Drinking Water Protection during Algal Mat Occurrence

Published on August 02, 2024

(Winchester, Va.) – The City of Winchester and the Virginia Department of Health (VDH) are monitoring potential impacts to drinking water related to algal mats present in the North Fork Shenandoah River. Algal mats have been observed in locations upstream of the City’s drinking water intake on the river. The City is taking every precaution to prevent impacts to drinking water, including monitoring both untreated river water and treated drinking water. At this time, the City’s water is safe to drink and there are no restrictions on drinking water use while monitoring continues. 

Due to normal increases in algae during the summer and especially this summer with very low water flows and high temperatures, the City has been conducting routine screening of the untreated river water for the presence of toxins that can be associated with harmful algal blooms (HABs).  These blooms are caused by cyanobacteria, which can produce a variety of toxins known as cyanotoxins. Normally, cyanotoxins are not present in the river, so sampling is typically seasonal. Cyanotoxins can affect human health if consumed in excessive quantities over an extended period of time.

A sample of untreated river water collected on July 23 showed the presence of anatoxin-a, a neurotoxin produced by HABs that can impact health. Since learning of this detection, the City has collected additional samples of both untreated river water and treated drinking water. It has also optimized its treatment process for the removal of cyanotoxins.  

There is not a federally established health advisory level for anatoxin-a. VDH has established a level of 0.4 micrograms per liter as a very conservative level.  If anatoxin-a is measured to exceed this threshold for ten or more consecutive days, then VDH will require the City to take additional action. Samples of treated drinking water collected on July 29 and July 30 both measured below this threshold, with an anatoxin-a level of 0.189 micrograms per liter detected in the July 29 sample and no anatoxin-a detected in the July 30 sample.

Enhanced water monitoring and optimized treatment activities will continue for as long as potentially harmful algal mats are observed, which could continue through October or longer. The City will advise the public if any protective action is warranted.

Customers with questions should contact Don Riggleman at the City of Winchester at (540) 773 – 1340.