More than 90 percent of Northern Virginia 10th graders passed the 1982 state minimum competency test, but for the first time the statewide student pass rate dropped and the gap between statewide black and white student pass rates widened, according to a state report released yesterday.

The student pass rate dropped in every Northern Virginia jurisdiciton from 1981 to 1982. Statewide, 93 percent of 10th graders in regular programs passed the 1982 test, down from 95 percent last year.

In recent years the difference between white and black pass rates narrowed dramatically locally and statewide fell from 31 percentage points when the test was first given in 1978 to less than 10 points in 1981. The statewide gap increased to 13 points this year. The gap between black and white pass rates also increased in every Northern Virginia juridiction except Falls Church, where the figures were not available.

"We will be looking for an explanation, if there is one," said Beatrice H. Cameron, assistant superintendant for the Fairfax Public Schools, of the difference between black and white pass rates.

The competency test in reading and mathematics measures practical skills such as using a telephone book and basic academic skills such as finding the main idea in a story or adding fractions. Though students must pass the test to graduate, only a few have not graduated because they failed it. Students may take the test repeatedly.

"There's a possibility people did so well last year that teachers have become complacent about teaching those skills and have shifted back to teaching the regular curriculum more," said state test expert Donald S. Sale.

The statewide black pass rate on the test dropped more than four points from last year to 83 percent this year, while the white pass rate declined less than one point to 96 percent.

Virginia testing director Gerald W. Bracey said the gap between black and white pass rates narrowed from 1978 to 1981 in part because he believed basic skills teaching efforts were aimed at blacks more than whites. Blacks also were more likely than whites to barely pass the 1981 test, Bracey said, so a small decline in individual test scores in 1982 could have accounted for the lower overall black pass rate. Bracey said the test results also could have been a fluke.

"It may not necessarily be a trend," said James P. Akin, the chief testing analyst for the Alexandria public schools. "It may be an isolated thing." CHART:Virginia minimun competency results for tenth grade students.