Deposits at commercial banks in the Washington metropolitan area rose about 6 percent in the past 12 months to $12.93 billion on March 31 compared with $12.24 billion a year earlier, according to a survey by The Washington Post.

However, first-quarter deposits were down about $310 million from the record of $13.24 billion at the end of 1979. This decline reflects not only a normal withdrawal of money for income tax payments during the initial quarter, but also a decline of bank time (savings) deposits in some institutions, during a period in which higher interest rates could be earned on other invesments.

Moveover, the new deposit figures reflect a steady and definite slowing of regional economic activity. The 6 percent annual growth rate in overall deposits is down substantially from 9 percent growth during calendar year 1979 and 13 percent growth during 1978.

Local check volume as processed through the D.C. Clearinghouse Association was up just 5.34 percent in the first four months of 1980 compared with a year ago to $6.72 billion.

Separately, a new investment company report on banking firms in the Washington region shows that the earnings growth of area banks averaged 15.64 percent last year, with D.C. banks outperforming those in both Maryland and Virginia with earnings up 18.21 percent.

The report, compiled by Johnson, & Co., shows that Virginia banks had the highest return on equity (14.43 percent) while D.c. had the highest average return on assets (0.99 percent). Maryland banks had the highest growth in compound annual earnings from 1974 through 1979, at 22.54 percent.

Individual banks with the greatest growth in earnings last year were Madison National Bank, up 35.45 percent; Security National Bank, 26.77 percent; First American Bank of Virginia (controlled by Financial General Bank shares), 26.67 percent; and First Maryland Bancorp; 22.67 percent.

Among other general figures on area banks:

In terms of deposit, Maryland banks expanded the most in 1979 (up 10.81 percent), with D.C. banks up 8.91 percent and Virginia banks up 4.83 percent.

By far the best performer in compound annual earnings per share increase from 1974 through 1979 was First American Bank of Maryland, based in Silver Spring, and another Financial General unit. Recovering from a depressed period, First American of Maryland's earnings per share shot up at a compound annual rate of 127 percent (9 cents a share in 1975 vs $2.38 in 1979).

The slowest bank firm in earnings growth has been Union Trust Bancorp., of Baltimore, up 0.2 percent to $3.61 a share from $3.58 a share.

Among banks whose recent stock prices (mid-March) were close to or above book value are Madison ($37 vs. book value of $33.30); Security ($28 vs. book value of $30.85); Finanical General ($17 and higher vs. book value of $18); First American of Virginia ($42 vs. book value of $37.10); National Bank of Fairfax ($34 vs. book value of $33.70); New Virginia Bancorporation ($7.50 vs. book value of $8.22) and Citizens Bank & Trust Co. of Riverdale ($40 vs. book value of $44.39).

A key measure of business profitability is operating earnings as a percentage of stockholders' equity. By this measure, the best area performers in 1979 were: New Virginia Bancorporation, 17.6 percent; National Bank of Fairfax and Dominion Bankshares Corp., 16 percent; United Virginia Bankshares Inc., 15.9 percent; Citizens Bank & Trust and First Virginia Banks Inc., 15.8 percent; Madison National, 15.7 percent; First Maryland Bancorp and Suburban Bancorporation, 15.6 percent; Virginia National Bankshares Inc., 14.8 percent; Central Fidelity Banks Inc., 14.3 percent; and American Security Corp., 14.1 percent.

The Johnston, Lemon study covers 26 area banking firms, providing data on earnings, deposits, loans, capitalization and other operating measures.