How the Swimmable are Calculated
The beach grading system was created in consultation with scientists who study water quality in Long Island Sound. Our approach is designed to capture, for each beach, how frequently water quality was found to be unsafe for swimming (frequency) according to state water quality criteria, and a measure of how high the level of contamination is (magnitude) on the worst sampling day of the season. Because sources and concentration of contamination can vary with weather, the frequency and magnitude grades are provided for both dry and wet weather conditions.
The beach grades are a combination of four sub-category scores, equally weighted. Those sub-category scores are:
Frequency Dry (FD) = represents the percentage of samples, collected at the site during periods of prolonged dry weather, that meet the state water quality criteria for safe swimming (see above). This indicates how often a site is likely to be unsafe for recreation in dry weather. A high percentage of FD failure would indicate a consistent source of pollution that is unrelated to wet weather (e.g. groundwater discharge).
Frequency Wet (FW) = represents the percentage of samples, collected at the site after rain (greater than 1/4 inch of rain in prior 48 hours), that fail to meet the state water quality criteria for safe swimming. This indicates how often a site is likely to be unsafe for recreation in wet weather. A higher percentage of FW failure than FD failure would indicate the presence of pollution sources triggered by precipitation (e.g. CSO or urban stormwater).
Magnitude Dry (MD) = represents the highest concentration of fecal indicator bacteria measured in any sample collected at the site during periods of prolonged dry weather. Higher bacteria levels are associated with more risk of illness to swimmers, and therefore MD represents a measure of water quality on the worst dry weather sampling of the season.
Magnitude Wet (MW) = represents the highest concentration of fecal indicator bacteria measures in any sample collected at the site after rain (greater than 1/4 inch of rain in prior 48 hours). Higher bacteria levels are associated with more risk of illness to swimmers, and therefore MW represents a measure of water quality on the worst wet weather sampling of the season.
NY & CT State Water Quality Criteria
There are multiple sources of water quality concern for beachgoers (e.g. garbage, algae, pharmaceuticals, murkiness/turbidity), but the most common risk when swimming in polluted water is coming in contact with, or ingesting, disease-causing microorganisms such as bacteria, viruses, and protozoa associated with fecal pollution. Collectively, these agents are known as pathogens. This is why fecal bacteria concentration measured at beaches is used to determine if the water is safe for swimming.
Due to the wide variety of potential pathogens, it is not practical to test for them directly. Instead, beach water quality is assessed by testing for the bacteria Enterococci (Entero), which reliably indicates the presence of feces in water.. Following is the criteria used by the health departments in New York and Connecticut for coastal beach monitoring and management.*
Marine Beach Criteria: Single Sample Maximum
Enterococcus (“Entero”) = greater than or equal to 104 cfu/100mL
Any sample equal to or greater than 104 Entero colony-forming units per 100 milliliters (cfu) is considered unsafe for swimming and should result in a beach closure. Once closed, the beach should not be reopened until acceptably low bacterial counts have been restored.
Marine Beach Criteria: Geometric Mean
Enterococcus (“Entero”) = greater than or equal to 35 cfu/100mL
A geometric mean is a weighted average used to track water quality overtime. Beach managers typically track a rolling geometric mean average for each beach (each new sample updates the average, which is based on 5 samples). When a geometric mean is equal to or greater than 35 Entero that beach is considered unsafe for swimming and should be closed until the average returns to acceptable levels.
The levels in the guidelines are based on an anticipated illness rate of 19 or more illnesses per 1,000 swimmers. This means that at concentrations of 104/100 ml Enterococcus, approximately 19 out of 1,000 swimmers can be reasonably expected to contract a waterborne illness. Therefore, below the acceptable level of 104/100 ml there is still a chance of contracting a waterborne illness, but the risk decreases with lower bacteria levels.
Wet weather sample: cumulative rain fall equal to or greater than 1/4 inch in prior 48 hours.
Dry weather sample: cumulative rain fall of less than 1/4 inch in prior 48 hours.
Fecal contamination: water pollution that is the result of high concentration of fecal matter in the water. The source could be human or animal.
Pathogens: disease-producing agents including viruses, bacteria, and parasites.
Enterococcus (“Entero”): fecal-indicating bacteria that lives in the intestines of warm-blooded animals.
Colony-forming unit (CFU): a unit used to estimate the number of viable bacteria in a sample. Usually measured as CFU per 100 milliliters of water when evaluating bacterial water quality.
* New York and Connecticut follow the federal guidelines for recreational water quality that EPA issued in 2004. In 2012, based on new scientific research, EPA updated and reissued their guidelines for beach monitoring and management practices (Recreational Water Quality Criteria). The 2012 federal guidelines have not yet been adopted by New York or Connecticut.