The Nantucket County Department of Health, and the Massachusetts Department of Conservation and Recreation collect water quality samples from May through September. Some beaches are tested weekly on Tuesday, while others are tested monthly (Cliffside, Sconset, and Surfside 2). Infrequently used beaches or beaches that historically have had very few, if any, water quality issues are tested less often, while high-use or historically problematic beaches are tested more often.

The water is tested for Enterococci to determine water quality in brackish and marine water environments. Enterococci is a group of bacterial species typically found in human and animal intestines and are therefore present in sewage. Enterococci is also referred to as indicator organisms. Indicator organisms are used to predict the presence of pathogenic, or disease-causing, organisms associated with fecal contamination.

Swim Guide checks for the latest information daily, Monday - Friday during the swimming season.

A beach is marked Green when less than 104 units of Enterococci / 100 ml of water is present in a single sample, or when the geometric mean of 5 samples contains less than 35 units / 100ml of water.

A beach is marked Red when more than 104 units of Enterococci / 100 ml of water is present in a single sample, or when the geometric mean of 5 samples contains more than 35 units / 100ml of water.

A beach is marked Grey when there is no current or reliable monitoring information available.