All of Marin County’s beaches received high marks for water quality on dry summer days last year. Heal the Bay, a Santa Monica-based environmental nonprofit that grades California beaches according to the likelihood of falling ill from contaminated water, gave A grades to all 24 beaches tested in the county. The grades consider levels of coliform, E. coli, and Enterococcus bacteria, as reported by the county. “The beaches we monitor are doing exceptionally well,” said Arti Kundu, a project manager with the county’s Environmental Health Services division, which provided the data for the report card. The county is required by state law to monitor water quality weekly from April through October. The monitored beaches all have more than 50,000 annual recreational users, and each has a storm drain discharging into the water. In the summer months, water quality is generally good across California: Last year 95 percent of the state’s beaches got A or B grades from Heal the Bay. Water quality is worse in winter months because of runoff, but counties are not required to test in the rainy season. A five-year average from Heal the Bay shows that on rainy days during the summer season, the number of A or B grades drops from 99 percent to just over 80 percent. Parts of coastal Marin got 15 inches of rainfall last year, less than half the historical average, and the county is now in a severe drought. Lack of rain, which accounts for the county’s drought-related problems, may also account for low levels of water contaminants. Tests conducted on two rainy winter days earlier this year in the Point Reyes National Seashore found excessive levels of E. coli and Enterococcus. The private tests, conducted around Kehoe Lagoon, Abbotts Lagoon and Schooner Creek, were commissioned by the Western Watersheds Project, a nonprofit involved in efforts to end ranching in the park. The sites are not part of the county’s regular beach monitoring program, and they are not swimming areas. Ms. Kundu noted that the private testing was conducted during the first major rain of the year, and took place over only two days. “That’s why we do weekly monitoring,” she said. “From one snapshot, you cannot generalize the water quality.” Ms. Kundu added that the county does monitor water quality at Drakes Beach and Drakes Estero, both within the seashore, through the Environmental Action Committee of West Marin. She called those sites “very clean.” Luke Ginger, a water quality scientist at Heal the Bay, cautioned that the report card does not reflect day-to-day water quality, and he said beachgoers should consult his organization’s weekly online beach reports or the county’s water quality testing site for a more up-to-date picture. Some freshwater and brackish swimming spots in Marin have not tested as well as the beaches: Last summer, the county posted consistent advisories against swimming at Chicken Ranch, the Green Bridge and the Inkwells. Those advisories have been in place off and on this summer.