediment quality
Secchi depth
Secchi Disk Transparency is a measure
of the clarity of the water, and a quick, simple, and accurate method for estimating
lake water quality. A black and white disk (called a secchi disk) is lowered
into the water until it just disappears from sight--this depth measurement is
recorded. The deeper the measurement, the clearer the water.
Secchi disk measurements give a general indication of problems with algae, zooplankton, water color and silt.
The animated diagram (right) illustrates the Secchi Disk in action.
Nutrients
Two water samples are collected to determine the concentration of nutrients:
one for soluble and the other for total nutrients. The sub-sample for soluble
nutrients is filtered through 0.45µm cellulose acetate filters, while the total
nutrient sub-sample is not filtered. Both samples are stored at temperatures
below 4°C and transferred to the laboratory within 24 hours where they were
frozen at -20°C.
The concentration of filterable reactive phosphorus (FRP) is determined directly from the filtered sample. It should be noted that "while reactive phosphorus is largely a measure of orthophosphate, a small fraction of any condensed phosphate present usually is hydrolysed unavoidably" and will also be included (APHA, 1989 Standard Methods for the Examination of Water and Wastewater. 17th edition, prepared and jointly published by American Public Health Association, American Waterworks Association and Water Pollution Control Federation). Total phosphorus is determined from the unfiltered sample and comprises all phosphorus fractions (orthophosphates, condensed phosphates and organically-bound phosphorus).
Total kjeldahl nitrogen (organic nitrogen plus ammonia) is also determined from the unfiltered sample. The concentrations of nitrogen occurring in the oxidised state (NO2 and NO3) and as ammonia are determined separately using the filtered sample. From these measurements, the concentrations of organic nitrogen and of total nitrogen (comprising oxidised nitrogen, organic nitrogen and ammonia) are determined.
Chlorophyll-a
Water samples are filtered through 0.45µm Whatman GF/C filter papers in known
volumes. Each filter paper with the retentate (i.e. the material retained on
the paper after filtration) is stored in a plastic tube with preservative, covered
in aluminium foil to prevent light entering, and transported on ice to the laboratory.
The amount of chlorophyll-a in the retentate is determined using a spectrophotometer
and the concentration of chlorophyll-a in the original sample calculated in
µg/L.
Suspended solids
A known volume of water is filtered through a pre-weighed filter paper (0.45µm
Whatman GF/C). The filter paper and retained sediment is then oven-dried at
105°C for one hour, cooled to room temperature in a desiccator, then weighed.
The drying, desiccation and weighing steps are then repeated, to give an average
final weight (filter paper plus retained sediment) based on two measurements.
A final concentration in mg/L is calculated by subtracting the initial weight
of the filter paper from the final weight (filter paper plus retained sediment),
and dividing by the volume of water that was filtered.
Faecal coliforms
Samples are collected from 20cm below the water surface, in 250mL sterilised
containers. They are stored on ice or in a refrigerator and sent to a certified
laboratory for analysis within 24 hours of collection.
At the laboratory, faecal coliform concentrations are determined using a standard membrane filtration method. A measured volume of water sample is passed through a membrane filter, retaining any bacteria on the membrane. The membrane is placed onto the surface of a growth medium (plate) and incubated at 44.0 - 44.5 °C; the number of colonies on the plate is then counted. The final concentration of faecal coliforms is calculated in colony- forming units per 100mL, by dividing the number of colonies counted by the volume of water that was filtered.
Turbidity, dissolved oxygen,
pH, conductivity;
These indicators are all measured in situ (on site) using a water quality
meter (YSI Grant 3800 Water Quality Loggers). This instrument is calibrated
and operated in accordance with EPA (Environmental and Technical Services) quality
assurance procedures.
Toxicants in sediments;
Sediment sample collection, preparation and storage are undertaken according
to methods outlined in the EPA Water Quality Sampling Manual 3rd
Edition (1999); chemical analysis was undertaken by specialist laboratories,
including state and commonwealth government laboratories.
A composite sample of fine-grained (muddy) sediments was taken from the left bank, midstream, and the right bank of the waterway; sediments were thoroughly mixed using a stainless steel bucket and spoon. Sub-samples for metals and metalloids, organic compounds and nutrients were spooned into an acid-washed bottle, a solvent-washed bottled and a thick-walled plastic bag, respectively. Samples were stored on ice or in a freezer until delivery to the laboratory.
View the latest version of the Queensland water quality guidelines, and information comparing water quality data to guidelines, and integrated water quality assessments.
Last updated: 16 February 2007
