Deposit growth at banks based in the District of Columbia slowed modestly in the third quarter of 1979, the first such easing in the past two years and a significant indicator that economic activity here has started to cool.

According to a survey by The Washington Post, D.C. bank deposits on Sept. 30 were a record $6.13 billion, an increase of about 14 percent over the same date last year.

The total was not much higher than the previous record of $6.03 billion on June 30. Moreover, the rate of growth was down from an 18 percent level in the 12 months ended at mid-year.

In Northern Virginia, deposits on Sept. 30 were $3.2 billion, virtually unchanged from mid-year and up by less than 9 percent from the same date in 1978. The table of Virginia deposits in the area was published in The Post yesterday but the figures for the Nothern Virginia Bank were inadvertently deleted. Northern Virginia's Sept. 30 total was $146,687,606.

According to figures from the Federal Reserve Bank of Richmond, regional banks in recent weeks have experienced relatively miniscule growth in negotiable certificates of deposits and other large savings deposits as interest rates have soared.