For decades, those who summered around Annapolis rarely questioned whether or not to dive in. The beaches along the Severn, South, West and Rhode rivers were crowded with city families trying to escape the heat. Children dove off wooden piers adjacent to their family cottages and stayed in until the smells of barbeque called to them in the late afternoon.

But by the 1970s, the area began to change. The summer cottages came down, replaced with larger homes that now sell for millions of dollars. More people living year-round on the land led to changes in the water, too. It got murkier, and people would occasionally complain of earaches, diarrhea and stomach discomfort. Some newer residents built pools or joined existing ones, unwilling to trust the rivers. Even the stalwarts often wondered, is it still safe to swim in the rivers?

For nearly 22 years, Sally Hornor has been trying to answer that question. The microbiologist runs Operation Clearwater, a testing service that looks at the water quality of six river systems in Anne Arundel County. With two students from Anne Arundel Community College helping her, Hornor tests about 50 beach sites - some every week, some every other week. She is looking for evidence of enterococci, bacteria typically found in human and animal waste.

Often, Hornor is the bearer of bad news. A safe measurement for swimming is 104 enterococci per 100 milliliters, or about half a cup, of water. On some beaches, the counts have been as high as 50,000. Last year, some of her beaches reached 38,000. Not surprisingly, they are highest after a rain. And the hotter the temperatures, the more likely bacteria will thrive. Several beaches post their off-the-charts counts in the sticky heat of mid-August.

The day after she collects her samples, Hornor posts her results on the Operation Clearwater web site. There, residents can locate their beach on a map and view the history of counts. Communities pay $27 for each sample. Some individuals have hired her to test their private beaches as well. She conducts the test in a brand new lab at Anne Arundel Community College, a clean and bright room filled with stainless steel appliances and petri dishes and state-of-the-art equipment.

The enterococci she finds are not necessarily the cause of illnesses, Hornor said, but they are indicators that something is out of balance in the water. Their presence can also indicate high nitrogen counts and a hospitable environment for other harmful organisms.

Some Riverkeepers also send the results to community residents or post the results on their own web site. Within 24 hours of the test, Hornor said, people know whether to take a dip.

"I think people are really concerned about the water," Hornor said. "They really want to know if the water is safe to swim in. They see these high counts, and they say, 'how can we fix this?' We're letting people know that it is a problem."

Anne Arundel County's rivers have big problems. Since the University of Maryland began measuring river systems throughout the Chesapeake Bay in 2006, the rivers have scored among the unhealthiest. They have never scored above a D-minus; in both 2008 and 2010, they earned an F.

The rivers have little agricultural activity, nor are they home to heavy industry. Their problems come from failing septic systems and stormwater runoff, Hornor said. The Severn River alone is home to more than 40,000 septic systems - many holdovers from the area's summer cottage days, when the systems weren't used daily. Waste from pets and wild animals also can contribute to both high nitrogen counts and high levels of bacteria.

Hornor, 62, came to Operation Clearwater after earning her Ph.D. in ecology at the University of Connecticut. The Severn River Association, which began the testing program in 1974, was looking for someone to take it over. Since then, Hornor, who also teaches a full load of classes at the college, has won multiple accolades for her work, including the National Oceanic and Atmospheric Administration's Environmental Hero Award in 2002.

Hornor sees a couple of silver linings to the slog of bad news. Trends indicate slight improvements in water quality over the last 10 years, which she attributes to better education. Many beaches now have bins for pet waste, and many areas have put in stormwater retrofits that absorb pollution before it enters the waterways.

Herring Bay, a community near Deale with three swimming beaches, hired Hornor four years ago to test their water. Since then, the community has become more aware of its contributions to the problem, said Sharon Brewer, a volunteer with Advocates for Herring Bay, which brings Hornor water samples every week.

"People are a lot more aware of pet waste now," Brewer said. "It's really made a big difference."

Brewer said that residents check the Operation Clearwater site to determine whether they should swim. Even though the results are not in real time every day, they can make an educated guess on their safety based on the weather. The health department also tests, but it only does it once a month.

"It's nice to at least get an idea of what's going on," Brewer added. "We have a big beach, and people use it all the time."

And that is the other bright spot among the bad news: People in these communities refuse to give up on their rivers. For decades, Anne Arundel County has hosted a competitive swim team in the Severn River, with hundreds of children participating each year. Sherwood Forest campers swim daily in the river - although they check Operation Clearwater to make sure it's safe.

Camp Director Billy Moulden calls it the "Old School Huck Finn Summer" flavor and says the people in his community are "parents, grandparents, great-grandparents who sorely do not want to surrender the freedom of natural resources." From time to time, when conditions warrant it, Moulden has had to shut down river swim practice, but he has no plans to abandon it.

Having answered the question of whether it is safe to swim, Hornor is now trying to solve another mystery: What does it take to make someone sick?

For years, Hornor has collected anecdotal evidence of residents getting sick after swimming at times when bacterial counts were high. Now, she and her daughter, Kathryn Caperna, who recently earned her masters in public health at George Washington University, have embarked on a study to determine a correlation between illness and high bacteria counts. She will be distributing surveys to families who use the rivers and posting information on the beaches on how to reach her to report illnesses.

Hornor's work is gaining international attention. NOAA is thinking about duplicating her methods in India, and the World Bank is using some of her work to develop a citizen testing program in Liberia.

As for Hornor's own swimming plans, she says, she would take a dip - in June and maybe in July.

"At this point," she said, "I don't think I want to go swimming in August."