Water quality at San Diego County beaches was excellent again over the summer and contributed to one of the cleanest peak-use seasons on record along the states coastline, according to a report card issued Tuesday by Heal the Bay.

A major sewage spill in Tijuana caused closures along Imperial Beach shortly before Labor Day, but otherwise the Santa Monica-based environmental group praised the countys beaches with a slate of A grades.

Heal the Bay based its assessment on bacterial pollution data collected by agencies along Californias coast. The summer of 2012 marked the sixth consecutive year of excellent water quality statewide from Memorial Day through Labor Day. Ninety-six percent of all spots tested received A or B grades a slight improvement from last year.

Heal the Bay did find that a relatively small number of locations statewide posed health risks; 20 sites received fair-to-poor water quality grades over the summer. Chronic problem spots such as Malibu Pier and the Avalon were among those tagged with failing grades.

The Southern California Coastal Water Research Project is trying to shed more light on the interaction between stormwater and bacteria in San Diegos Mission Bay. Water quality at the popular destination has improved markedly during dry weather, according to Heal the Bay, but researchers want to look at it more closely because of periodic pollution problems.

Several possible sources of bacteria pollution have been essentially eliminated, according to the environmental research agency. Previous source tracking studies indicated that birds were substantial contributors and pollution from human sources was insignificant, said a summary by the research group.

The bacteria-tracking project is expected to include daily water tests at several spots in Mission Bay, and it should conclude in October.

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SD, state beaches fare well during summer tests