Beach water quality at the Santa Monica Pier has dramatically improved, but overall water quality at Los Angeles County beaches dropped slightly in 2010-11, according to a recently released report by Heal the Bay.

In the Santa Monica-based environmental organization’s 21st annual Beach Report Card, A-to-F letter grades were assigned to 92 beaches in the county for the dry-weather period from March 2010 through April 2011 based on levels of weekly bacterial pollution. Approximately 75 percent of sites earned A or B grades this year, compared to 80 percent in last year’s report.

Making one of the biggest jumps was the Santa Monica Pier, which moved off the list of top 10 “Beach Bummers” to earn an annual A grade for the beach south of the pier during dry weather. Heal the Bay officials credited the significant advancement to more than $2 million in projects and efforts by staff to improve the water quality.

The overall decline in county beach water quality can be attributed to factors such as higher than usual rainfall totals during the reporting period, according to Heal the Bay. Some chronically polluted county beaches that had seen marked improvement experienced a drop this year despite millions of dollars being spent on water quality improvements, the report found.

The statewide average for beaches receiving A or B grades is 90 percent. Despite significant improvements over the past two decades, Los Angeles County continues to have the greatest number of beaches with poor water quality grades of any county in the state.

“Despite numerous individual beach success stories, this year demonstrated that there hasn’t been progress reducing major beach pollution sources like the Los Angeles River, Malibu Creek and Topanga Creek,” said Mark Gold, president of Heal the Bay.

The Beach Report Card is a study of coastal water quality based on daily and weekly samples taken from sites along the entire coast of California. 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

Heal the Bay officials noted that some significantly polluted beaches helped drag down the county’s overall grades, including those in Avalon, which was ranked second on the Beach Bummers, and Malibu.

Eight beaches in the county received year-round F grades, and the county led the top 10 Beach Bummer list, with four locations in the ranking of the state’s most polluted beaches.

Still, seven beaches in Los Angeles County were placed on Heal the Bay’s Honor Roll, meaning they scored perfect A+ grades by not having any bacterial exceedances in year-round dry weather.

Heal the Bay officials believe that one of the reasons that L.A. County lags in water quality is because many monitoring agencies, unlike most others in the state, collect samples directly in front of flowing storm drains and creeks. Monitoring at “point zero” locations, where polluted runoff often pools, is the best way to ensure that health risks to swimmers are captured in water quality data, officials said.

Summertime water quality at Santa Monica Bay beaches was excellent, with 91 percent of the beaches from Palos Verdes to Leo Carillo receiving A or B grades, the report found. Wet weather water quality in L.A. County fell significantly, with 29 percent of beaches receiving A or B grades compared to 50 percent last year.