Harmful Algal Blooms (HABs) in Drinking Water

What are HABs?

Freshwater harmful algal blooms (HABs), most notably Cyanobacteria blooms, were formerly known as blue-green algae blooms, and are found in lakes, rivers, ponds and other surface waters. Cyanobacteria are common in freshwater and are an important part of aquatic life. Excessive growth of Cyanobacteria producing HABs have become a growing concern in the United States and worldwide. HABs can produce toxins referred to as cyanotoxins that pose health risks to humans, animals, fish and shellfish. HABs can also create taste and odor problems in drinking water sources, producing earthy and musty smells and off-putting taste. The impacts of HABs have increased significantly over the past several years with blooms affecting the drinking water supplies in numerous communities in multiple states.

Cyanobacteria have the capability to synthesize metabolites known as cyanotoxins during their exponential growth. Not all strains of Cyanobacteria possess the genes necessary to produce cyanotoxins. Simply possessing the necessary genes does not ensure expression of those genes. Cyanotoxins are classified into three categories by chemical structure: cyclic peptides, alkaloids and lipopolysaccharides. The most commonly occurring are cyclic peptides, of which microcystins are the most common in freshwater. Microcystin-LR is the most toxic of the known variants of microcystin.

What are the Health Impacts of HABs?

In 2015, The U.S. Environmental Protection Agency (EPA) issued 10-day Health Advisories for two cyanotoxins: Microcystin and Cylindrospermopsin. If these cyanotoxins occur in drinking water over the EPA national 10-day Health Advisory level (see Table below), people are at risk for various adverse health effects including headache, incoherent speech, drowsiness, loss of coordination, abdominal pain, upset stomach, vomiting and diarrhea, as well as liver and kidney damage. The Health Advisories (Do Not Drink) provide guidance for the cyanotoxin levels in drinking water at which adverse human health impacts are likely to occur when exposed to these levels over a 10-day time period. Health Advisories are not enforceable safe drinking water standards.

10-day Health Advisories Level


The risk of adverse health impacts from cyanotoxins are higher for infants, young children under the age of six, and other vulnerable populations including:  pregnant women, nursing mothers, those with pre-existing liver conditions, those receiving dialysis treatment, the elderly, and sensitive populations.

What causes HABs?

Though all the factors that directly perpetuate HABs are not fully understood, increasing nutrient pollution and climate change are believed to be linked to the increase of occurrence and locations. Freshwater HABs occur most often where there are high levels of nutrients such as nitrogen and phosphorus present in warm, still waters such as reservoirs, lakes, and ponds. HABs can also occur in rivers, particularly in summer months. The sources of excess nutrients can be fertilizers from agricultural activities, sewage and industrial discharges, and stormwater. Climate change is also believed to be a contributor to the increased occurrence of HABs due to warming water temperatures, more frequent droughts, and increased flooding creating more polluted runoff into freshwater bodies. In recent years, Lake Erie algal blooms have lasted into December and January. Similarly, during California’s recent drought period higher incidences of HABs were experienced.

How do HABs Impact Drinking Water?

Cyanobacteria blooms are a significant concern to drinking water suppliers utilizing surface water sources. HABs can be difficult to identify. The shape, size, location, color and cyanotoxin production can vary from bloom to bloom. Seasonal and year-to-year fluctuations can make predicting the occurrence of HABs extremely difficult. While conventional water treatment facilities providing coagulation, sedimentation, filtration and disinfection can generally remove the cyanobacteria cells and very low levels of toxins, during a severe HAB event, when high levels of cyanobacteria and cyanotoxin occur, they can disrupt the treatment processes (floc formation, filtration and chlorination) and require considerable intervention to produce drinking water below the cyanotoxin health advisory levels. Some cyanobacteria may also produce color, unpleasant taste and odor, and may increase the production of potentially harmful disinfectant byproducts (DBPs) precursors. Each HAB event is unique and proper treatment measures need to be considered on a case by case basis. Control measures must be selected carefully, as applying the wrong treatment process could rupture the cyanobacteria cells and result in the release of cyanotoxins rather than the removal of cyanotoxins.

How to protect drinking water from HABs?

Be prepared!  Drinking water suppliers need to evaluate and understand their surface water sources and the conditions that could produce HABs including high nutrient levels, water temperature, flow, and pH. HABs can be transported to drinking water intakes by wind or water currents. Water suppliers should evaluate the ability to vary raw water withdrawals from different levels within the raw water source to reduce or eliminate drawing a HAB into the water treatment plant. Depending on the HAB potential for any public water supply source, water suppliers should consider evaluating alternative sources of supply unaffected by HABs. Water suppliers should also effectively monitor for cyanobacteria and cyanotoxin in the raw water source, as well as evaluate methods to treat the raw water source. Additionally, suppliers should evaluate the ability of the treatment plant processes to effectively remove cyanobacteria and cyanotoxin and consider additional treatment technologies or processes to address HABs dependent on the HAB occurrence risk.

How are HABs regulated in Drinking Water?

There are currently no Clean Water Act regulations for cyanobacteria in surface waters or Safe Drinking Water Act enforceable standards or limits for cyanotoxins, although cyanotoxins are currently on EPA’s priority list of drinking water contaminants of potential concern. The EPA Health Advisories for the two cyanobacteria toxins which were issued in 2015 and discussed herein are not enforceable, but provide the technical guidelines needed to assist public water suppliers in protecting their consumers.


FOR MORE INFORMATION | Serena DiMagno, Sr. Environmental Consultant