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Port Phillip Bay EPA water quality

The table below shows the average of all samples of the EPA’s beach report enterococci (bacterial) sampling results from December 2013 to February 2024.

It is interesting to note some locations receive extraording readings at certain times.  I assume this is from large rain fall events flushing out rivers or drains.  

You can click locations marked with * to see the more detailed data graph. (This shows reading averages by Month, not every single reading). 

If you are interested in a location data not shown, please let me know via the comments and I will update it.  

Also, what do you think about the high readings for your location?  Do they correspond to extreme rain fall events?  Please let us know in the comments too.

Location *click to show dataAverage (orgs/100ml)Num of Samples
St.Kilda *235214
Mornington Life Saving Club *208200
Port Melbourne Beach *188217
Elwood *186188
Frankston Surf Life Saving Club182220
Frankston Coast Guard172193
Carrum Surf Life Saving Club149178
Mordialloc *147210
Mentone *139214
The Dell135184
Werribee South134185
Canadian Bay112209
Eastern Beach95188
South Melbourne LSC *88199
Hampton *73182
Seaford *69206
St Leonards69184
Brighton Life Saving Club *63154
Safety Beach59207
Williamstown *58183
Aspendale Life Saving Club5553
Black Rock Life Saving Club51166
Half Moon Bay *42207
Mount Martha33163
Santa Casa27169

Is it worse in summer?  Below we averaged all EPA sample readings for all locations by month (scale on left).  Taken as an average all across the Bay it does appear that spring is worse.  The bacterial sample readings for October, November and December are higher in these months, where as the sea water temperatures are highest in January, February and March. So it’s worse in Spring., not worse in Summer.  

We can compare it somewhat against the Melbourne mean rainfall by month which is shown in yellow (the scale is in mm on the right).   Although, this is the  Melbourne area rainfall, not the whole Port Phillip Bay area.  Port Phillip Bay receives water from six major catchments: Yarra, Werribee, Maribyrnong, Dandenong, Mornington, and Bellarine.

Port Phillip Bay Circulation and Wind Direction.  

I do not know what difference the general circulation of currents in the Bay makes to how long rain water run off would stay closer to shore.  

For example, from December to February there is a general clockwise rotation of currents around the Bay.

From June to August there are two opposing eddies.  See image on the right.

Further, the wind strength and direction would have impacts depending on whether there are offshore or onshore winds in respective locations as these create waves energy and circular motions. 

Further analysis is required, perhaps at a specific location, to determine the relationship between rainfall, temperature and bacterial levels.  Would you like to do that for your favourite swim site? 

December to February
June to August

Mentone Averages

A regular swimmer from Mentone asks, (below in the comments), is bacterial sample readings getting worse over time?  

On the right I have produced 2 graphs of the readings from Mentone.  The first graph contains the average of all samples by year from Mentone from 2014-2023. 

To better determine any trend over time, I wanted to exclude data outside the standard deviation of the dataset for Mentone.  This is the second graph.  From both these graphs, I don’t think we can conclude that the sample readings of bacterial org/100ml are getting worse over time.   

What do you think?

Article by John Scanlon.  

You can also use ‘About – Contact us’ for feedback or further questions.  Below is the link to the source EPA data. 

This Post Has 10 Comments

  1. Peter Y

    Hi – Can you please provide the Port Melb beach and Sth Melb beach detailed graphs too? Thank you

  2. Jurgen

    I wonder if others would be interested in a graph that shows both the ecoli count and temp??

  3. Don W

    John, congrats on all your work to compile these figures for us to see. The number of drains feeding into the bay close to where the readings are being taken presumably has a major bearing on these figures. Similarly, proximity to the mouth of our rivers (Yarra, Patterson, Mordy Creek ) would influence readings. After 4 years of regular swimming at Parkdale, I can confirm that really heavy rainfalls definitely impact our water quality. There is a large drain close to Mordialloc horse beach, a smaller one below Parkdale Beach Cafe and Mordy Creek itself, about 1.5 km south of where we enter/exit the water. Further north, there is a large drain and a smaller drain on either side of Mentone LSC. Water quality at Mentone is no doubt also impacted by the nearby Mentone dog Beach and the build-up of seaweed in ‘the corner’. Thanks for your time in undertaking this study and for sharing with us. Great stuff.

    1. Hi Don – Below is the bacterial sample readings for Mordialoc interspersed in date order with your local rain readins:
      2/01/2024 6:00 10 orgs/100ml
      7/01/2024 26 mm rain
      8/01/2024 31.5 mm rain
      9/01/2024 6:00 300 orgs/100ml
      15/01/2024 31 mm rain
      17/01/2024 16.6 mm rain
      16/01/2024 6:00 4600 orgs/100ml
      19/01/2024 00:00 290 orgs/100ml
      23/01/2024 6:04 10 orgs/100ml

      There was 50 ml in the immediate days preceeding the reading on the 9th and a sample of 300 orgs/100ml, yet almost the same amount of rain preceeding the reading on the 16th yields an extraordinary reading of 4600 ml!

      What’s going on here? Is it water coming out of the creek and/or the wind and tide factors? This data does tell us much except the extraordinary reading dissipated after 7 days. What do you think?

  4. Julia

    This is a great initiative John has undertaken. It’s certainly an issue close to our hearts! I have a couple of suggestions:

    1. In the table of figures and testing sites, in the headings “Average” and “Count” add “(orgs/100ml)”

    2. You have inspired me to do a bit of brief analysis on the EPA data and it looks like the averages dating back to 2013 may be hiding a gradual overall increase in E. coli/100ml with time. I’d be interested in seeing the location average for each year, as well as the current figure for that location. I realise this would require some mucking around with Excel and maybe macros, but the results could reveal a trend worth knowing about. A bit easier would be just displaying an average for 2013 and for 2024 so we can compare the two.

    1. John S

      Hi Julia,
      As per the updated artilce. Plus as per your request, I’ve added Hampton now. I tried a different graph style for that.

  5. Peter M

    Many thanks John. I would be fascinated to see what the water testing results are like in the channel at the Brighton yacht club, particularly during sailing season. I would be willing to contribute to regular testing of the water quality for12 months. If there are other like-minded people that would be interested to contribute their time, I would be interested to contribute financially to learn more about this.

    1. Hi Peter,
      It is possible to get water kits for EColi measuring, but they vary in cost, time & accuracy. It is something to consider.

      Also, the EPA also does have other water quality sensors in 11 spots in Port Phillip Bay. These don’t do bacterial levels, they do nitrates & oxygen levels, etc. I’ll do a short article on this later too.

  6. Peter M

    Thanks for the tip. I’ll investigate this further.

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