Water quality at Los Angeles County beaches continues to show improvement but remains below the statewide average, an annual beach report found.

In its 22nd annual Beach Report Card, the Santa Monica-based environmental organization Heal the Bay assigned A to F letter grades to 91 beaches in the county for the dry-weather period from April to October 2011, based on levels of weekly bacterial pollution.

According to the report, approximately 82 percent of sites earned A or B grades, compared to 75 percent in last year’s report. The 82 percent figure is seven percentage points better than the county’s five-year summertime average.

Of the 407 beaches sampled throughout the state, 92 percent received A or B grades, a 2 percent increase from last year.

The improvement during all three phases of the 2011-12 study can be attributed to a number of factors, most notably ongoing infrastructure improvements aimed at curbing bacterial pollution, Heal the Bay officials said.

But officials note that despite significant improvements over the past two decades, the county continues to have the greatest number of beaches with poor water quality grades of any county in the state.

“We are heartened by numerous individual beach success stories, but this year highlighted that there is still a long way to go in addressing stormwater pollution,” said Kirsten James, Heal the Bay’s director of water quality. “Locally, Heal the Bay will be focusing efforts on advocating for a strong municipal stormwater permit to address this critical beach water quality issue.”

The county has the dubious distinction of being home to seven of the 10 beaches listed on Heal the Bay’s annual “Beach Bummer” list, which ranks the most polluted beaches in the state. Avalon Beach on Catalina Island tops this year’s list. Malibu claimed four of the 10 spots on the Beach Bummer list.

The Beach Report Card is a comprehensive evaluation of coastal water quality based on daily and weekly samples taken from sites along the entire U.S. Pacific Coast. A poor grade means beachgoers face a higher risk of contracting illnesses such as stomach flu, ear infections, upper respiratory infections and skin rashes than swimmers at cleaner beaches.

A total of six county beaches were placed on Heal the Bay’s Honor Roll, with perfect A or A+ grades. However, 11 beaches in Los Angeles County received F grades for the summer reporting period, up from last year’s nine.

During wet weather conditions, only 34 percent of county beaches received A or B grades, although it was a slight improvement to last year’s 29 percent. Los Angeles wet weather water quality remained well below the statewide average of 64 percent A or B grades.

Heal the Bay officials noted that one reason the county scores lower in water quality is that its monitoring agencies – unlike most others in the state – collect samples directly in front of flowing storm drains and creeks.