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Update Regarding Water Quality Issues at Scottsdale Pond

Post Date:08/22/2017 8:55 AM

 8/30/17 Update: The City of Novato first noticed an algae bloom at Scottsdale Pond in late July.  Algae blooms can occur when water temperatures and nutrient levels rise in a body of water.  Water samples from Scottsdale Pond were taken and sent to a lab to test for algae-produced cyanotoxins.  EPA method 546 was used by the independent lab to determine total microcystin-LR cyanotoxin in the water.  These common cyanotoxins are produced by freshwater algae blooms and can be harmful to people and animals in high doses.

The results of the three samples taken are as follows:  2.880 micrograms per liter, <7.50 micrograms per liter, and 6.650 micrograms per liter.  When microcystin-LR concentrations exceed 6 micrograms per liter, The California Water Quality Control Board and Department of Public Health recommend that signs be placed warning against swimming in the water, drinking or cooking with the water, eating shellfish from the water, and advising that pets or other animals should not enter the water or drink the water. 

The Scottsdale Pond algae-produced cyanotoxin test results have slightly exceeded the state warning threshold, and Scottsdale Pond Park visitors are reminded not to drink or cook with the pond water, not to eat any fish or shellfish caught in the pond, that there is never any swimming allowed in Scottsdale Pond, and pets and other animals should be kept away from the water. Updated warning signs are being posted at Scottsdale Pond today.  

Similar situations occur every few years at Scottsdale Pond—typically during periods of drought or particularly warm summers. Cooler temperatures and rain events in the coming months will help to restore the water levels, reduce the algae, and improve the overall quality of the water.

More information about algae blooms and cyanotoxins can be found here:   http://www.mywaterquality.ca.gov/habs/what/index.html

Scottsdale Pond Warning Sign (English)
Scottsdale Pond Warning Sign (Spanish)

8/22/17: The City of Novato first noticed an algae bloom at Scottsdale Pond in late July.  In response the City sampled the water and sent it off to a lab for testing. We are awaiting those results.
 
Additionally, Staff is assessing the situation that includes: 

  • There is no fresh water inflow to the pond during summer months – and the pond has become stagnant.
  • Water levels are very low, on average less than 3-feet deep, due to increased evaporation in our recent hot summer months, and the water depth is too low to run the aerator fountains (the intakes are into the mud).
  • The reduced volume and shallower depth of pond water has heated up and has very little capability to hold oxygen.
  • The decomposing algae is creating a high level of “biological oxygen demand” that is lowering the available oxygen for fish (carp).
  • Carp are likely the only fish that have survived in the shallow, warm water pond.  They churn up the sediment and algae material on the bottom of the pond exacerbating all of the above.
  • As of this morning there were 30-50 dead carp along the shore, there are an estimated 1,000+ fish remaining in the pond that remain at risk.

Next steps:

  1. City staff will post signs to residents to keep pets out of the water until the green algae “slick” on the surface has diminished.  Pets should be given fresh water before and during exercise, and rinsed off if they do go in the pond.
  2. We will take steps, as possible, to aerate the shallow water in an effort to increase dissolved oxygen to reduce fish die-off.
  3. Aeration efforts will continue until daytime temperatures drop, the water cools, and the oxygen levels naturally increase.
  4. We will monitor the pond, and remove the dead carp on a daily basis as needed.

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