Each summer for the past few years, toxic blue green algae has been found in in our region. The problem appears to be growing worse. This past summer the Virginia Department of Health issued a Harmful Algae Bloom (HAB) Advisory for a 53-mile stretch of the North Fork of the Shenandoah River in August. Samples taken from algal mats on the river bottom contained harmful levels of toxins produced by cyanobacteria. Other areas of Virginia reported non-toxic algae blooms and there were several crowd sourced reports of algae in August.
Algae blooms also called harmful algal bloom (HAB) or dead zones form in summers when higher temperatures reduce the oxygen holding capacity of the water, the air is still and especially in years of heavy rains that carry excess nutrient pollution from cities, suburban lawns and farms. The excess nutrient pollution combined with mild weather encourages the explosive growth of algae fed by excessive nutrient pollution. However, toxic algal blooms are relatively new.
Not all algal blooms are toxic or hazardous. Only the certain species of blue-green algae form the toxin, for reasons that aren't fully understood. Toxic bacteria were not a problem until the 21st century, though algae blooms have been a problem on Lake Erie, the Gulf of Mexico, the Chesapeake Bay and other areas for over half a century. Only algae that contains microcystine or cyanobacteria, a toxin produced by microcystis, a type of blue-green algae that spreads in the summer are hazardous. In 2014 routine water testing in Toledo, Ohio found two samples that tested positive for microcystin at concentrations higher than the VDH advisory level of 1 microgram per liter for potable water. This shut down their water supply for days.
In the 21st century toxic or hazardous algal blooms have become a concern in our region. They occur when algae grow out of control when there are favorable environmental conditions. Hazardous algal blooms, the ones that contain the toxins, can lead to the poisoning of fish, shellfish, birds, livestock, domestic pets and other aquatic organisms that can lead to human health impact from eating fish or shellfish exposed to toxins as well as drinking water contaminated by toxins. Our existing water treatment plants do not remove the toxins and toxic algal blooms could disrupt water supply in our region.
This past summer in mid-July the Virginia Department of Environmental Quality (DEQ) found extensive multi-species benthic algal mats in the North Fork Shenandoah River. Analysis found that the mats contained several cyanobacteria and high levels of cyanotoxins. Tropical Storm Ida came through the North Fork Shenandoah River watershed on September 2, 2021, with severe winds and dumping almost 4 inches of rain on parts of the watershed sharply raising river flow rates.
Anticipating the storm’s potential to scour the algal mats from the river and wash them downstream, the Interstate Commission on the Potomac River (ICPRB) ran its emergency spill model to attempt to track the algae as it was carried downstream by storm flows. The ICPRB then implemented sampling to determine if measurable algal toxin levels reached the Potomac River mainstem after the storm. Samples were collected near the mouth of the Shenandoah River when their models indicated the scoured North Fork algal material would be passing.
The North Fork Shenandoah River is the raw water source for several towns including Woodstock, Strasburg, and Winchester, VA. Two toxins were detected in raw and/or finished water samples at Strasburg on August 3rd, 9th , and 12th but not detected in subsequent samples. Cylindrospermopsin was detected in one finished water sample at Strasburg on August 3rd (0.062 µg/L).
Neither toxin exceeded VDH’s advisory thresholds. Microcystin, nodularin and saxitoxin were not detected in any samples. The relatively low cyanotoxin levels in the water column and water supply intake samples suggested to the scientists at the ICPRB that the algal mats were still intact for the most part and were not releasing measurable toxins to the water column.
The Scientists believe that the North Fork algae bloom was washed out by Tropical Storm Ida before the die-off of the bacteria and release of the toxins and that the rainfall associated with the storm served to dilute the concentration of toxic bacteria. Tropical Storm Ida’s very high flows diluted the cyanotoxins in the scoured algal mats to non-detectable levels before they reached the Potomac mainstem.
However, the scientists suggest that “this might not be the case when streamflows are lower, as is more typical of late summer and early autumn, and cyanobacteria blooms are senescing. Further investigation of the downstream transport of cyanotoxins in the Potomac River and its tributaries could be done with river flow models that are more advanced than the spill model used.” While the spill model was adequate for the purposes of this rapid response sampling, the spill model is a relatively simple with built-in assumptions that cannot be changed to reflect actual river conditions. “Additional field observations and algal bloom sampling would also provide a better understanding of how rapidly cyanotoxins decompose once they are released into the water column and exposed to different river conditions.”
A new model needs to be built to reflect the behavior of the river and the cyanotixins to help prepare all the local water agencies for future toxic or hazardous algal events. With the increase in these events in our region we must be prepared for toxic bacteria impacting our regional water availability. Sadly, we need to be able to predict when toxic bacteria may force the Washington Aqueduct, WSSC, Fairfax Water and Loudoun Water to close their Potomac River intakes. Our water companies will need to be prepared with enough water storage to ride out a potential annual toxic bacteria event. Welcome to our new world.
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