Before going for a dip in the bay or hopping on water skis this summer, there's a way to know how much bacteria is in the water.
The Maryland Department of the Environment posts swimming advisories on an interactive online map at marylandhealthybeaches.com. Beaches that are closed due to high bacteria readings are shown with red markers; safe beaches are shown in blue.
A smartphone app is in the works for the MDE beach data.
All Maryland counties with shorelines test water. Anne Arundel County has the most robust government testing program, with about 90 testing sites along the more than 500 miles of shoreline in the county.
Arundel's health department tests waterfront locations on a weekly, biweekly or monthly basis to measure enterococci, a bacteria that indicates that there is waste from a warm-blooded animal on the water.
In addition, neighborhood associations and environmental groups pay Anne Arundel Community College to test about 40 more sites on the Magothy, Severn, South, West and Rhode Rivers.
Tests begin before Memorial Day and continues through Labor Day. Results for both programs are posted online.
"People can use that information to adjust their use," said Sally Hornor, a biology professor who has run Anne Arundel Community College's Operation Clearwater for 28 years. "It doesn't mean they can't use the water, just they need to be careful."
Swimming in bacteria-laden water can cause gastrointestinal illnesses.
Health officials say people should not swim in natural bodies of water if they have compromised immune systems, ear infections, a perforated eardrum, open cuts or skin lesions. Everyone who swims in natural bodies of water should wash well afterward.
Bacteria can get into waterways in a number of ways, including sewage spills, leaky septic systems, untreated waste from boat toilets, animals (such as geese or dogs) and from stormwater runoff.
In Arundel, the county and college programs test water samples and count colonies of bacteria. For county-tested sites, officials will close beaches to swimming if counts are too high, based on U.S. Environmental Protection Agency standards. Usually, the limit is 104 or 158 colonies of bacteria per 100 ml of water, depending on frequency of testing and how many people swim at the site.
Closed beaches are marked with yellow warning signs and noted on e-mail and Twitter alerts, as well as listed on a recorded phone line.
The county also offers a pre-emptive warning against swimming in creeks, rivers or the Chesapeake Bay for at least 48 hours after any rainfall that totals a half-inch or more. That's because stormwater carries bacteria and other pollutants in the water. The county also does not do bacteria testing within 48 hours of a rainfall, because bacteria counts predictably rise after rainfalls.
The county does not officially close down beaches that are tested through the college's Operation Clearwater program. The college works with its participating groups, including riverkeepers, to spread the word of high bacteria levels. Some communities post the data on Facebook or bulletin boards at beaches.
For information on the county testing, call 410-222-7999 or visit aahealth.org/beach.
For Operation Clearwater testing results for the Magothy and Severn, visit ola2.aacc.edu/tldomanski. For South River, visit southriverfederation.net under "river health," and for West and Rhode rivers, visit westrhoderiverkeeper.org under "programs."
The nonprofit riverkeepers have their own nationwide website and app that shares their own independent test results and government results called the Waterkeeper Swim Guide.