Algal Blooms

Recreational Water Contact Health Advisory Issued August 18, 2026

Blue-green algae (or cyanobacteria) is a type of naturally occurring algae that can be present in water bodies, including lakes and reservoirs. When algae accumulate to high concentrations or bloom, they can harm other aquatic life by reducing dissolved oxygen levels.  These are known as Harmful Algal Blooms (HABs).  Also, some HABs contain certain cyanobacteria known to produce harmful chemicals (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. WSSC Water closely monitors these algal activities in our Potomac River and Patuxent Reservoirs as they are used as drinking water supply sources as well as for recreational purposes.
 

These and other naturally occurring seasonal algae can also form algal mats and give the water an earthy or musty  odor. We carefully monitor water temperature, dissolved oxygen, and cyanotoxin levels to protect drinking water and recreational users. 

Cyanotoxins in Drinking Water

WSSC Water monitors both treated and untreated water for cyanotoxins to ensure these toxins do not enter our water system. If we detect increasing  algal activity in the source water, we increase 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 to HABs 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 under which adverse human health impacts are not anticipated over a 10-day time period of exposure.  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 enforceable health standards, but recommended guidelines for water utilities to manage the risks if found.

HABs in Recreational Water

We also monitor the health of our reservoirs as they are often used for recreation. Accidentally consuming reservoir water can cause harmful health effects if HABs are present. Signs will be placed around each recreation area when water contact is unsafe. Please adhere to these signs. 

People can have adverse health effects from HABs if they:

• Accidentally ingest reservoir water (e.g., splashed onto face)
• Consume tainted fish flesh or internal organs
• Wade in 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

WSSC Water watershed regulations prohibit pets from being in the water at any time.  Dogs are particularly vulnerable to becoming extremely ill or dying after contact with the water containing cyanotoxins.  

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

During a HABs event, if you, someone you know, or a pet has contacted or ingested water at the affected reservoir, you should call the appropriate county office from the following list:

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

When HABs occur, WSSC Water will post water contact advisory or warning signs at all affected recreation areas and alerts on the WSSC Water website. This is to limit or prevent recreational water contact until concentrations decrease to an acceptable level. Examples of the signs are below.

[Image]		An advisory means that there are high levels of blue-green algae in areas of the reservoir and water contact may pose negative health effects. During an advisory, water contact is not advised.        During a health advisory: Recreation areas remain open Boating and fishing are allowed Any body parts that come in contact with reservoir water should be rinsed with drinking water Keep pets away from the water
		A warning means that there are extremely high levels of blue-green algae in areas of the reservoir and water contact is not allowed due to likely negative health effects. The affected reservoir will be closed to recreation during a warning. During a health warning: Recreation areas are closed Boating and fishing are not allowed Any body parts that come in contact with reservoir water should be rinsed with drinking water Keep pets away from the water