Each site has been given a water quality condition rating, calculated using the results of weekly bacteria samples. The rating is based on a statistical average of E.coli (the geometric mean) over the last five samples. This average is compared to rating categories based on Victoria's current environmental quality objectives for primary and secondary contact recreation. To give users current information on recreational water quality, the ratings are based on samples taken under all weather conditions and are updated weekly.
The ratings are indications of recreational water quality over the previous five weeks, and do not give a measure of risk to the health of river users on any given day. River users should take care at all times and not swallow river water. They should also be mindful of other potential hazards such as high flows, unseen objects and litter.
The rating system has four categories:
High water quality - The E.coli geometric mean, based on the last five samples, has remained below the primary contact objective of 200 organisms per 100mL. These sites are generally considered suitable for all forms of recreation. Although the water quality is high, there may still be some potential health risk for some people. On occasions, such as after high rainfall, there may be an increased possibility of contamination from runoff, which may contain animal or human waste.
Medium water quality - The E.coli geometric mean, based on the last five samples, is in the range between 201 and 1000 organisms per 100mL. Sites with water quality in this range are considered suitable for boating, but not generally suitable for swimming. There is a potential health risk for some people from exposure to the water, particularly if swallowed. High rainfall events may increase the risk of contamination from animal or human waste.
Low water quality - The E.coli geometric mean, based on the last five samples, is in the range between 1001 and 5000 organisms per 100mL. Sites with water quality in this range may be used for boating, but they are not suitable for swimming. There is a greater potential health risk to river users from exposure to the water, particularly if swallowed. High rainfall events may further increase the risk of contamination from animal or human waste.
Unsuitable for recreation - The E.coli geometric mean, based on the last five samples, is greater than 5000 organisms per 100mL. Sites with water quality in this range are considered unsuitable for recreational activities, with a greatly increased potential risk to human health. This rating will remain in place until the E.coli geometric mean returns to below 5000 organisms per 100mL.
In addition to the condition rating for each site, EPA will issue alerts or advisories when required:
River user's alert - In the event of a pollution incident or single, very high E.coli result, that could affect the recreational water quality of the river. The alert will remain in place until E.coli levels in follow up samples return to a range considered suitable for recreational activities.
Stormwater advisory - If high rainfall events occur that may affect the recreational water quality of the river. EPA monitors rainfall at gauges throughout the catchment. When rainfall exceeds 20mm over the previous 48 hour period in the Lower and Middle Yarra or 40mm over the previous 48 hours in the Upper Yarra, a stormwater advisory will be issued. The advisory will apply to individual sections of the river, Lower, Middle or Upper, if rainfall has been localised, or to the entire river if rainfall is widespread. The advisory will be removed when conditions return to those considered suitable for recreational activities, usually around five days after heavy rain ceases.