The NJ Department of Health (NJDOH) assesses water quality at approximately 180 ocean and 35 bay monitoring stations along the NJ coast beginning mid-May and ending in mid-September. These samples are tested for Enterococci, a type of bacteria in animal and human feces. The presence of these bacteria could indicate the possible contamination of waters people use for enjoyment and relaxation. Swimming advisories are issued based on the amount of bacteria per 100mL of sample. If the concentration of bacteria in a sample exceeds the state standard (greater than 104 colonies), a swimming advisory is put into place and further sampling is done each day. If two consecutive daily samples are greater than the standard amount, the beach closes until the subsequent results are below the standard. The NJDOH notes that regional health or enforcement agencies can close beaches at any time they feel necessary to ensure public and health safety.
(The NJ Department of Environmental Protection manages the NJ Cooperative Coastal Monitoring Program referenced above where county health departments conduct water quality monitoring on recreational beaches in NJ.)