Algal Blooms

Blue-green algae (or cyanobacteria) is a type of naturally occurring bacteria that can be present in water bodies, particularly lakes and reservoirs. These are known as Harmful Algal Blooms (HABs) because some of them can produce harmful chemical (called cyanotoxins) that cause adverse health effects if ingested or from coming in contact with skin. HABs tend to increase during spring and summer, as the water temperature warms up. WSSC Water closely monitors these algal activities in our Rocky Gorge Reservoir as it is used as a drinking water source for Patuxent Water Filtration Plant (WFP) as well as for recreational purposes.

Additionally, other naturally occurring seasonal algae can give the water an earthy or musty taste and odor. Do not be alarmed, these are not harmful algae. We carefully monitor water temperature, oxygen levels, and HABs levels to watch for such activity.

Cyanotoxins in Drinking Water

WSSC Water monitors both treated and untreated water for cyanotoxins during the warmer months to ensure it does not enter our water system. If we see any increase in algal activity in the source water, we step up this monitoring program and have treatment measures we can employ. While not required by regulatory agencies, we implemented this monitoring program in 2015 as a proactive measure to address growing concern related with HAB, and have not detected any cyanotoxins in the treated tap water.

The U.S. Environmental Protection Agency (U.S. EPA) published national drinking water Health Advisories for the cyanotoxins (see Table 1). The Health Advisories provide the cyanotoxins levels in drinking water that may pose adverse human health impacts when exposed to these levels over a 10-day time period. The Health Advisories are lower for younger children because they consume more water relative to their body weight and thus are at increased risk of adverse health impacts.  These Health Advisories are not an enforceable health standards based on sound science, but a recommended guideline for water utilities to manage the risks if found.




Children pre-school age and younger (under 6 years old)

0.3 ug/L

6 years and older

1.6 ug/L


Children pre-school age and younger (under 6 years old)

0.7 ug/L

6 years and older

3.0 ug/L

HAB in Recreational Water

We also monitor the health of our reservoirs, as they are often used for recreation. Accidentally consuming reservoir water, such as while boating, can cause harmful health effects if HABs are present. Please check signs for whether or not the water is safe for recreation, and follow advisories. WSSC Water watershed regulations prohibit pets from being in the water at any time.

People can have adverse health effects from harmful algal blooms if they:

  • Accidentally ingest reservoir water (e.g., splashed onto face)
  • Consume tainted fish flesh or internal organs
  • Wade or contact affected waters (e.g., while launching kayaks, canoes or boats)

Some of the adverse health effects can include:

  • Skin irritation or rashes
  • Nausea or other gastrointestinal distress
  • Disorientation
  • Numbness
  • Fatigue

Harmful algal blooms can also adversely affect pets and there are reports that some pets have died from contact with such toxins. WSSC Water watershed regulations prohibit pets from being in the water.

Harmful algal blooms can also create dead zones in bodies of water, which may kill aquatic life, raise treatment costs for drinking water and hurt businesses and jobs that depend on clean water.

If high levels of cyanobacteria are detected, WSSC Water increases water-testing efforts to closely monitor the status. Because the reservoirs also serve as a drinking 

water sources, we closely monitor toxin levels at the treatment plant as well.

In the event that you, someone you know or a pet has contacted or ingested water at either reservoir, you should call your local Health Department, listed below.

  • Howard County: 410-313-1773 (Community Hygiene Department)
  • Montgomery County: 240-777-0311 (Montgomery County 311)
  • Prince George’s County: 301-883-4748 (Prince George’s County 311) 

WSSC Water will post signs at that site’s recreation areas and post alerts on the WSSC Water website in order to limit or prevent recreational water contact until concentrations decrease to an acceptable level.

There are two levels of alerts: advisory and warning. Examples of the signs are below.


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